The dimensions of the bars for a gable roof. Rafter leg: design and installation

The construction of the roof frame is carried out according to the developed project, which indicates all the necessary parameters, including the type of construction, the pitch of the rafters, the section of the elements and the method of mounting the nodes.

System calculation principles

During the operation of the roof, its frame experiences high loads of various types.:

• constants (the weight of the rafter system itself and the roofing pie);
• periodic (wind and snow load, the weight of a person servicing or repairing a roof or chimney).

In order to correctly calculate and make a reliable roof, it is necessary to determine its configuration, choose the type of roofing, and calculate the optimal angle of inclination of the slopes. The degree of complexity of the frame and the dimensions of its elements to a certain extent depend on the parameters of the design load, the main part of which falls on the rafters. It is advisable to choose such dimensions of a wooden rafter as a section with a certain margin of safety.

How to calculate the length of the rafters? For calculations, it is required to apply the Pythagorean theorem (if the length of the end wall and the height of the ridge are known), or the sine theorem (if, in addition to the length of the end wall, the angle of inclination of the roof slope is known).

For the manufacture of rafters, you can use boards or timber. To build a roof frame, designed for high loads, will help additional elements that give the structure rigidity.

Determine the pitch of the rafters

To make a calculation of such a parameter as the pitch of the rafters, it is necessary to take into account the weight of the roofing, the angle of inclination of the slopes, wind and snow loads. On average, the step (the distance between adjacent legs forming the roof slope) ranges from 70 to 120 cm.

To eliminate the risk of deformation of the rafter legs under high loads, it is recommended to use dry lumber when installing the rafter system. Usually it is a beam or a board with a thickness of at least 50 mm. The exact dimensions of the wooden rafters and other elements are determined based on the requirements for structural strength.

The pitch of the rafters depends on the degree of slope of the roof and the length of the rafter legs. To build a solid roof by covering a large span between the ridge and the top of the wall, the pitch of the rafters should be reduced. For example, for a roof with a slope of 45 °, the maximum step should be no more than 80 cm. The rafter step should also be reduced when using heavy roofing materials, which include ceramic tiles, cement-sand tiles, asbestos-cement slate.

Features of calculating the section of the elements of the truss system

If you have to build a roof with your own hands, you need to calculate the size of the rafter legs - calculate the required section of the rafters. Considering how to calculate the rafters, you should pay attention to the characteristics of the material from which the rafter legs are made.

Regulatory documents regulate the bearing capacity of wood various breeds. If the cross-section of rafters from timber or boards, weakened by cuttings and / or holes for bolted connections, is considered, the load-bearing capacity of wood is calculated with a factor of 0.8 of the standard value. It is also necessary to pay attention to the type of wood for manufacturing - defects reduce its resistance to stress. The cross section of the rafters is selected taking into account standard sizes lumber. Make continuous load-bearing structure follows from a bar or board with a length of not more than 6.5 m.

Having calculated the system and determined the dimensions of the rafter legs and crossbars, it is required to calculate the total weight of these elements and add the resulting value to the calculated loads:

• the total volume of lumber required for the roof frame is multiplied by the volumetric weight of the wood;
• the resulting value (own weight of the rafters, kg / m2) is added to the calculated load;
• the calculation scheme of the structure is recalculated using the result obtained above.

Treatment of truss elements with an antiseptic

In private construction, the construction of a truss system is most often carried out from lumber, since wood is affordable and allows you to make structures with your own hands without the use of complex tools. Wood material prepared for installation (such as timber, logs) often gets to the construction site already treated with protective agents in a production environment. But a board or a bar that is not impregnated with special compounds is usually used for manufacturing.

How to process the rafters before installing the roof frame? Treatment is required to protect the wood from rotting and prevent fire hazards. Treatment with antiseptic and flame retardant can be carried out separately. With the use of a complex fire and bioprotective agent, processing will take half the time.

Treatment with an antiseptic or a combined composition should be performed in two steps. It is necessary to impregnate the top layer of wood with a special liquid, applying it with a brush or roller. After the first layer has dried, the antiseptic treatment is repeated.

pitched roof rafters

How to make rafters for pitched roof? Construction of a truss system single-pitched or gable roof do-it-yourself requires a careful approach to the manufacture of rafter legs. Dimensions are calculated at the design stage of the roof. To properly make these structural elements, it is necessary to use lumber of a section and length regulated by the project.

The degree of complexity of the work largely depends on which design is chosen for installation. If it is required to make layered rafters from boards or timber, each element is adjusted at the installation site when attaching it to the ridge run and Mauerlat. It is important to strictly monitor the observance of the geometry of the entire structure.

Hanging roof trusses are more convenient to make according to a template in order to achieve an exact match in the dimensions of each structure. For this, cutting in boards and assembling trusses is recommended to be done on the ground. Then it is necessary to check the horizontalness of the Mauerlat or support beams, the geometric dimensions of the building box. Having eliminated possible shortcomings, you can proceed with the installation of roof trusses on the house.

Diagonal rafters

Do-it-yourself arrangement of the hip roof truss system requires installation various kinds rafters, such as:

• sloping (diagonal beams forming a triangular slope);
• central hip;
• side;
• shortened (spidermen).

The side rafter legs are made of board and are installed similarly to the elements of a conventional pitched roof with a hanging or layered structure. The central hip rafters are layered elements. To make sprigs, bars or boards are used that are attached to diagonal beams and Mauerlat.

How to make rafters for a hip roof? To properly mount this species roof structure, it is required to accurately calculate the cross section and the angle of inclination of the sloping beams. The dimensions of the elements depend on the length of the overlapped span. It is important to observe symmetry when installing diagonal rafters, otherwise the roof may deform under load.

Making rafters to a given size

The use of unified lumber for the manufacture of various elements of the truss system allows you to optimize construction costs and simplify the calculation and installation of roof units. In particular, if it is necessary to make rafter legs of a certain section and length, a solid beam, its segments or boards can be used.

To make a rigid beam with your own hands, the method of rallying boards is used - they are connected by wide sides and pierced in a checkerboard pattern with nails. A long beam of a given section can be made from four or more properly cohesive boards - interconnected with a shift of half the length of the board. Such a beam is highly durable and can be used as a diagonal rafter.

When deciding how to lengthen the rafters, you can apply the liner method. In this case, a third board is placed between the two boards, protruding to a certain length. To connect the boards, nails driven in a checkerboard pattern are used. It is important not only to carefully align the boards, but also to put fragments of the board (inserts) corresponding in thickness to the central board into the empty gap between the extreme elements. This method allows you to increase the length of standard rafter legs (not hip).

Principles of fastening rafters

To ensure the reliability of the truss system, erected by one's own hands, it is necessary to decide in advance how to fix the rafters in the ridge and to the roof support. If it is planned to make a fastening that will prevent deformation of the roof during shrinkage of the building, it is necessary to fasten the rafters together at the top with a bolt with a nut or a hinge plate, and install a special one at the bottom fastener- sliding support.

A hanging truss requires a rigid attachment of the rafters to each other in the ridge: you can use a nail connection, a cutting method, overhead plates made of board or metal. The layered system does not provide for the connection of the rafter legs to each other - they are mounted to the ridge run.

How to attach the rafters to the base of the roof? In order to properly fix the rafters on the Mauerlat, a cut must be made in the rafter leg so as not to weaken the roof support. When installing roof trusses on floor beams, cutouts should also be made, but in this case, the notch is also made in the support beam.

How to make roof rafters: step and size, how to fix and calculate the length

How to calculate rafters: we calculate correctly

The weather conditions of our country are fickle, so the rafter system of a house under construction must have sufficiently high reliability and durability. This article describes how to calculate the rafters and truss system, various loads on them and provides an example of such a calculation.

Regardless of the chosen shape of the future roof, its rafter system must be strong enough, for which it is necessary, first of all, to correctly and correctly calculate the truss system.

The primary task of the designer and architect is not to design the appearance of the building, but to carry out a qualitative calculation of the strength of the planned house, including its rafter system.

The calculation of the rafter system includes a number of different parameters, which include:

• weight of roofing materials used to cover the roof, for example - soft roof, ondulin, natural tiles, etc.;
• weight used for interior decoration materials;
• the weight of the structure of the rafter system itself;
• calculation of beams and rafters;
• external weather effects on the roof and others.

In the process of calculating the truss system, it is imperative to calculate the following positions:

1. Calculation of the section of the rafters;
2. Rafter pitch, i.e. the distance between them;
3. Spans of the rafter system;
4. Designing a truss truss and choosing which rafter attachment scheme - layered or hanging - will be used during construction;
5. Analysis of the bearing capabilities of the foundation and supports;
6. Calculation of such additional elements as puffs that connect the structure of the rafters, preventing it from “driving around” and braces that allow “unloading” the rafters.

When using a typical project, there is no need to think about how to calculate the truss system, since all calculations have already been completed. In the case of construction individual project all necessary calculations should be done in advance.

Doing roofing work with your own hands and calculations should be a specialist with sufficient qualifications and having the necessary knowledge and skills.

Requirements for structural elements of rafters

Installation of the truss system

For the manufacture of structural elements of the rafters, coniferous wood is used, the moisture content of which should not exceed 20%.

Modern roofing wooden material pre-treated with special protective preparations. Parameters such as the thickness of the rafters are selected in accordance with the calculations discussed below.

Loads that affect the design of the rafters and in connection with which it may be necessary to strengthen the truss system, according to the duration of the impact, are divided into two categories: temporary and permanent:

1. Dead loads include loads created by the own weight of the rafter structure, the weight of roofing materials, battens, thermal insulation and materials used in finishing the ceiling. They are directly affected by the size of the rafters;
2. Live loads can also be divided into short-term, long-term and special. Short-term loads include the weight of workers performing roofing, as well as the weight of the tools and equipment they use. In addition, short-term loads include wind and snow loads on the roof. Special loads include rather infrequent actions such as earthquakes.

Snow load calculation

Snow load map

The most complete calculated value of the snow cover load is calculated using the formula:

• where Sg is the calculated value of the mass of snow cover per 1 m 2 of the horizontal earth's surface, taken from the table;
• µ is a coefficient that determines the transition from the weight of the snow cover on the ground to the snow load on the roofing.

The value of the coefficient µ is selected depending on the angle of slope of the roof slopes:

µ=1 if the slope angles of the roof slope do not exceed 25°.

µ=0.7 in the case when the slope angles of the slopes are in the range of 25-60°.

Important: if the slope angle roof slope exceeds 60 degrees, the snow load value is not taken into account when calculating the rafter system.

Wind Load Calculation

Wind load map

To calculate the design value of the average wind load at a certain height above ground level, the following formula is used:

Where Wo is the value of the wind load established by the standards, taken from the table according to the wind area;

k - taking into account the change in wind pressure depending on the height, the coefficient selected from the table, depending on the area in which construction is being carried out:

1. Column "A" indicates the values ​​of the coefficient for such areas as open coasts of reservoirs, lakes and seas, tundra, steppes, forest-steppes and deserts;
2. Column "B" includes values ​​for urban areas, forested areas and other areas covered evenly by obstacles that are greater than 10 meters high.

Important: the type of terrain when calculating the wind load on the roof may vary depending on the wind direction used in the calculation.

Calculation of sections of rafters and other elements of the rafter system

The cross section of the rafters depends on the following parameters:

• The length of the rafter legs;
• The step with which the rafters of the frame house are installed;
• Estimated value of various loads in a given area.

The data given in the table is not a complete calculation of the rafter system, they are only recommended for use in calculations when rafter work will be carried out for simple roof structures.

The values ​​given in the table correspond to the maximum possible loads on the rafter system for the Moscow region.

We give for the rafter system the size of other structural elements of the rafters:

• Mauerlat: bars with a section of 150x150, 150x100 or 100x100 mm;
• Diagonal valleys and legs: bars with a section of 200x100 mm;
• Runs: bars with a section of 200x100, 150x100 or 100x100 mm;
• Puffs: bars with a section of 150x50 mm;
• Crossbars acting as supports for racks: bars with a section of 200x100 or 150x100 mm;
• Racks: bars with a section of 150x150 or 100x100 mm;
• Boards of the cornice box, struts and fillies: bars with a section of 150x50 mm;
• Hemming and frontal boards: section (22-25) x (100-150) mm.

An example of the calculation of the rafter system

We give a specific example of the calculation of the rafter system. We take the following as initial data:

The crossbars are fastened to the legs of the rafters using bolts to avoid “grinding” its ends with nails. In this regard, the bending resistance value of the second grade weakened wood material is 0.8.

Direct calculation of the rafter system:

• Calculation of the load acting on one meter of linear length of the rafter:

• If the slope of the roof slopes does not exceed 30 degrees, the rafters are calculated as bending elements.

According to this, the maximum bending moment is calculated:

Note: The minus sign indicates that the direction of bending is opposite to the applied load.

• Next, the required required moment of resistance to bending is calculated for rafter leg:

W \u003d M / R izg \u003d 21500/104 \u003d 207 cm 3

• For the manufacture of rafters, boards are usually used, the thickness of which is 50 mm. Take the width of the rafter equal to the standard value, i.e. b=5 cm.

The height of the rafters is calculated using the required moment of resistance:

h \u003d √ (6xW / b) \u003d √ (6x207 / 5) \u003d √249 \u003d 16 cm

• Received following sizes rafters: section b \u003d 5 cm, height h \u003d 16 cm. Referring to the dimensions of the lumber according to GOST, we select the nearest size that fits these parameters: 175x50 mm.
• The resulting value of the cross section of the rafters is checked for deflection in the span: L 1 \u003d 300 cm. The first step is to calculate the rafter leg of this section at the moment of inertia:

J \u003d bh 3 / 12 \u003d 5 × 17.5 3 / 12 \u003d 2233 cm 3

f norm \u003d L / 200 \u003d 300/200 \u003d 1.5 cm

f = 5 x q n x L 4 / 384 x E x J = 5 x 1.94 x 300 4 / 384 x 100,000 x 2233 = 1 cm

The value of the calculated deflection of 1 cm is less than the value of the standard deflection of 1.5 cm, therefore the previously selected section of the boards (175x50 mm) is suitable for the construction of this rafter system.

• We calculate the force acting vertically at the convergence of the rafter leg and the strut:

This effort is then decomposed into:

• rafter axis S \u003d N x (cos b) / (sing g) \u003d 357 x cos 49 ° / sin 79 ° \u003d 239 kg;
• strut axis P \u003d N x (cos m) / (sin g) \u003d 357 x cos 30 ° / sin 79 ° \u003d 315 kg.

where b=49°, g=79°, m=30°. These angles are usually set in advance or calculated using the scheme of the future roof.

In connection with small loads, it is necessary to constructively approach the calculation of the cross section of the strut and check its cross section.

If a board with a thickness of 5 cm and a height of 10 cm is used as a strut (the total area is 50 cm 2), then the compression load it can withstand is calculated by the formula:

H \u003d F x Rszh \u003d 50 cm² x 130 kg / cm² \u003d 6500 kg

The value obtained is almost 20 times higher than the required value, which is 315 kg. Despite this, the cross section of the strut will not be reduced.

Moreover, to prevent its eversion, bars will be sewn to it on both sides, the cross section of which is 5x5 cm. This cruciform section will increase the stiffness of the strut.

• Next, we calculate the thrust perceived by the puff:

H \u003d S x cos m \u003d 239 x 0.866 \u003d 207 kg

The thickness of the crossbar-scrum is set arbitrarily, b = 2.5 cm. Based on the calculated tensile strength of wood, equal to 70 kg / cm 2, we calculate required value section height (h):

h \u003d H / b x R race \u003d 207 / 2.5x70 \u003d 2 cm

The cross section of the scrum got rather small dimensions of 2x2.5 cm. Let's say that it will be made of boards 100x25 mm in size and fastened with screws with a diameter of 1.4 cm. For the calculation, it is necessary to use the formulas used when calculating screws for shear.

Then the value of the working length of the capercaillie (a screw whose diameter exceeds 8 mm) is taken depending on the thickness of the board.

The calculation of the bearing capacity of one screw is performed as follows:

T hl \u003d 80 x d hl x a \u003d 80x1.4x2.5 \u003d 280 kg

Fastening the scrum requires the installation of one screw (207/280).

In order to prevent the wood material from being crushed at the place of the screw fastening, the number of screws is calculated using the formula:

T ch \u003d 25 x d ch x a \u003d 25x1.4x2.5 \u003d 87.5 kg

In accordance with the value obtained, the fastening of the screed will require three screws (207/87.5).

Important: the thickness of the tightening board, which is 2.5 cm, is chosen to demonstrate the calculation of the screws. In practice, in order to use the same parts, the thickness or section of the tightening usually corresponds to the parameters of the rafters.

• Finally, the loads of all structures should be recalculated, changing the estimated dead weight to the calculated one. To do this, using the geometric characteristics of the elements of the rafter system, the total volume of lumber required for the installation of the rafter system is calculated.

This volume is multiplied by the weight of wood, the weight of 1 m 3 of which is approximately 500-550 kg. Depending on the area of ​​​​the roof and the pitch of the rafters, the weight is calculated, which is measured in kg / m 2.

The rafter system provides, first of all, the reliability and strength of the roof being erected, therefore its calculation, as well as various related calculations (for example, the calculation of rafters and beams) should be performed competently and carefully, without making the slightest mistake.

Board for rafters: selection of building materials, calculations of parameters, installation nuances

Building a strong and reliable truss system is not an easy task. In the construction of the frame of the future roof, every little thing is important, because it is only in hot countries that you can put an armful of hay on a simple structure, and it will serve faithfully. But in the conditions of the Russian climate, where a strong wind can tear off the roof from an entire plant, and snow accumulates up to several tons, completely different requirements are imposed on the roof and its “skeleton”. And therefore, even at the design stage, the focus is on the rafter board with all its parameters, such as length, thickness and material of manufacture.

Therefore, if you want to be calm for new roof and its durability, study the basic principles for selecting the elements of its design: which board is designed to carry and distribute the main load, what kind of timber is needed between the rafters, and what material to purchase for arranging the ridge run and internal puffs of trusses. And our article will help you understand all these subtleties.

Boards for the truss system: to make or order?

So, let's start with where exactly you will buy the rafters - it depends on how meticulously it will be necessary to calculate the parameters of each board in the structure.

So, ideally, if you can entrust the purchase of boards and beams to an experienced specialist, because. he is even able to assess the quality of the material by eye and understand whether the purchased quantity will be enough. Although, guided by our advice, you yourself will completely cope with this task.

In total, you have three options for purchasing rafter boards, and we will analyze the advantages and disadvantages of each of them.

Method number 1. Cubic meters of lumber

So pay attention to these important points. If you are offered wood in cubic meters - this is quite an acceptable way, you simply indicate the number of cubic meters of boards and timber in a specialized company. And already on the spot, you will process all this with protective impregnations and build a roof.

The only point is that there will be a lot of work, as well as waste. Therefore, you will need to purchase 10% more boards than you originally planned according to your calculations, so that there is a margin for trimming, waste and unexpected defects that were not detected before.

Method number 2. Custom cut boards

Another option is to purchase ready-made rafters after cutting. Many companies custom-cut wood to the desired parameters and provide already ready-made version, it remains only to assemble the structure itself on the roof. There will already be a minimum of waste here, work will go faster, it is only important to first guess with the size of the rafters. But there is one minus: in the general set, one or two boards may be missing. After all, especially attentive people do not always work at such enterprises, and it is not a fact that the same workers will not hide a bar from each order. And you will have to roll up your sleeves and make additional templates for the missing elements.

How to order ready-made cut boards for rafters? You simply make a paper version of the roof project or a special program, show it to the designer, and then to the company representative. Pretty simple!

Method number 3. Finished roof trusses

The third option is to purchase ready-made roof trusses. They are easier to install on the walls directly on the roof than to assemble separately, and the whole structure is assembled in 1-3 days. And for this you no longer need a carpenter! Such truss trusses are assembled at the factory and turn out to be quite high quality. You just have to install them and connect them in the skate.

You won’t have any waste here at all, but, of course, ready-made roof trusses will cost more than just rafter boards. In addition, usually a measurer is even sent from the manufacturer to the site so that he personally checks the distance between the walls of the house and corrects something in the project documentation. And this is already something of a safety net for you, especially if the calculations of the truss system seem complicated for you (they are not easy for professionals, believe me).

Self-made rafters from boards

So, if you don’t like the idea of ​​ordering the processing and cutting of boards, then get ready to be a designer, an architect, and a carpenter at the same time. In fact, all you need to know about your rafters is their angle of inclination, section, distance, and method of attachment.

If you have ever watched the work of professionals, then you must have admired how masterfully and effortlessly they measure the desired length of the rafters and make them into complex cutouts. At the same time, all they need to know is the parameters of the roof. And at the same time, they use the most elementary tools: a carpenter's square, their own recorded calculations, or a "smart" book. But, in a good way (and by all the rules), you should initially calculate everything using special construction calculators. By the way, special applications for smartphones are also popular today. Although you can easily do the most basic calculations yourself.

So, since if you took up the manufacture of rafters from boards on your own, then you already know for sure the width of the building and the thickness of the ridge board. And here it is important not to make a mistake when calculating the future length of the rafter: be sure to subtract the thickness of the ridge board from the full width of the building. Believe me, many people forget about it for some reason. Now divide the result by two and you will get the run of each rafter leg.

The angle of inclination of the roof can be determined using geometric formulas, and the type of formula depends on what shape of roof you are going to build: a classic gable, or a more complex four-slope. After all, their structural units are different, and the requirements for the ability to withstand certain loads are also different.

Rafters for a gable roof: a simple calculation

Rafters for a gable roof are the easiest to make, because, in fact, they are standard triangles, which are only important to arrange in a row strictly vertically, with a certain step, and fix the tops with a ridge run:

Rafter for hipped roof: triangles + rectangles

It will be a little more difficult for you to make rafters for a standard four-pitched roof (hip, hip) because. more needs to be done here accurate calculations:

The rafter system of a hipped roof is distinguished by the fact that rafters are installed here, which are directed to the corners of the walls - diagonal. And even more: other rafters of the slopes rest on such boards. Because of this, slanting (another name is “diagonal”) rafters carry a load one and a half times more than ordinary ones, and their length also exceeds the standard length of the boards.

Most often, such rafters are made paired:

Rafters for unusual roof shapes: complex design

For pitched and flat roofs standard rules manufacture of rafters, calculation of loads and structures. But a completely different approach in the construction of outbuildings, gazebos and other household facilities. Here, the roof does not always have to be something like a miniature copy of a large and real one. On the contrary, it is important to think over everything in order to invest in the budget and correctly combine a small design with a large one.

For example, as an option, special elongated rafters are made for the roof of an extension to the house:

The demonstrated rafters do not create point loads on the edge of the roof (as is often the case, unfortunately, in the practice of private construction), and as a result, the roof does not heel over the years to one side. Here, the load, due to the unusual length of the rafters, is distributed evenly, with less pressure and over all slopes. Yes, and this design looks more competent and professional.

But such rafters are made for round roofs small houses, outbuildings and gazebos:

And, finally, in some cases, part of the rafters does not play any functional role at all in the entire structure - only decorative. Making them takes a lot of time, but you don’t need to calculate strength at all. And see how just one additional board in the truss system completely changes the whole look of the roof:

Bent boards for gabled roof rafters

And finally, if you like unusual and fashionable lancet roofs, then you should know that all this is also possible to bring to life. Such boards are really made: they are steamed up to 100 degrees in the factory, and, with high humidity, giving elasticity, bent according to a special template. Only after that they are glued together and dried in special chambers.

Also, in addition, special cuts are made along the fold line of such bars, which also causes its own difficulties, and therefore such a rafter system will cost more. And if we talk about private construction, it will be difficult for you to complete this entire technological stage. But there is such a possibility.

And we figured out the design and parameters of the rafters, and now let's move on to the most difficult step - calculating the sum of all loads that affect such roofs. Thanks to the answer to such questions, you can calculate not only the length of the rafters, but also their cross section, which will be reliable.

How to calculate the load on the truss system?

So, you will need to build on what kind of area you live in. After all, the board for the rafters depends on this: the dimensions are selected so that the roof maintains maximum integrity and at the same time successfully resists both static and dynamic loads.

Here is the whole set of loads that you will have to consider when designing the truss system and its parameters:

What are roof limit states?

And now - in more detail. In order to accurately determine which section of the rafters is necessary for the roof device, you first need to calculate the sum of all-all possible loads. This calculation is done according to the so-called "limit states" method, when the roof structure loses its ability to resist the external influence of wind, a large amount of snow, or receives significant deformation.

When does it happen? When the roof structure has already exhausted its bearing capacity, stability and endurance. Those. this is the very moment when there is so much snow on the roof that the truss system cannot withstand it and breaks, or the structure of the roof is such that a strong wind can eventually tear it off. And here it is necessary to take into account both the data of both static and dynamic loads:

• when snow, insulation and roofing put pressure on the roof, this is a static load;
• wind and other influences that cause the opening of the roof nodes or the deflection of the rafters are dynamic loads.

If the roof does not cope with all this individually or in combination, then this will lead to negative consequences. In addition, it is dangerous in itself when the rafters have cracks, deflections and other initially invisible problems that adversely affect the bearing capacity of the structure. It is very important to prevent all this.

How to find out the typical loads of a certain area?

In order to meticulously calculate the pressure on the roof, we have a separate article. You just need to look at the map of your area for the average wind and snow loads, use them to determine the minimum thickness of the rafter section and their minimum spacing along the slope:

You have two ways to work with all this data:

• Method number 1. Calculate everything down to the millimeter using special formulas, an online calculator or the mind of a hired roof specialist.
• Method number 2. Focus on the traditions of construction in a particular region and make the simplest calculations with a margin.

The fact is that it is not without reason that in every country the houses are similar to each other. For example, in the snowiest regions of Russia, high sharp roofs have long been built, and in especially windy regions, they are more gentle. If the standard snow depth in your area is higher than the national average, then either you'd better build a steeply pitched roof, or make the rafters twice as thick and strong as possible to support the required load. Therefore, if you want to purchase rafter boards with standard values, organize the same roof slope as your neighbors - that's the whole trick.

So, if you know what the load on the roof will be approximately during its service, then you will also find out the one that will affect each individual rafter. How? Just divide the load by their number, and here it is important to know whether the rafters themselves will cope with the responsibility assigned to them. And this, in turn, can also be found out.

How to calculate bending and tensile strength?

Absolutely every rafter is affected by a variety of physical forces. And the more rafters at each gap, the easier it is for each of them individually, and vice versa. But after all, it makes no sense to put continuous rows, and therefore it is more rational to calculate and insure everything.

Stress conditions at rafters made of different wood

How exactly the rafter will work for bending and stretching during operation depends not only on its thickness, but also on what material it is made of. Namely - from what grade, breed and moisture content of wood. The following data tables will help you understand this:

Here are some features of wood species suitable for rafters:

• Hardwood trees are less flexible than pine trees, and their properties are different in all directions. Therefore, boards from this wood are less often used specifically for rafters. On the other hand, larch has such good characteristics that it can be compared with oak in terms of durability.
• Pine is relatively inexpensive, but has large quantity knots. On the other hand, resinous wood will always provide high resistance to decay, and pine timber is excellent for arranging a truss system due to its lightness and strength.
• Spruce is ideal for floor beams and rafters. Moreover, dry spruce is as strong as pine. This wood is perfect for those areas that are all the load.
• Oak wood is stronger and more durable, and therefore more expensive, because in order to be used in construction, its age must be about 120 years! Whereas pine has only 30.
• All other types of wood should be used for the manufacture of rafters carefully, carefully studying their properties.

So, to summarize: the main property of wood as a material is its deflection strength. But, if this is not enough, the boards simply increase in length. In addition, according to SNiPs “Loads and Impacts”, the load on all rafters is uneven, the rafter on the left can be loaded much more than the rafter on the right, and therefore you always need to play it safe and take such values ​​with a margin.

The choice of the length and section of the truss elements

All rafter legs are made of the same thickness and length, depending on the pitch of the rafters and the load that will act on them:

AT different forms roof trusses are attached in their own way, and two main designs are most popular today: with hanging and inclined rafters.

• Rafters rigidly interconnected, lie in the same plane and have only two points of support - these are the outer walls. And the rafters at the same time rely on the Mauerlat or the upper crown of the log house.
• hanging trusses, on the contrary, consist of rafter legs that work on compression and bending, and under the weight of the structure they expand, which is transferred to the wall. To compensate for these forces, the rafter legs must be tightened with puffs - these are horizontal beams that will need to be fixed at the base of the rafters or higher. They are also important to calculate correctly.

And the length of the rafters of both variants of the truss system depends directly on the angle of the roof:

And even more: knowing the height of the future roof and the width of the end of the house, you will immediately determine the length of the rafters, using the most common formulas from the school geometry course.

Combination of wood and metal

If you have to make the truss elements thinner than necessary, then they need to be strengthened with struts, crossbars and racks. But, sometimes it is necessary to stiffen the entire truss system in view of the design of the building itself. For example, the spans will be large, or the house is located in an area with unfavorable climatic conditions. For this purpose, a combined rafter system made of wood and metal is used, where the metal takes over the most critical areas.

In this case, the natural deformation of the wood of the rafter boards will no longer play its negative role, and you can safely refer to the choice of parameters. metal elements in such a system, both independent rafters are installed, and as elements of strengthening the rafter legs. As a result, metal inserts significantly increase the rigidity of the entire truss system, although they complicate its operation.

The thing is that the condensate likes to settle just on the metal, and upon contact with the wooden parts, it causes their gradual decay. That is why, in the combined system, boards for rafters must first be treated with moisture-proofing agents, and even if necessary, film insulation of the contact between wood and metal is used. In addition, under such a roof, competent ventilation of the under-roof space and constant monitoring of the condition of wooden rafters are necessary.

Of course, the simpler the roof truss system, the more clear and understandable the requirements for the parameters of all its elements, while complicated roof even an experienced specialist does not always feel confident. But the main task for both a beginner in this business and a professional is to achieve the final quality of the entire structure!

Rafter board: dimensions, how to choose, calculations, thickness and length

Rafter dimensions and material requirements

The acquisition of the right material is one of the most important aspects of any construction. When building a truss system, you need to understand that it has a great responsibility, therefore, to create such a structure, you should use only reliable, high-quality raw materials. You can buy ready-made roof trusses, or you can make the necessary details from logs or other material on your own. The dimensions of the rafters, the methods of their installation, fasteners and other nuances should be chosen individually for each building and truss system.

Options for purchasing material for rafters

There are three standard options for purchasing rafter boards, it is worth considering the pros and cons of each of them:

• cubic meters of lumber. Buying wood in cubic meters is a completely acceptable way, for this you just need to order the required number of cubic meters of timber or boards from a specialized company. Further, they will still need to be cut and processed, only after that you can begin construction. In this case, it is worth considering waste when processing wood, so it’s better to take somewhere 10 percent more than calculated according to the construction plan.
• Custom cut boards. The next option is to purchase ready-made rafters after cutting. Now many companies are engaged in cutting wood to order according to the given parameters, the builders will only have to assemble the truss system on the roof. There will be practically no waste here, and the process of constructing the structure will go much faster.
• Finished roof trusses. This option is easier than the first two. Such structures are easier to mount on the walls right on the roof, which will reduce the assembly time of the entire structure to one or several days. This farm needs only to be installed and connected in the ridge. The amount of waste is a minimum, but also the price this option noticeably higher.

For self-manufacturing rafters, it is enough to know their necessary parameters, such as the angle of inclination, distance, section, fastening method, and so on. In order to calculate right size rafters, use carpentry tools. Modern special building calculators carry out these calculations extremely quickly.

lumber requirements

When calculating roof structure, which provides for the production of rafter legs from a board, in addition to all sizes, it is also extremely important to take into account the quality of the lumber used. First you need to decide what kind of wood is best suited for rafters. Here the answer is unequivocal - it is hardwood and coniferous lumber that meets the requirements of GOST 2695-83 and GOST 8486-86. Based on these standards, a board for the manufacture of rafters may have:

• Non-through cracks not exceeding half the board in length;
• No more than three knots per linear meter of material, while the maximum allowable knot diameter is 30 mm;
• Humidity is not more than 18% (from the moisture meter).

According to the requirements of SNiP, when buying lumber for the production of rafters and other components of the roof frame, it is imperative to check the documents informing about the quality of the purchased products. They must indicate:

• The name of the product and its standard number;
• Name of the manufacturer with basic data about him;
• Type of wood, size of lumber, degree of humidity;
• Date of release of the consignment;
• The number of units in the package.

Wood is a natural material, so it is quite susceptible to biological degradation. To reduce the risk of damage to the structure assembled from this lumber, it must be prepared before installation. Preparation involves carrying out various constructive and protective measures.

Protective ones include:

• Impregnation of wood with special flame retardants to prevent the likelihood of fire;
• Treatment of wood with antiseptic compounds to prevent premature decay;
• Processing rafters and others wooden elements pest control agents.

Construction activities include:

• Formation of waterproofing under the roof and vapor barrier from the side of the room;
• Installation of waterproofing pads at the junction of wooden structures to brick ones;
• Roofing pie ventilation equipment.

The rafters made from the board, subject to all prescribed roof construction technologies, will last a long time without any repair.

Rafter dimensions

The choice of section and length of truss elements is extremely important. All rafter legs must be created the same length and thickness, which depend on the pitch of the rafters and the expected load on them. The materials for the rafters are logs, timber or boards. The disadvantages of log rafters include their considerable weight and the need to make complex cuts for fasteners, which significantly weakens the bearing capacity of the structure. The beam is more suitable for the manufacture of rafters, but it costs more than boards.

Important! For rafters most the best option is a beam that has already aged for some time: it does not change shape after the construction of the roof, which ensures the stability of the structure. A beam with a width of 10–15 cm is used, while its thickness should be about 5 cm.

Rafter legs from the board - best choice, this material is very popular and is actively used to create side rafter legs. It is also convenient to make side rafters of an elongated type and strong sloping rafter legs from the board.

The thickness of the board significantly affects its characteristics. An ordinary board suitable for the manufacture of rafters has a thickness of 40-60 mm.

Important! It should be noted that during the construction of the roof on outbuildings it is better to take a board with a thickness of 40 mm, this will help reduce costs. When erecting the roof of a residential building, this parameter should not be lower than 50 mm.

The width of the rafter board is selected depending on the length of the opening to be blocked - the longer the length of the rafter leg, the wider the board is needed for its production. When the length of the rafters does not exceed 6 meters, for their manufacture, you can take a board 150 mm wide - that is, the minimum size of the rafter section for the roof of a residential building is 50 × 150 mm. If the rafter leg is more than 6 meters, then the width of the board must be at least 180 mm. The elongated leg is made from cohesive boards, the width of which is 150 mm.

Another important parameter is the cross section of the rafters. It depends on the loads on the roof: both from external weather and from the severity of the truss system itself. Also, the calculation of the section is affected by the angle of inclination of the slope, the width of the building and the length of the overlapped opening. For calculations, special formulas and tables are used; there are many corresponding computer programs. Having determined the value of the section of the rafter legs, it is worth paying attention to the installation features of this design.

Features of mounting rafter legs

After choosing the optimal material for the rafter system, adjusting the size of the rafters, going through all the stages of their processing, it is also necessary to correctly install the rafter legs on the Mauerlat. The strength and reliability of the entire structure largely depends on this connection. There are two mounting options - sliding and rigid. Each of them is applicable to a certain type of rafters, and its choice depends on many factors.

Rigid fastening eliminates the possibility of any turns and bends of the rafters. For him, cuts are created and the rafter legs are fixed on the Mauerlat with the help of various fasteners.

A sliding joint, also often referred to as a swivel joint, has a couple of degrees of freedom. It is usually used in the construction of roofs over wooden house, since it allows the roof to settle over time on the frame, giving a certain shrinkage in the first few years. In this case, the connection of the ridge with the rafters is not so rigid. The rafter leg is fixed on the Mauerlat by washing down and strengthened with nails on both sides.

When erecting a hip roof, the rafter is more than 6 meters in size. Because of this, the rafters increase in length. To strengthen the rafter legs, struts are made for them from vertical racks. More than two racks are rarely installed at the same time.

Many people have such questions: how to make rafters with your own hands, how to properly install rafters, what is the best material for them to choose, and so on. Now you can find a lot of information about this in various sources and do the work yourself. If there are any questions and it is not possible to cope on your own, then it is better to turn to the services of professionals, whose reliability there is no doubt.

Rafters - board and log dimensions: thickness and installation of rafters

To draw up a technical project for a house, it is necessary to calculate the rafters. There are several options for roof structures.

Rafter legs that rest on two supports, while not having any additional stops, are called rafters without struts. They are used for shed roofs, the span of which is about 4.5 meters or for gable roofs, the span of which is about 9 meters. The rafter system is used either with the transfer of the thrust load to the Mauerlat, or without transfer.

Sloped rafters without spacers

The bending rafter, which does not transfer the load to the walls, has one support firmly fixed and freely rotating. The other support is movable and rotates freely. Three options for attaching rafters can meet these conditions. Let's consider each in detail.

The hemming of the top of the rafter leg or the upper support cut is installed in a horizontal position. It is enough just to change the method of supporting the run, and the rafter leg will immediately show the thrust. This calculation of the rafter leg, due to the rigidity of the conditions for creating the upper node, is usually not used for gable roofs. Most often, it is used in the construction of shed roofs, since the slightest inaccuracy in the manufacture of the assembly will turn the non-expansion scheme into a spacer. In addition, in gable types of roofs, if there is no spacer on the Mauerlat, due to the deflection of the rafters under the action of the load, the destruction of the roof ridge assembly may occur.

At first sight this system may seem unrealistic in execution. Since an emphasis on the Mauerlat is created on the lower part of the rafter, in fact, the system must exert pressure on it, that is, a horizontal force. However, it does not show the expansion load.

Thus, in all three options, the following rule is observed: one edge of the rafter is installed on a sliding support, which allows you to turn. The other is on a hinge that only allows rotation. Fastening rafter legs on sliders are installed using a variety of designs. Most often they are performed using mounting plates. It is also possible to fasten with nails, self-tapping screws, using overhead bars and boards. It is only necessary to choose the right type of fastener that will prevent the rafter leg from sliding in the support.

How to calculate rafters

In the process of calculating the truss structure, as a rule, an “idealized” calculation scheme is adopted. Based on the fact that a certain uniform load will press on the roof, that is, an equal and identical force that acts evenly along the planes of the slopes. In reality, there is no uniform load on all roof slopes. So, the wind sweeps snow on some slopes and blows off from others, the sun melts from some slopes and does not reach the rest, the same situation with landslides. All this makes the load on the slopes completely uneven, although outwardly it may not be noticeable. However, even with an unevenly distributed load, all three of the above options for truss mounts will remain statically stable, but only under one condition - a rigid connection of the ridge run. In this case, the run is either supported by slanting rafter legs, or inserted into the gables of the wall panels of the hip roofs. That is, the truss structure will remain stable only if the ridge run is firmly fixed from possible horizontal displacement.

In the case of manufacturing a gable roof and supporting the purlin only on posts, without relying on the walls of the fronts, the situation worsens. In options numbered 2 and 3, with a decrease in the load on any slope, opposite the calculation on the opposite slope, the roof may move in the direction where the load is greater. The very first option, when the very bottom of the rafter leg is made with a notch with teeth or with a hemming of a support bar, while the top of the horizontal notch is laid on a run, will hold an uneven load well, but only if the uprights that hold the ridge run are perfectly vertical.

In order to give stability to the rafters, a horizontal scrum is included in the system. It is slightly, but still increases stability. That is why in those places where the fight intersects with the racks, it is fixed with a nail fight. The statement that the fight always works only on stretching is fundamentally not true. The fight is a multifunctional element. So, in a non-thrust truss structure, it does not work in the absence of snow on the roof, or it only works in compression when a slight uniform load appears on the slopes. The structure works in tension only during subsidence or deflection of the skate run under the influence of the maximum load. Thus, the fight is an emergency element of the truss structure, which comes into operation when the roof is littered with a large amount of snow, the ridge run turns out to be bent to the maximum calculated value, or uneven unforeseen subsidence of the foundation occurs. The consequence may be uneven subsidence of the ridge run and walls. Thus, the lower the contractions are set, the better. As a rule, they are installed at such a height that they would not create obstacles when walking in the attic, that is, at a height of about 2 meters.

If in options 2 and 3 the lower rafter support unit is replaced with a slider with the edge of the rafter leg extended beyond the wall, then this will strengthen the structure and make it statically stable with completely different combinations of the structure.

Also one in a good way to increase the stability of the structure is a fairly rigid fixing of the bottom of the racks that will support the run. They are installed by cutting into the bed and fixed with overlaps by any accessible ways. Thus, the lower strut support assembly is converted from a hinged assembly to a rigid pinch assembly.

How to calculate the length of the rafters does not depend on the method of attaching the rafter legs.

The cross section of contractions, due to the development of rather small stresses in them, does not take into account the rafters, but is taken quite constructively. In order to reduce the size of the elements that are used in the construction of the truss structure, the cross section of the scrum is taken to be the same size as the rafter leg, while thinner disks can be used. Contractions are installed either on one or both sides of the rafters and fastened with bolts or nails. When calculating the section of the truss structure, contractions are not taken into account at all, as if they do not exist at all. The only exception is the bolting of the contractions to the rafter legs. In this case, the load-bearing capacity of the wood, due to the weakening of the bolt hole, is reduced by using a factor of 0.8. Simply put, if holes are drilled in the rafter legs for the installation of bolt fights, then the calculated resistance must be taken in the amount of 0.8. When fixing the fights on the rafters only with a nail fight, the weakening of the resistance of the rafter tree does not occur.

But it is necessary to calculate the number of nails. The calculation is made on the cut, that is, the bending of the nails. For the calculated force, they take the spacer, which occurs in the emergency position of the truss structure. Simply put, in the calculation of the connection between the nails of the scrum and the rafter leg, a spacer is introduced, which is absent during the standard operation of the truss system.

The static instability of the trussless system manifests itself only on those roofs where it is not possible to install a ridge run that protects against horizontal displacement.

In buildings with hipped roofs and gables made of stone or brick, non-bracing rafter systems are quite stable and there is no need to take measures to ensure greater stability. However, to prevent accidents of structures, contractions should still be installed. When installing bolts or studs as fasteners, you should pay attention to the diameter of the holes for them. It should be the same as the diameter of the bolts or slightly smaller. When emergency the wrestling will not work until the gap between the wall of the hole and the stud is selected.

Please note that in this process the bottoms of the rafter legs will move apart at a distance of several millimeters to several centimeters. This can lead to shifting and scrolling of the Mauerlat and to the destruction of the wall cornice. In the case of spacer truss systems, when the mauerlat is firmly fixed, this process can cause the walls to move apart.

Expanded rafters

The rafter that performs bending work and transfers the thrust load to the wall panelsmust have at least two fixed supports.

To calculate this type of truss systems, we replace the lower supports with different degrees of freedom in the previous schemes with supports with a single degree of freedom - hinged. To do this, where they are not, bars for support are nailed to the edges of the rafter legs. As a rule, a bar is used, the length of which is at least a meter, and the cross section is about 5 by 5 cm, given the nail connection. In another embodiment, it is possible to arrange a support in the form of a tooth. In the first version of the calculation scheme, when the rafters abut horizontally against the run, the upper ends of the rafters are sewn either with nails or with a bolt. Thus, a hinged support is obtained.

As a result, the calculation schemes practically do not change. Internal bending and compression stresses remain unchanged. However, a spacer force appears in the former supports. In the upper nodes of each rafter leg, the oppositely directed spacer, originating from the end of the other rafter leg, disappears. Thus, it does not cause much trouble.

The edges of the rafters, which abut against each other or through the run, may be checked for material collapse.

In rafter spacer systems, the purpose of the fight is different - in emergency situations, it works in compression. In the process of work, it reduces the thrust on the walls of the edge of the rafters, but does not completely exclude it. She will be able to remove it completely if she fixes herself at the very bottom, between the edges of the rafter legs.

Please note that the use of spacer layered truss structures requires careful consideration of the effect of the spacer force on the walls. It is possible to reduce this thrust by installing rigid and durable ridge runs. It is necessary to try to increase the rigidity of the run by installing racks, cantilever beams or struts, or to build a construction lift. This is especially true for houses made of timber, chopped logs, lightweight concrete. Concrete, brick and panel houses are much easier to bear the force of thrust on the walls.

Thus, the truss structure, erected according to the spacer option, is statically stable under various combinations of loads, it does not require a rigid attachment of the Mauerlat to the wall. In order to keep the thrust, the walls of the building must be massive, equipped with a monolithic reinforced concrete belt around the perimeter of the house. In the event of an emergency, inside the spacer system, which works in compression, the fight will not save the situation, but will only partially reduce the spacer, which is transmitted to the walls. In order to avoid an emergency situation, it is necessary to take into account all the loads that can act on the roof.

Thus, no matter what shape the roof of the house is chosen, the entire truss system must be calculated in such a way as to satisfy the provisions of reliability and strength. To make a complete analysis of the truss structure is not an easy task. A large number of different parameters must be included in the calculation of wooden rafters, including thrust, bending, and possible weight loads. For a more reliable arrangement of the truss system, it is possible to install more suitable fastening methods. At the same time, one should not take the dimensions of the rafters without making a full analysis of their technical and functional abilities.

Calculation of the section of the rafters

The cross section of the truss beams is selected taking into account their lengths and the load received.

So, a beam up to 3 meters long is selected with a section diameter of 10 cm.

A bar, up to 5 meters long, with a section diameter of 20 cm.

A beam, up to 7 meters long - with a cross-sectional diameter of up to 24 cm.

How to calculate rafters - an example

Given a two-story house measuring 8 by 10 meters, the height of each floor is 3 meters. Corrugated asbestos-cement sheets were chosen for the roof. The roof is gable, the supporting posts of which are located along the central load-bearing wall. The pitch of the rafters is 100 cm. It is required to choose the length of the rafters.

How to calculate the length of the rafters? As follows: the length of the rafter legs can be chosen so that three rows of slate sheets can be laid on them. Then the required length: 1.65 x3 = 4.95 m. The roof slope in this case will be 27.3 °, the height of the triangle formed, that is, the attic space, is 2.26 meters.

1. Calculation of the bearing elements of the coating

Rafter legs are calculated as free-lying beams on two supports with an inclined axis. The load on the rafter leg is collected from the cargo area, the width of which is equal to the distance between the rafter legs. The calculated live load q must be located in two components: normal to the axis of the rafter leg and parallel to this axis.

2.1.1. Lathing calculation

We accept a crate of boards with a section of 50x50 mm (r = 5.0 kN / m), laid with a step of 250 mm. Wood - pine. The pitch of the rafters is 0.9 m. The slope of the roof is 35 0.

Calculation of the lathing under the roof is carried out according to two loading options:

a) Self-weight of the roof and snow (calculation for strength and deflection).

b) Self weight of the roof and concentrated load.

Initial data:

1. We accept bars of the 2nd grade with a calculated resistance Ru=13 MPa and modulus of elasticity E=1´ 10 4 MPa.

2. Operating conditions B2 (in the normal area), min=1 ; mn=1,2 for mounting load in bending.

3.Reliability factor for the intended purpose g n=0,95 .

4. Density of wood r \u003d 500 kg / m 3.

5. Reliability factor for load from the weight of galvanized steel g f=1,05 ; from the weight of the bars g f=1,1 .

6. Normative weight of snow cover per 1 m 2 of the horizontal projection of the earth's surface S 0 \u003d 2400 N / m 2.

Calculation scheme of the crate

Table 2.1

Load collection per 1 r.m. lathing, kN/m

where S 0 - the standard value of the weight of the snow cover per 1 m 2 of horizontal

the surface of the earth, taken according to the table. 4, for IV snow paradise-

she is S 0 = 2.4 kPa;

m- coefficient of transition from the weight of the snow cover of the earth to

snow load on the coating, taken according to clauses 5.3 - 5.6.

When loading a beam with a uniformly distributed load from its own weight and snow, the largest bending moment is equal to:

Kn m

At angles of inclination of the roof a³10 °, it is taken into account that the self-weight of the roof and the lathing is evenly distributed over the surface (slope) of the roof, and snow - along its horizontal projection:

M x = M cos a = 0.076 cos 29 0 = 0.066 kN´m

M y = M sin a = 0.076 sin 29 0 = 0.036 kN´m

Moment of resistance:

cm

cm

The strength of the battens of the crate is checked taking into account the oblique bend according to the formula:

,

where Mx and M y- components of the calculated bending moment about the main axes X and Y.

Ry=13 MPa

gn=0,95

,

The moment of inertia of the bar is determined by the formula:

cm 4

cm 4

Deflection in a plane perpendicular to the slope:

m

Deflection in a plane parallel to the slope:

m,

where E=10 10 Pa- the modulus of elasticity of wood along the fibers.

Full deflection:

= m

Deflection test: ,

where = - the maximum allowable relative deflection, determined from the table. sixteen .

When loading the beam with its own weight and concentrated load, the largest moment in the span is:

Checking the strength of normal sections:

where Ry=13 MPa- design resistance of wood to bending.

gn=0,95 - reliability coefficient for the intended purpose.

The conditions for the first and second combinations are met, therefore, we accept the crate with a section b´h=0.05´0.05 with a step of 250 mm.

2.1.2. Calculation of rafter legs

We calculate the layered rafters from the beams with a single-row arrangement of intermediate supports under the galvanized roof. cr. iron. The base of the roof is a crate made of bars with a section of 50-50 mm with a step =0.25 m. Step of rafter legs =1.0 m. The material for all wooden elements is pine of the 2nd grade. Operating conditions - B2.

Construction area - Vologda.

Calculation scheme of the rafter leg

The bars of the crate are placed along the rafter legs, which are lower

the ends rest on the Mauerlats (100 100) laid along the inner edge of the outer walls. In the ridge knot, the rafters are fastened with two plank overlays. To pay off the thrust, the rafter legs are pulled together with a crossbar - two paired boards. Roof slope angle 29 0 .

We collect loads on 1 m 2 of the inclined surface of the coating, the data is entered in table 2.2.

Table 2.2
Load collection per 1 r.m. rafter leg, kN/m

where S 0 - the standard value of the weight of the snow cover per 1 m 2 of the horizontal surface of the earth, taken according to table. SNiP 4, for the IV snow region S 0 = 2.4 kPa;

m- coefficient of transition from the weight of the snow cover of the earth to the snow load on the cover, taken according to clauses 5.3 - 5.6.

We perform a static calculation of the rafter leg as a two-span beam loaded with a uniformly distributed load. The dangerous section of the rafter leg is the section on the middle support.

Bending moment in this section:

The vertical pressure at point C, equal to the right support reaction of a two-span beam, is:

=0.265 kN

With a symmetrical load of both slopes, the vertical pressure at point C doubles: kN.

Expanding this pressure in the direction of the rafter legs, we find the compressive force in the upper part of the rafter leg:

kN

Collectionloads

Previously, to determine the loads, we set the cross section of the rafter leg 75x225 mm. The constant load on the rafter leg is calculated in Table. 3.2.

Table 3.2 Estimated constant load on the rafter leg, kPa

Estimated maximum load on the rafter leg (combination of constant plus snow)

The geometric scheme of the rafters

Schemes for calculating the rafter leg are shown in fig. 3.2. With the width of the corridor in the axes = 3.4 m distance between the longitudinal axes of the outer and inner walls.

The distance between the axes of the Mauerlat and the lying, taking into account the binding to the axis (

\u003d 0.2 m) m. We set the brace at an angle β = 45° (slope 2 = 1). The slope of the rafters is equal to the slope of the roof i 1 \u003d i \u003d 1/3 \u003d 0.333.

To determine the dimensions necessary for the calculation, you can draw a geometric diagram of the rafters on a scale and measure the distances with a ruler. If the mauerlat and the bed are on the same level, then the spans of the rafter leg can be determined by the formulas

Node heights h 1 =i 1 l 1 \u003d 0.333 * 4.35 \u003d 1.45 m; h 2: = i 1 l\u003d 0.333 * 5.8 \u003d 1.933 m. Height mark: we take the crossbar 0.35 m below the intersection point of the axes of the rafter leg and the rack h = h 2 - 0.35 (m) = 1.933 -0.35 = 1.583 m.

Forces in the rafter leg on the crossbar

The rafter leg works like a three-span continuous beam. Support settlements can change the support moments in continuous beams. If we assume that the bending moment on it has become equal to zero due to the subsidence of the support, then it is possible to conditionally cut the hinge into the place of the zero moment (above the support). To calculate the rafter leg with a certain margin of safety, we consider that the subsidence of the strut lowered the support bending moment above it to zero. Then the design scheme of the rafter leg will correspond to Fig. 3.2, c.

Bending moment in rafter leg

To determine the thrust in the crossbar (puff), we consider that the supports have sagged in such a way that the reference moment over the strut is equal to M 1 and above the racks - zero. We conditionally cut the hinges into places of zero moments and consider the middle part of the rafters as a three-hinged arch with a span l cp = 3.4 m. The thrust in such an arch is

Vertical component of the strut reaction

Using the diagram in Fig. 3.2.d, determine the force in the strut

Rice. 3.2. Schemes for calculating rafters

a-cross section of the attic; b - diagram for determining the estimated length of the rafter leg; c - design scheme of the rafter leg; d - scheme for determining the thrust in the crossbar; l - also for a scheme with one longitudinal wall; 1 - Mauerlat; 2 - bed; 3 - run; 4 - rafter leg; 5 - rack; 6 - brace; 7 - bolt (tightening); 8 - spacer; 9, 10 - persistent bars; 11 - filly; 12 - overlay.

Calculation of the rafter leg according to the strength of normalsections

Required moment of resistance run

App. M accept the width of the rafter leg b = 5 cm and find the required section height

App. M accept a board with a section of 5x20 cm.

There is no need to check the deflections of the rafter leg, since it is located in a room with limited access to people.

Calculation of the joint of boardsrafter leg.

Since the length of the rafter leg is more than 6.5 m, it is necessary to make it from two boards with an overlap joint. We place the center of the joint at the place of support on the brace. Then the bending moment at the joint during the subsidence of the strut M 1 = 378.4 kN * cm.

The joint is calculated similarly to the joint of the runs. We accept the length of the overlap l nahl =1.5 m= 150cm, nails diameter d= 4mm=0.4cm and long l guards = 100 mm.

Distance between axes of nail connections

150 -3 * 15 * 0.4 \u003d 132 cm.

The force taken by the nail joint

Q \u003d M op / Z \u003d 378.4 / 132 \u003d 3.29 kN.

Estimated nail pinching length, taking into account the normalized limiting gap between the boards δ W = 2 mm with a board thickness δ D = 5.0 cm and a nail tip length l, 5d

a p = l gv -δ d -δ w -l,5d \u003d 100-50-2-1.5 * 4 \u003d 47.4 mm \u003d 4; 74 cm

In the calculation of the dowel (nail) connection:

is the thickness of the thinner element a= a p =4,74 cm;

- the thickness of the thicker element c = δ d = 5.0 cm.

Finding a relation a/c = 4,74/5,0 = 0,948

App. T, we find the coefficient k n \u003d 0.36 kN / cm 2.

We find the bearing capacity of one seam of one nail from the conditions:

– wrinkling in a thicker element

\u003d 0.35 * 5 * 0.4 * 1 * 1 / 0.95 \u003d 0.737 kN

– crumpling in a thinner element

\u003d 0.36 * 4.74 * 0.4 * 1 * 1 / 0.95 \u003d 0.718 kN

- nail bending

= (2,5* 0,4 2 + 0,01* 4,74 2)

/0.95=0.674 kN

- but not more than kN

Of the four values, choose the smallest T = 0.658 kN.

Find the required number of nails P guards ≥ Q/ T =2,867/0,674=4,254.

Accept P guards = 5.

We check the possibility of installing five nails in one row. The distance between the nails across the wood fibers S 2 \u003d 4d \u003d 4 * 0.4 \u003d 1.6 cm. The distance from the extreme nail to the longitudinal edge of the board S 3 \u003d 4d \u003d 4 * 0.4 \u003d 1.6 cm.

According to the height of the rafter leg h = 20 cm should fit

4S 2 + 2Sz \u003d 4 * 1.6 + 2 * 1.6 \u003d 9.6 cm

Calculation of the junction of the crossbar with the rafter leg

According to the assortment (app. M), we take a crossbar from two boards with a section bxh = 5x15 cm each. The force at the joint is relatively large (N = 12, kN) and may require the installation of a large number of nails in the conditions of the construction site. To reduce the complexity of the installation of the coating, we design a bolted connection of the crossbar with the rafter leg. We accept bolts with a diameter of d = 12 mm = 1.2 cm.

In the rafter leg, dowels (bolts) crush the wood at an angle to the fibers α = 18.7 0 . App. Ш we find the coefficient corresponding to the angle α =18.7 0 coefficient k α =0.95.

In the calculation of the dowel joint, the thickness of the middle element is equal to the width of the rafter leg c \u003d 5 cm, the thickness of the extreme element is equal to the width of the crossbar board a = 5 cm.

We determine the bearing capacity of one seam of one dowel from the conditions:

– crumpling in the middle element

= 0.5*5* 1.2*0.95* 1 *1/0.95 = 3.00 kN

– collapse in the end element

\u003d 0.8 * 5 * 1.2 * 1 * 1 / 0.95 \u003d 5.05 kN;

- bending of the dowel = (l.8 * 1.2 2 + 0.02 * 5 2)

/0.95=3.17 kN

- but not more than kN

From the four values, we choose the smallest T = 3.00 kN.

We determine the required number of dowels (bolts) with the number of seams n w = 2

We accept the number of bolts n H =3.

There is no need to check the cross section of the crossbar for strength, since it has a large margin of safety.

4. PROVISION OF SPATIAL RIGIDITY AND GEOMETRIC STABILITY OF THE BUILDING

The main element of the roof, perceiving and resisting all types of loads, is rafter system. Therefore, in order for your roof to reliably withstand all environmental influences, it is very important to make the correct calculation of the truss system.

For self-calculation of the characteristics of the materials necessary for the installation of the truss system, I give simplified calculation formulas. Simplifications are made in the direction of increasing the strength of the structure. This will cause some increase in the consumption of lumber, but on small roofs of individual buildings it will not be significant. These formulas can be used when calculating gable attic and mansard, as well as shed roofs.

Based on the calculation methodology below, programmer Andrey Mutovkin (Andrey's business card - Mutovkin.rf) developed a truss system calculation program for his own needs. At my request, he generously allowed me to post it on the site. You can download the program.

The calculation methodology was compiled on the basis of SNiP 2.01.07-85 "Loads and impacts", taking into account the "Changes ..." of 2008, as well as on the basis of formulas given in other sources. I developed this technique many years ago, and time has confirmed its correctness.

To calculate the rafter system, first of all, it is necessary to calculate all the loads acting on the roof.

I. Loads acting on the roof.

1. Snow loads.

2. Wind loads.

On the truss system, in addition to the above, the load from the roof elements also acts:

3. Roof weight.

4. The weight of the rough flooring and lathing.

5. The weight of the insulation (in the case of an insulated attic).

6. The weight of the rafter system itself.

Let's consider all these loads in more detail.

1. Snow loads.

To calculate the snow load, we use the formula:

Where,
S - the desired value of the snow load, kg / m²
µ is a coefficient depending on the slope of the roof.
Sg - normative snow load, kg/m².

µ - coefficient depending on the slope of the roof α. Dimensionless value.

You can approximately determine the angle of the roof slope α by the result of dividing the height H by half the span - L.
The results are summarized in the table:

Then if α is less than or equal to 30°, µ = 1 ;

if α is greater than or equal to 60°, µ = 0 ;

if 30° is calculated by the formula:

µ = 0.033 (60-α);

Sg - normative snow load, kg/m².
For Russia, it is accepted according to map 1 of mandatory annex 5 of SNiP 2.01.07-85 "Loads and impacts"

For Belarus, the normative snow load Sg is determined
Technical code of GOOD PRACTICE Eurocode 1. EFFECTS ON STRUCTURES Part 1-3. General impacts. Snow loads. TCH EN1991-1-3-2009 (02250).

For example,

Brest (I) - 120 kg/m²,
Grodno (II) - 140 kg/m²,
Minsk (III) - 160 kg/m²,
Vitebsk (IV) - 180 kg/m².

Find the maximum possible snow load on a roof with a height of 2.5 m and a span of 7 m.
The building is located in the village. Babenki, Ivanovo region RF.

According to map 1 of the mandatory annex 5 of SNiP 2.01.07-85 "Loads and impacts", we determine Sg - the standard snow load for the city of Ivanovo (IV district):
Sg=240 kg/m²

We determine the angle of the roof slope α.
To do this, we divide the height of the roof (H) by half the span (L): 2.5 / 3.5 \u003d 0.714
and according to the table we find the slope angle α=36°.

Since 30° , calculation µ will be produced according to the formula µ = 0.033 (60-α) .
Substituting the value α=36° , we find: µ = 0.033 (60-36)= 0.79

Then S \u003d Sg µ \u003d 240 0.79 \u003d 189 kg / m²;

the maximum possible snow load on our roof will be 189kg/m².

2. Wind loads.

If the roof is steep (α > 30°), then because of its windage, the wind presses on one of the slopes and tends to overturn it.

If the roof is flat (α, then the lifting aerodynamic force that occurs when the wind bends around it, as well as turbulence under the overhangs, tend to raise this roof.

According to SNiP 2.01.07-85 "Loads and actions" (in Belarus - Eurocode 1 IMPACTS ON STRUCTURES Part 1-4. General actions. Wind actions), the standard value of the average component of the wind load Wm at a height Z above the ground should be determined by the formula :

Where,
Wo - normative value of wind pressure.
K is a coefficient that takes into account the change in wind pressure along the height.
C - aerodynamic coefficient.

K is a coefficient that takes into account the change in wind pressure along the height. Its values, depending on the height of the building and the nature of the terrain, are summarized in Table 3.

C - aerodynamic coefficient,
which, depending on the configuration of the building and the roof, can take values ​​from minus 1.8 (the roof rises) to plus 0.8 (the wind presses on the roof). Since our calculation is simplified in the direction of increasing strength, we take the value of C equal to 0.8.

When building a roof, it must be remembered that wind forces tending to lift or tear off the roof can reach significant values, and therefore the bottom of each rafter leg must be properly attached to the walls or to the mats.

This is done by any means, for example, using annealed (for softness) steel wire with a diameter of 5 - 6 mm. With this wire, each rafter leg is screwed to the mats or to the ears of the floor slabs. It's obvious that the heavier the roof, the better!

Determine the average wind load on the roof one-story house with the height of the ridge from the ground - 6m. , slope angle α=36° in the village of Babenki, Ivanovo Region. RF.

According to map 3 of Appendix 5 in "SNiP 2.01.07-85" we find that the Ivanovo region belongs to the second wind region Wo = 30 kg / m²

Since all buildings in the village are below 10m, coefficient K= 1.0

The value of the aerodynamic coefficient C is taken equal to 0.8

standard value of the average component of the wind load Wm = 30 1.0 0.8 = 24 kg / m².

For information: if the wind blows at the end of this roof, then a lifting (tearing) force of up to 33.6 kg / m² acts on its edge

3. Roof weight.

Different types of roofing have the following weight:

1. Slate 10 - 15 kg/m²;
2. Ondulin (bituminous slate) 4 - 6 kg/m²;
3. Ceramic tiles 35 - 50kg/m²;
4. Cement-sand tiles 40 - 50 kg/m²;
5. bituminous tiles 8 - 12 kg/m²;
6. Metal tile 4 - 5 kg/m²;
7. Decking 4 - 5 kg/m²;

4. The weight of the rough flooring, lathing and truss system.

Draft flooring weight 18 - 20 kg/m²;
Lathing weight 8 - 10 kg/m²;
The weight of the rafter system itself is 15 - 20 kg / m²;

When calculating the final load on the truss system, all of the above loads are summed up.

And now I will tell you a little secret. Sellers of certain types of roofing materials as one of the positive properties they note their lightness, which, according to their assurances, will lead to significant savings in lumber in the manufacture of the truss system.

As a refutation of this statement, I will give the following example.

Calculation of the load on the truss system when using various roofing materials.

Let's calculate the load on the truss system when using the heaviest (Cement-sand tile
50 kg/m²) and the lightest (Metal tile 5 kg/m²) roofing material for our house in the village of Babenki, Ivanovo region. RF.

Cement-sand tiles:

Wind loads - 24kg/m²
Roof weight - 50 kg/m²
Lathing weight - 20 kg/m²

Total - 303 kg/m²

Metal tile:
Snow loads - 189kg/m²
Wind loads - 24kg/m²
Roof weight - 5 kg/m²
Lathing weight - 20 kg/m²
The weight of the truss system itself is 20 kg / m²
Total - 258 kg/m²

It is obvious that the existing difference in design loads (only about 15%) cannot lead to any tangible savings lumber.

So, with the calculation of the total load Q acting on square meter We got the roof!

I especially draw your attention: when calculating, carefully follow the dimension !!!

II. Calculation of the truss system.

truss system consists of separate rafters (rafter legs), so the calculation is reduced to determining the load on each rafter leg separately and calculating the section of a separate rafter leg.

1. We find the distributed load per linear meter of each rafter leg.

Where
Qr - distributed load per linear meter of the rafter leg - kg / m,
A - distance between rafters (rafter pitch) - m,
Q - total load acting on a square meter of roof - kg / m².

2. We determine the working area in the rafter leg maximum length Lmax.

3. We calculate the minimum cross section of the material of the rafter leg.

When choosing a material for rafters, we are guided by a table of standard sizes of lumber (GOST 24454-80 Softwood lumber. Dimensions), which are summarized in Table 4.

A. We calculate the cross section of the rafter leg.

We set the width of the section arbitrarily in accordance with the standard dimensions, and the height of the section is determined by the formula:

H ≥ 8.6 Lmax sqrt(Qr/(B Rbend)), if the slope of the roof α

H ≥ 9.5 Lmax sqrt(Qr/(B Rbend)), if the roof pitch α > 30°.

H - section height cm,


B - section width cm,
Rizg - resistance of wood to bending, kg / cm².
For pine and spruce Rizg is equal to:
Grade 1 - 140 kg / cm²;
Grade 2 - 130 kg / cm²;
Grade 3 - 85 kg / cm²;
sqrt - square root

B. We check whether the deflection value fits into the standard.

The normalized deflection of the material under load for all roof elements should not exceed the value L / 200. Where, L is the length of the working area.

This condition is satisfied if the following inequality is true:

3.125 Qr (Lmax)³/(B H³) ≤ 1

Where,
Qr - distributed load per linear meter of the rafter leg - kg / m,
Lmax - working section of the rafter leg of maximum length m,
B - section width cm,
H - section height cm,

If the inequality is not met, then increase B or H .

Condition:
Roof slope angle α = 36°;
Rafter pitch A = 0.8 m;
The working section of the rafter leg is maximum length Lmax = 2.8 m;
Material - pine 1 grade (Rizg = 140 kg / cm²);
Roof - cement-sand tiles(Roof weight - 50 kg/m²).

As it was calculated, the total load acting on a square meter of the roof is Q \u003d 303 kg / m².
1. We find the distributed load per linear meter of each rafter leg Qr=A·Q;
Qr=0.8 303=242 kg/m;

2. Let's choose the thickness of the board for the rafters - 5cm.
We calculate the cross section of the rafter leg with a section width of 5 cm.

Then, H ≥ 9.5 Lmax sqrt(Qr/B Rbend), since the slope of the roof α > 30°:
H ≥ 9.5 2.8 sqrt(242/5 140)
H ≥15.6 cm;

From the table of standard lumber sizes, select a board with the nearest section:
width - 5 cm, height - 17.5 cm.

3. We check whether the deflection value is within the standard. For this, the inequality must be observed:
3.125 Qr (Lmax)³/B H³ ≤ 1
Substituting the values, we have: 3.125 242 (2.8)³ / 5 (17.5)³ = 0.61
Meaning 0.61, then the cross section of the material of the rafters is chosen correctly.

The cross section of the rafters, installed in increments of 0.8 m, for the roof of our house will be: width - 5 cm, height - 17.5 cm.

1.
2.
3.

The truss system is a structure that provides strength to the roof and serves as the basis for laying roofing material. She is shown in the photo.

The roof is a supporting structure that performs the following functions:

• gives the building a beautiful appearance;
• takes on external loads;
• protects the attic from the outside world;
• transfers the load from the crate and the material on it to the walls of the building and internal supports.

The main elements of the roof are lathing, rafters and Mauerlat. Also, the supporting structure includes additional fasteners - crossbars, racks, struts of rafters, struts, and so on. The reliability and strength of the roof is most affected by the rafter system. Rafters are the main load-bearing part of the roof. The rafter system accounts for the weight of not only the roofing, but also the snow cover, wind pressure. It must withstand all these impacts, so the calculation is made taking into account the type of roofing material and the climatic features of the region.

Rafter system design

The connection of the rafters with each other gives rigidity to the roof frame, and the result is a solid truss structure. The load on the rafters can be quite significant, for example, during strong winds, so the frame is tightly connected to the building box.

In the construction of private houses and cottages, wooden truss systems are usually used, which are easily manufactured and installed. If mistakes were made during the construction of the walls, these products can be easily processed: shortened, built up, hung, etc.

During installation, fasteners of the truss system are used: bolts, screws, clamps, nails, staples. They are also used to reinforce the supporting roof structure. The interconnected roof elements create a truss truss, which is based on triangles, which are the most rigid geometric figure.

When choosing a material for the manufacture of a rafter system, it is necessary to take into account the design and architectural nuances of the project. Do not forget about the antiseptic and fire impregnation for them, as this affects the durability of the roof.

The system consists of rafter legs. Install the rafters at an angle of slope of the roof slopes. The lower sections of the rafter legs rest on the outer walls with the help of a mauerlat, which contributes to an even distribution of the load. The upper ends of the rafters rest on a beam under the ridge or on intermediate fittings. With the help of a rack system, the load is transferred to the load-bearing internal walls.

Types of rafters

The design transfers to the walls a significant bursting force horizontally. In order to reduce the load, a stretch is used to connect the rafter legs. Do it either at the base of the rafters, or at a higher height. Stretching at the base of the rafters is at the same time a floor beam - this is relevant when creating mansard roofs. With an increase in the height of the stretch, it is necessary to increase its power and make sure that it is securely attached to the rafters.

Part layered rafters includes: rafter leg, mauerlat, headstock, brace, puff. This type of rafter is installed in buildings that have an average load-bearing wall or intermediate supports in the form of pillars. Elements of this design work only for bending, performing the function of attendants. The weight of the rafter system is less, the materials are also required in less quantity, so it is cheaper than the hanging system.

Installation of a layered system is done if the supports are no more than 6.5 meters apart from each other. If there is an additional support, the rafters sometimes cover a width of 12 meters, and if there are two supports, up to 15 meters.

Rafter legs most often do not rest on the walls of the building, but on a special beam - Mauerlat. This element can be located along the entire length of the house or be placed only under the rafter legs. If the structures are wooden, a log or timber is taken for the Mauerlat, which is the upper crown of the log house.

At brickwork Mauerlat walls are a beam installed flush with the inner surface of the walls, fenced from the outside with a masonry protrusion. A layer of waterproofing is laid between this element and the brick - for example, roofing material can be put in two layers.

If the width of the rafters is small, over time they may sag. To prevent this from happening, use a lattice consisting of a rack, crossbar and struts. A run is laid in the upper part of the structure, which connects the rafters or trusses. This is done regardless of the type of roof. Subsequently, on this run, a roof ridge is made. In places where there is no bearing walls, the heels of the rafters rest against the side runs - longitudinal beams of considerable power. The dimensions of these parts depend on the expected load.

In the construction of private houses, log rafters are used - they are lighter. To create roofs on multi-storey residential buildings and industrial buildings metal rafters are used.

Installation of truss systems

The slope angles of the slopes are selected based on the type of building and the purpose of the attic space. The amount of slope is also influenced by the material chosen to create the roofing.

If rolled products are to be laid, the angle of inclination should be 8-18 degrees. for tiles required angle equal to 30-60 degrees, for roofing steel or asbestos cement sheets - 14-60 degrees.

The installation of the rafter system begins after the erection of the load-bearing walls of the house (more: ""). Construction of wooden rafters chopped house significantly different from systems for houses made of foam concrete, brick, frame wooden or panel houses. The differences are significant even with the same shape, type and type of roof. As for how to treat the truss system, it is necessary to use antiseptic and fire-fighting agents so that the roof lasts for a long time.

The main elements of the supporting structure are the crate. The roof is the outer part of the roof, which is laid on a supporting structure, consisting of battens and rafters.

For the production of rafters, material of a certain size is taken. So, the thickness of the rafters (section) is most often 150x50 and 200x50 millimeters. For the crate, they usually take bars and boards measuring 50x50 and 150x25 millimeters. The distance between the rafter legs is on average 90 centimeters. If the roof slope is more than 45 degrees, this step is increased to 100-130 centimeters, and if a huge amount of snow falls in the region, then reduced to 60-80 centimeters.

In order to make more accurate calculations regarding the gap between the building legs, it is necessary to take into account their cross section, the step between the supports (struts, ridge run, racks), and the type of roofing material.

The floating rafter system is fastened with special brackets, which allow the rafters to “sit down” along with the shrinkage of the gables and not hang over the ridge log.

In mountainous areas, the chalet truss system is popular (more details: ""). A feature of this design is a significant protrusion of the roof beyond the bearing walls. Sometimes such a ledge reaches two or three meters, and the angle of the roof slope is small. Snow does not linger on such a roof, so it lasts a long time. But the best option is a roof protrusion of 1-1.5 meters (read also: "Characteristics and design of roofs: truss systems").

Installation of the truss system must be carried out, strictly observing all the requirements. If there is no experience in construction, it is better to entrust the construction of the roof to specialists, since this is not an easy task, and the slightest mistake can lead to its collapse.

Any rafter system is formed from numerous rafters, for the creation of which timber or boards are used. Boards are most often chosen due to their affordable cost, but their strength is considered not too high compared to timber.

Important! The durability of the roof and the safety of living in the house depend on the quality of the selected lumber.

Requirements for rafter boards

Roof rafters withstand significant impacts from snow, wind and roofing, therefore, in the process of their creation, certain rules must be taken into account, which they must comply with.

Important! During the formation of rafters, not only right choice their size and section, but also the material used to create them.

It is optimal to choose a timber for rafters , but this material has a high cost, so boards are often bought to reduce costs. Only high-quality wood is selected, and often the choice falls on needles or larch.

When searching for boards used to create rafters, the basic requirements for them are taken into account:

Boards are purchased only from trusted sellers who provide buyers with information about the product. To do this, there must be special accompanying documentation, and it contains information:

• the type of wood used for the manufacture of boards;
• name and number of the product standard;
• name of the production organization engaged in its manufacture;
• number of units in one package;
• date of release of the boards;
• lumber dimensions, as well as its moisture content.

Important! Wood is a natural material, so various biological influences lead to its destruction, so it is important to choose the right boards, as well as protect them with special protective compounds.

Rafter board must be processed before use different formulations:

• processing with high-quality antiseptics that will not allow the material to rot;
• impregnation with flame retardants that protect wood from fire;
• treatment with pest and insect repellents.

Only with the right choice of boards and after their high-quality processing is it possible to make rafters that are not only of high quality, but also resistant to various influences.

What dimensions should the rafters have?

After the optimal board for the rafters is selected, you can begin to form a special drawing and diagram of the future truss system. For this, the section, length, width and other parameters of the rafters are determined, which, after manufacturing, will be connected to each other in the correct sequence.

The size of the rafters can vary significantly, since many factors influence this parameter. These include the dimensions of the house and roof, the chosen design of the truss system, possible wind loads and other similar effects. The best recommendations would be:

• the minimum size is 50x150 mm;
• if significant spans are created, then a size of 150x150 or 250x100 is selected;
• often larger rafters are used if it is planned to build a significant in size trade pavilion or other large building.

Important! To accurately know the optimal dimensions of the rafters for the roof, you need to correctly calculate this indicator.

For the calculation, it is important to determine what load will affect the roof as much as possible, which allows you to choose the section and other parameters of the rafters. Additionally, it is possible to use special standard values, but they do not take into account certain climatic conditions of different regions, so experts prefer to make correct calculations, materials on the topic:,.

Proper determination of the dimensions of the rafters

When determining optimal sizes rafters, it is important to consider how thick the board used to create the rafters should be.

Important! The thickness of the board has a direct impact on the strength of the products created from it.

It is advisable to use a board for these purposes, the thickness of which varies from 4 to 6 cm. If the truss system is being built not in small buildings intended for household use, then to reduce costs it is allowed to use boards 3.5 cm thick. For a residential building, it is recommended to choose products , the thickness of which will not be less than 5 cm.

When choosing the width of the board, it is certainly taken into account how long the opening has, overlapping with rafters. The longer the rafters should be, the wider the board is used to create them:

• if the length of the rafter is approximately 6 m, then it is advisable to use a board whose width is approximately 15 cm;
• if the length of the legs exceeds 6 m, then the width of the board should be 18 cm at all;
• if it is required to get an even longer rafter leg, then an extension of the elements is used, and the places where there is an overlap should be located next to the ridge part of the roof.

The cross section of the rafters is calculated depending on a certain optimal distance between them, and the length of the elements is also taken into account. During work, it is necessary to decide what constant loads will affect the roof from wind and snow. The mass of the created truss structure, the angle of inclination of the slope, and also how long the opening that needs to be covered is taken into account. When calculating, it is additionally taken into account what width the structure has.

Important! To facilitate calculations, it is recommended to use special computer programs that are freely available on the Internet, and with their help, not only a quick result is obtained, but also the accuracy of the values ​​\u200b\u200bis guaranteed.

After determining the cross section of the rafters, it is necessary to decide at what distance from each other they will be installed. is directly related to their cross section, so if these parameters are not correctly determined, this can adversely affect the reliability and durability of the roof.

Important! It is allowed to reduce the cross section of the rafters if special struts are used.

Basic rules for choosing boards

The rafter board must meet the numerous requirements and conditions described above. To really choose quality material, it is recommended to use certain expert advice. These include:

Important! If a material with a high moisture index is purchased, then it is not allowed to use it for the construction of the truss system, as it will be fragile, and there is also a danger for living in a house with such a roof, because after a short period of time the geometry of the structure will be violated.

If several unsuitable elements are found in one batch of boards, it is not recommended to use them to create important parts of the roof, so they are used for additional elements.

home » materials » The dimensions of the bars for a gable roof. Rafter leg: design and installation