The roof of a house is a load-bearing structure that takes on all external load (weight roofing pie, its own weight, the weight of the snow cover, etc.) and transmits it to everything load-bearing walls at home or on internal supports.

In addition to its aesthetic and load-bearing functions, the roof is an enclosing structure, separating the attic space from the external environment.

The basis of the roof of any house is the rafter system.

This is the frame to which the roof is attached.

All loads are taken by this skeleton.

The rafter system consists of:

• rafter legs;
• Mauerlat;
• side purlins and ridge purlins;
• struts, diagonal braces, braces.

When all these elements (except for the Mauerlat) are connected to each other, a roof truss is obtained.

The basis of such a truss is a triangle, which is the most rigid of the geometric shapes.

The main element of the roof frame is the rafters.

Calculation of rafters

Before you begin directly calculating the rafters, you should find out what loads will affect the roof of the house.

That is, on the rafter legs.

Loads acting on the roof frame are usually divided into constant and variable.

Constant loads are those loads that act constantly, regardless of the time of day, season, etc.

This is the weight of the entire roofing cake, the weight additional equipment, which can be installed on the roof (fencing, snow retainers, aerators, antennas, etc.).

Variable loads appear at certain times of the year.

For example, snow.

When snow falls on the roof, this is a very decent weight.

In any case, it should be taken into account.

It's the same with the wind.

It's not always there, but when there's a strong wind, there's quite a bit of wind force acting on the roof frame.

And an inexperienced person is unlikely to succeed.

It's worth a try though.

When making calculations, you just need to remember a large number of different factors that influence the roof.

At least the weight itself rafter system with all elements and fasteners.

Therefore, professionals use special computer programs and calculators to calculate rafters.

How to find out the load on the rafter legs?

The collection of loads should begin with determining the weight of the roofing pie.

If you know what materials will be used and the area of ​​the slopes, then calculating everything is easy.

It is customary to calculate how much 1 square meter of roofing weighs.

And then multiply by the number of squares.

Let's calculate the weight of the roofing pie as an example.

The roofing material is ondulin:

1. Ondulin. A square meter of ondulin weighs 3 kg.
2. Waterproofing. If polymer-bitumen insulation is used, then it weighs 5 kg/square meter.
3. Insulation. The weight of one square of basalt wool is 10 kg.
4. Lathing. The boards are 2.5 cm thick. Weight square meter 15 kg.

We sum up all the weights: 3+5+10+15= 33 kg.

Then the value obtained as a result of the calculations should be multiplied by a factor of 1.1.

This is a correction factor.

It turns out 34.1 kg.

How much does 1 sq. meter of our roofing cake.

And if the total area of ​​our roof is 100 square meters, then it will weigh 341 kg.

Snow load calculation

There is a snow load map.

It shows the amount of snow cover in each region.

We calculate the snow load using the following formula: S = Sg x µ.

Sg is the mass of the snow cover.

µ—correction factor.

And this coefficient depends on the angle of inclination of the slopes of your roof.

The larger this angle, the lower the value of this coefficient.

At tilt angles greater than 60 degrees it is not used at all.

Since snow does not collect on the roof.

Calculating wind load

Just as the whole country is divided into regions according to the mass of snow, it is also divided according to the strength of the winds.

And there is also a special map on which the wind strength is indicated in each area.

To calculate wind loads, use the formula:

Wo - indicator taken from the map.

k is an adjustment factor depending on the type of terrain where the building is located and its height.

We calculate the cross-section of the rafter leg

The cross-section of the rafters depends on three factors:

• on the length of the rafters;
• on the distance between the railings;
• from loads acting on the roof.

Knowing these parameters, it is easy to determine from the table.

How to calculate the length of the rafter legs of a pitched roof

Of all the types of roofs, the shed roof is the simplest.

There are no complex elements in it at all.

And it is installed on load-bearing walls that have different heights.

This kind of roofing is installed on garages, bathhouses, and utility rooms.

To calculate what the length of the rafters will be one pitched roof, you should decide on the angle of inclination.

And the angle of inclination of the slope depends, first of all, on the type of roofing material which you want to use.

In the case when it is corrugated board, then optimal angle the inclination is 20 degrees.

But it is prohibited to make an angle less than 8 degrees!

Otherwise, in the cold season, the roof will not withstand the weight of the snow cover and will simply fail.

If you lay metal tiles, the minimum tilt angle increases to 25 degrees.

When using slate - 35 degrees.

If the roof is seam, then the angle of inclination can be different: 18 - 35 degrees.

After you have figured out the angle of the slope, you need to raise the back wall to such a height that you get the desired angle.

The most difficult thing in such calculations is to find the sine and tangent.

But for this they use the following sign:

Roof inclination angle, degrees	Tangent tgA	Sine sinA
5	0,09	0,09
10	0,18	0,17
15	0,27	0,26
20	0,36	0,34
25	0,47	0,42
30	0,58	0,5
35	0,7	0,57
40	0,84	0,64
45	1,0	0,71
50	1,19	0,77
55	1,43	0,82
60	1,73	0,87

For example, let's find the length of the rafters and the height of the facade wall for a house 5 meters long.

The tilt angle is 25 degrees.

To determine the height of the front wall, Lbc x tg 25 = 5 x 0.47 = 2.35 meters.

Accordingly, the length of the rafter leg is Lc = 2.35 x 0.42 = 5.6 meters.

And do not forget to add to the resulting length the length of the front and rear overhangs, which are necessary in order to provide the walls of the building with protection from slanting rain.

On average, the length of one overhang is 0.5 meters.

If required, this length can be longer.

But less than 0.5 meters is impossible.

This means that 1 meter should be added to the length of the rafter: Lc = 5.6 + 1 = 6.6 meters.

Calculation for a gable roof

Rafter system gable roof much more complex than the rafter system of a pitched roof.

There are more elements, and the principle of its operation is somewhat different.

To calculate the length of the rafter leg, we use the Pythagorean theorem.

If you look at the picture shown right triangle, then you can see that the hypotenuse b is our rafter.

And its length is equal to the length of the leg divided by the cosine of the specific angle of inclination of the slopes.

For example, if the width of the house is 8 meters, and the angle of inclination of the slopes is 35 degrees, then the rafter leg will have the length:

b= 8 / 2 / cos 35 = 8 / 2 / 0.819 = 4.88 meters.

Now all that remains is to add the length of the canopy, approximately 0.5 meters, to get the required length of the rafters.

It should be said that these are simplified versions of rafter calculations.

To get the most accurate data, it is best to use special programs.

For example, free program"Arkon".

The built-in calculator, based on the parameters specified by you, will automatically calculate both the cross-section of the rafter leg and the length of the rafter.

Video about the rafter calculation program.

Introducing free calculator according to gable roof calculations. Online calculation of the sheathing, the angle of the rafters and the required amount of materials.

Specify roofing material:

Select a material from the list -- Slate (corrugated asbestos cement sheets): Medium profile (11 kg/m2) Slate (corrugated asbestos cement sheets): Reinforced profile (13 kg/m2) Corrugated cellulose-bitumen sheets (6 kg/m2) Bitumen (soft , flexible) tiles (15 kg/m2) Galvanized sheet metal (6.5 kg/m2) Sheet steel (8 kg/m2) Ceramic tiles (50 kg/m2) Cement-sand tiles (70 kg/m2) Metal tiles, corrugated sheets (5 kg/m2) Keramoplast (5.5 kg/m2) Seam roofing (6 kg/m2) Polymer-sand tiles (25 kg/m2) Ondulin (Euro slate) (4 kg/m2) Composite tiles (7 kg/m2) ) Natural slate (40 kg/m2) Specify the weight of 1 square meter of coating (? kg/m2)

kg/m2

Enter roof parameters:

Base width A (cm)

Base length D (cm)

Lifting height B (cm)

Length of side overhangs C (cm)

Front and rear overhang length E (cm)

Rafters:

Rafter pitch (cm)

Type of wood for rafters (cm)

Working area of ​​the side rafter (optional) (cm) ">

Lathing calculation:

Sheathing board width (cm)

Sheathing board thickness (cm)

Distance between sheathing boards
F (cm)

Snow load calculation:

Select your region using the map below

1 (80/56 kg/m2) 2 (120/84 kg/m2) 3 (180/126 kg/m2) 4 (240/168 kg/m2) 5 (320/224 kg/m2) 6 ​​(400/280 kg/m2) 7 (480/336 kg/m2) 8 (560/392 kg/m2)

Wind load calculation:

Ia I II III IV V VI VII

Height to the ridge of the building

5 m from 5 m to 10 m from 10 m

Terrain type

Open area Closed area Urban areas

Calculation results

Roof angle: 0 degrees.

The tilt angle is suitable for of this material.

It is advisable to increase the angle of inclination for this material!

It is advisable to reduce the angle of inclination for this material!

Roof surface area: 0 m2.

Approximate weight roofing material: 0 kg.

Number of rolls of insulating material with 10% overlap (1x15 m): 0 rolls.

Rafters:

Load on the rafter system: 0 kg/m2.

Rafter length: 0 cm

Number of rafters: 0 pcs.

Lathing:

Number of rows of sheathing (for the entire roof): 0 rows.

Uniform distance between sheathing boards: 0 cm

Number of sheathing boards standard length 6 meters: 0 pcs.

Volume of sheathing boards: 0 m3.

Approximate weight of sheathing boards: 0 kg.

Additional information about the calculator

An online gable roof calculator will help you calculate the angle of the slope, the size and number of rafters, the amount of sheathing, and the volume necessary materials online. The calculation base includes in advance such common roofing materials as metal tiles, slate, ondulin, tiles made of ceramics, bitumen, cement and other materials.

Pay attention! Calculations are made based on SNiP “Loads and Impacts” and TKP 45-5.05-146-2009, taking into account the standards contained in these documents.

A gable roof (also spelled “gable roof”, “gable roof”) is a variant of a roof with two slopes running from the ridge to the outer walls of the building. Today this is the most common type of roof, due to its simplicity of execution, low cost and attractive appearance.

The rafters in the construction of such a roof rest on each other in pairs and are connected by sheathing. The end sides of a structure with such a roof have the shape of a triangle and are called gables (sometimes gables). Usually, an attic is installed under a gable roof, and small attic windows are made on the gables for lighting.

When filling out the fields of the calculator, pay attention to the “Additional information” icon, which hides explanations for each item.

The calculation results are also accompanied by explanations, which you can read below.

Explanations for the calculation results

Roof angle

This is the name of the angle at which the slope and rafters are inclined to the plane of the ceiling. The calculations were made taking into account the fact that it is planned to build a symmetrical gable roof. By entering an angle, you can not only calculate the required amount of materials for a given angle, but also check whether it is possible to build a roof at this angle from the materials you have chosen. You can decrease or increase the angle by changing the width of the base or the height of the rise: these parameters are strictly interconnected.

Roof surface area

The total area of ​​the roof slopes, including the area of ​​overhangs of a given length. Determines the amount of roofing and under-roofing material required during roof construction.

Approximate weight of roofing material

Estimated total weight of roofing material.

Number of rolls of insulating material

The required amount of under-roofing material, taking into account the required overlap of 10%. In our calculations, we assume rolls 15 meters long and 1 meter wide.

Load on the rafter system

The maximum possible load, taking into account wind and snow loads, on the rafters.

Rafter length

Rafters are measured from the base of the slope to the ridge of the roof.

Number of rafters

The total number of rafters required for a roof truss system at a given pitch.

Minimum rafter section

To ensure the roof has sufficient strength, it is necessary to select rafters with the section options suggested here.

Number of rows of sheathing

With the parameters you specify, you will need given quantity rows of sheathing. If you need to determine the number of rows for one slope, then given value must be divided by 2.

Uniform distance between sheathing boards

To eliminate waste of materials and save yourself from unnecessary trimming work, you need to choose a given distance between the sheathing boards.

Volume of sheathing boards

The number of boards required to sheath the entire roof (in cubic meters).

Please indicate the required dimensions in millimeters

X- width of the house
Y- roof height
C- overhang size
B- roof length
Y2- extra height
X2- extra width

Reference

The program is designed to calculate roofing building materials: quantities sheet material(ondulin, nulin, slate or metal tiles), roofing material (glassine, roofing felt), number of sheathing boards and rafters.
You can also calculate some useful roof dimensions.

The program works in two modes: a simple gable roof and a roof with two side gables (side roofs), type 1 and type 2.

Attention! If you have a roof with one side gable, then for the calculation use first type 1, then type 2. And from the data obtained, calculate the amount of building materials: rafters, sheathing boards, roofing and sheet materials.
Otherwise there may be an error in the calculation. After all, the program takes into account the cutouts in the main roof for the roofs of the side gables.

In the calculation you will see several numbers: the size or volume of the building material of half the roof and in brackets - the full size or volume.
In the calculation of an additional roof - the full size and volume, and in brackets there are two numbers: the size and volume of one and two additional roofs.

Attention! When calculating sheet roofing material, keep in mind that the program calculates by roof area.
For example, 2.8 rows times 7.7 sheets per row. During actual construction, 3 rows are laid.
To more accurately calculate the number of roofing sheets, you need to reduce the sheet height in the calculation until you get a whole number of rows.
Don't forget to set the amount of overlap more accurately.

When calculating the volume of material for the rafters of the main roof, in mode type 2, the program does not take into account the cutout for the side gable. This is due to some implementation difficulties in the program.
Maybe I'll solve this in the future.
However, the excess rafter material is unlikely to disappear, or make some adjustments to your calculations.
There will also be a separate program for more intelligent calculation of sheet roofing materials.

And don't forget what to buy building materials it is necessary with some reserve for waste.

The roof of a building is designed to hold external loads and redistribute them to load-bearing walls or supporting structures. Such loads include the weight of the roofing pie, the weight of the structure itself, the weight of the snow cover, and so on.

The roof is located on the rafter system. This is the name of the frame structure on which the roof is fixed. It accepts all external loads, distributing them across supporting structures.

The rafter system includes the following elements:

• Mauerlat;
• Struts and braces;
• Side and ridge purlins;
• Rafter legs.

A rafter truss is a structure that includes all of the listed elements with the exception of the Mauerlat.

Calculation of gable roof loads

Constant loads

The first type refers to those loads that always act on the roof (in any season, time of day, and so on). These include the weight of the roofing pie and various equipment installed on the roof. For example, the weight of a satellite dish or aerator. It is necessary to calculate the weight of the entire truss structure along with fasteners and various elements. To perform this task, professionals use computer programs, as well as special calculators.

Calculation gable roof is based on calculating the loads on the rafter legs. First of all, you need to determine the weight of the roofing cake. The task is quite simple, you just need to know the materials used, as well as the dimensions of the roof.

As an example, let’s calculate the weight of a roofing cake with ondulin material. All values ​​are taken approximately; high accuracy is not required here. Usually builders perform calculations of the weight per square meter of roofing. And then this indicator is multiplied by total area roofs.

The roofing pie consists of ondulin, a layer of waterproofing (in in this case- insulation on a polymer-bitumen basis), a layer of thermal insulation (the weight of basalt wool will be calculated) and sheathing (the thickness of the boards is 25 mm). Let's calculate the weight of each element separately, and then add up all the values.

Roof calculation gable roof:

1. A square meter of roofing material weighs 3.5 kg.
2. A square meter of waterproofing layer weighs 5 kg.
3. A square meter of insulation weighs 10 kg.
4. A square meter of sheathing weighs 14 kg.

Now let's calculate the total weight:

3.5 + 5 + 10 + 14 = 32.5

The resulting value must be multiplied by the correction factor (in this case it is equal to 1.1).

32.5 * 1.1 = 35.75 kg

It turns out that a square meter of roofing cake weighs 35.75 kg. It remains to multiply this parameter by the roof area, then you can calculate a gable roof.

Variable roof loads

Variable loads are those that act on the roof not constantly, but seasonally. A striking example is the snow in winter time. Snow masses settle on the roof, creating additional impact. But in the spring they melt, and accordingly, the pressure decreases.

Variable loads also include wind. This is also a weather phenomenon that does not always work. And there are many such examples. Therefore, it is important to take into account variable loads when calculating the length of the rafters of a gable roof. When calculating, you need to take into account many different factors affecting the roof of a building.

Now let's take a closer look at snow loads. When calculating this parameter, you need to use a special map. The amount of snow cover in different regions of the country is marked there.

To calculate this type of load, the following formula is used:

Where Sg is the terrain indicator taken from the map, and µ is the correction factor. It depends on the roof slope: the stronger the slope, the lower the correction factor. And here there is important nuance- for roofs with a slope of 60 o it is not taken into account at all. After all, the snow will simply roll off them and not accumulate.

The whole country is divided into regions not only by the mass of snow, but also by the strength of the winds. There is a special map on which you can find out this indicator in a certain area.

When calculating roof rafters, wind loads are determined using the following formula:

Where x is the correction factor. It depends on the location of the building and its height. And W o is the parameter selected from the map.

Calculation of the dimensions of the rafter system

When the calculation of all types of loads is finished, you can proceed to calculating the dimensions of the rafter system. The work performed will differ depending on what kind of roof structure is planned.

In this case, a gable one is considered.

Section of the rafter leg

The calculation of this indicator is based on 3 criteria:

• Loads from the previous section;
• Remoteness of the railings;
• Rafter length.

There is a special table of sections of rafter legs, in which you can find out this indicator based on the criteria described above.

Length of rafters in a gable roof

For manual calculations you will need basic knowledge geometry, in particular the Pythagorean theorem. The rafter is the hypotenuse of a right triangle. Its length can be found by dividing the length of the leg by the cosine of the opposite angle.

Let's look at a specific example:

It is required to calculate the length of the rafters of a gable roof for a house with a width of 6 m, in which the slope of the slopes is 45 o. Let L be the length of the rafters. Let's substitute all the data into the formula.

L = 6 / 2 / cos 45 ≈ 6 / 2 / 0.707 ≈ 4.24 meters.

You need to add the length of the visor to the resulting value. It is approximately 0.5 m.

4.24 + 0.5 = 4.74 meters.

This completes the calculation of the length of the rafters for a gable roof. It was manual method completing the task. There are special computer programs designed to automate this process. The easiest way is to use Arkon. This is a completely free program that even a person with little computer knowledge can easily understand.

It is enough to simply specify the input parameters based on the size of the house. The program will independently perform calculations and show the required cross-section, as well as the length of the gable roof rafters.

How to calculate the length of the rafters of a gable roof: roof calculation, load and design rules

We calculate the length of the rafters and overhangs of a gable roof

When designing a private house, it is necessary to take into account many different parameters. If they are calculated incorrectly, then the strength of the structure will be in great doubt. The same applies to the roof of the house. Here, even before the start of construction, you need to find out the height of the ridge, the area of ​​the roof and much more, including calculating the length of the rafters. And how to make the final calculations will be discussed in this article.

What type of roof

How to calculate the length of the rafters? This question will be of interest to everyone who builds a house on their own. But to answer it, you should first find out many other parameters. First of all, you should decide on the type of roof, because the length of the slope and rafters will depend on this. The most common option is considered gable design. But here there are several options, namely:

You can consider even more complex designs, for example multi-level ones. Such roofs will look very attractive. But to make calculations, and especially to build a rafter system, in this case, without the help of professionals will be almost impossible. Therefore, in most cases, we are limited to the three above-mentioned gable roof options.

System type

Calculating the length of the gable roof rafters will also depend on the system used. Here experts distinguish the following two main varieties:

1. Hanging system. This is the simplest option. In this case, the rafter legs rest only on the Mauerlat. Their upper part is simply connected to each other. This system is used if the width of the house is small. In this case, the length of the rafters should not exceed six meters. It is not advisable to use the hanging option with an asymmetrical gable roof.
2. The layered system is a more durable rafter system. It is used if there is an axial load-bearing wall running through the middle of the house. In this case, supports and a ridge girder are installed, onto which the upper part rafter legs.

Can also be used combined option. It is often used in the construction of houses with complex geometry. Here it will be more difficult to calculate the length of the rafters and other system parameters. If you have this option, then it is better to entrust everything to a specialist. In this case, there will be fewer mistakes, which means the roof will last longer and will not cause you problems during operation.

What else to consider

The type of roof and the system used are not all the parameters that will be required in order to calculate the length of the rafters of a gable roof. Before you calculate everything, you need to find out a lot more information, namely:

In addition, when calculating the length of the rafters, you should find out what overhangs should be. Not a single roof can do without this “additional” element. Overhangs play the role of protection, which protects the walls of the house and its foundation from being washed away by water flowing from the roof.

They can be a continuation of the rafters or made as independent elements. In the latter case, boards called “fillies” are attached to the main structure. At their core, they are an extension of the rafters.

What length to choose overhangs is up to the home owners themselves. According to existing building regulations, this parameter should be in the range from 50 to 60 centimeters. You shouldn’t do less, otherwise the walls and foundation may suffer. Sometimes overhangs are made more than one meter. In this case, a small canopy is created along the wall, which can be used for relaxing or storing things.

Making calculations

How is the length of the rafters calculated? If the roof has a symmetrical shape, then calculating this parameter is not difficult. To do this, use the formula of the Pythagorean theorem, namely: C is equal to the square root of A squared plus B squared, where:

• C is the required rafter length;
• A is the height at which the ridge is located (from the base of the roof);
• B is half the width of the house.

Moreover, using this formula you can calculate the length of the rafters only up to the mauerlat. The length of the overhangs is not taken into account here. If they are a continuation of the rafters, then their length must be added to the calculated parameter.

How to make a calculation if the roof is asymmetrical? In this case, the slopes will be different. But even here you can use the Pythagorean theorem. You can calculate the rafters for the roof using the same formula, only first find out the value of parameter “B” (in the first case it is equal to half the width of the house). If the roof is asymmetrical, then at the design stage you will calculate at what distance from the walls the ridge will be located. It is this value that is taken as the “B” parameter. As a result of the calculation, you will get the length of each of the rafter legs (on the left and right slope). As you can see, there are no problems with calculations here either.

There is another way to calculate rafters. In this case, the slope angle is used. This formula is a little more complicated than the previous one. The length of the rafters (for a gable symmetrical roof) will be equal to the sum of 0.5 and the height from the base of the roof to the ridge divided by the cosine of the slope angle.

No matter how the calculation is made, the main thing is to do it correctly and accurately. The strength of the entire rafter system will depend on this. If you cannot calculate the length of the rafters to a whole number, then it is better to round up. It is better to saw off a little excess during the installation itself.

Calculation of the length of the rafters of a gable roof depending on the type of roof (symmetrical, asymmetrical, broken) and the type of rafter system (hanging, layered). Basic nuances and calculations.

The roof is not only protection of the house from the external environment, but also a certain decorative element that gives the structure a finished look. That is why developers today are building the most unusual roofs with complex structures rafter systems.

The rafter system is the most important element arrangement of any roof. It bears the weight of the coating and precipitation. That's why correct execution Such a system, taking into account all the rules of construction art, is a guarantee of the reliability and durability of the roof. It is very important to correctly determine the length of the rafters and other structural elements. In this case, it is necessary to take into account such climatic features as:

What does the rafter system consist of?

Any structure of this kind is made in the form of interconnected elements that strictly correspond to the previously made calculations. The following elements can be distinguished as part of this system:

• slanting legs, which are also called rafter legs;
• stops, trusses and other fasteners that give the structure the necessary rigidity;
• vertical type racks;
• narozhniki.

Pay attention! It is necessary to take special responsibility when calculating the length of the rafters - any, even minor, mistake can lead to deformation of the roof geometry and, accordingly, its collapse.

If you do not understand the features of roofing construction, then it is better to contact qualified specialists. To do your own calculations, use special calculators and tables - this will help you avoid mistakes.

Types of rafter system

Types of rafter system

Rafter systems are divided into two groups depending on the material used:

• wooden structures;
• metal structures.

There are also reinforced concrete rafter systems, but they are used mainly in industrial buildings. In any case, whether the rafters are metal, wood or concrete, they must be firmly attached to the walls of the house.

Often for the construction of rafters in country houses wood is used mainly coniferous species. Compared to metal, wood is easier to process and install. Moreover, even if an error occurred during the calculations, then wooden parts easy to replace.

Before you start calculating, first measure the width of the house. The fact is that although small slanted legs do not require additional building, in some cases the special geometry of the roof requires reinforcement of the rafters, even if the house is of small size.

According to the design features, rafters are divided into:

In construction country houses Sloping rafters are more often used, but often builders combine both. As mentioned, extensions of the mowed legs may be required. This depends on the roofing material used during construction. So, slate or ceramic tiles in view heavy weight can only be installed on a high-strength rafter system.

Types of gable rafter systems

The cross-section of the boards used in the construction of the rafters can be 20x6 cm or 15x5 cm. But if the structure is strengthened, you can choose a beam with O larger cross-section (there is another way of strengthening - by splicing boards).

And now - directly to the calculations.

What to consider when calculating rafters

First, let's define the fundamental points.

1. The type and shape of the roof directly affect functional features rafter system. The fact is that calculations for hipped and gable roofs will differ from each other, because they need to be carried out according to different methods. Moreover, asymmetrical roofs (for example, broken roofs) require additional stabilization elements - crossbars, sleepers, struts, etc.
2. Future loads on the structure, mainly snow and wind, are also very important in the calculations. For example, in snowy regions of the country it is quite difficult to build a roof with a slope of less than 45°, and if you increase the slope or height of the structure, the wind load will increase. In short, you need to determine the very “ golden mean", but not to the detriment of attractiveness and... Very often only true masters can solve such a problem.
3. One more important point when calculating is the coating material. Many of these materials require certain conditions. So, flexible tiles laid exclusively on a solid surface (in extreme cases, a thin sheathing). Ceramic tiles require a reinforced frame.
4. Size and area are the main indicators that influence the choice of a particular type of roof. If the area is large, then the pitch of the rafters increases and, accordingly, the distance between them. Because of this, the cross-section of the timber used increases.

Pay attention! The distance between load-bearing walls is called a run. As the run increases, the number of changes in the design increases, in particular, the number of stabilizing and reinforcing elements.

How to calculate rafters for a roof

Now, having familiarized yourself with the starting points, you can take paper, a ruler and a pencil and start making calculations.

First stage. Roofing cake weight

First, determine how much the roof itself will weigh. This is very important, because the rafter system must withstand this weight for a long time. It is very easy to calculate: find out the weight per square meter of each layer, summarize the data obtained and add a correction of 10%.

Here is an example of such calculations.

1. A square meter of sheathing weighs 15 kg.
2. The roof covering will be, say, ondulin with a weight of 3.5 kg.
3. A square meter of bitumen waterproofing weighs another 6 kg.
4. Weight of a 10 cm layer mineral wool is approximately 10 kg per square meter.

Let's see what happens.

Add the correction 10%, it turns out 37.95 kg. This figure is an indicator of the weight of the roofing cake.

Pay attention! In most cases, this weight does not exceed 50 kg, but experienced specialists are confident that when making calculations, it is necessary to base their calculations on precisely this value – “for reserve”.

It turns out that the weight of the roofing cake should be 50 + 10% = 55 kg/m².

It is very important to take into account snow load, because snow can accumulate on the roof for quite some time large quantities. Use a special formula to determine this load:

S in this case, this is the snow load that you need to calculate;

µ – correction depending on the slope of the slope;

U flat roof, the slope of which does not exceed 25°, the correction will be equal to unity; if the slope of the slope is more than 25°, but does not exceed 60°, then the correction will be 0.7. If a very steep roof is being built, then the snow loads for it can not be calculated at all.

Sᶢ is the weight of a square meter of snow cover. This indicator depends on the climatic characteristics of a particular region; you can find out about it in SNiP.

Let’s say the roof slope is 25°, and the snow mass is 200 kgf/m².

To calculate wind load on rafters, use the formula below.

Wᵒ in this case, it is a standard indicator that you must determine from the table (it all depends on what region you live in);

TO- This is an amendment that takes into account the height of the house and the type of terrain.

Fourth stage. Calculation of pitch and length of rafters

Selecting the section and length of the rafter leg

To calculate the length of the rafters, you can remember geometry at school, namely the famous Pythagorean theorem. After all, a rafter structure is, in fact, a right triangle and measuring its diagonal is very simple. But do not forget to take into account when calculating:

• strength of beams;
• possibility of deformation - how much load the system can withstand without breaking.

Pay attention! According to GOST, rafters should not bend more than 1/250 of their length. For example, if the length of the rafters is 5 m, then multiply this number by 0.004 - this will give you maximum deflection, namely 2 cm.

Basic material requirements

According to GOST, wood must meet the following requirements:

• its humidity should not exceed 18%;
• the number of knots should not exceed three pieces per linear meter timber;
• there may be non-through cracks, but their length should not exceed half the total length;
• wood must be treated with an antiseptic, fire retardant and biological protection agent.

In addition, when purchasing bars, pay attention to:

• manufacturing company;
• date of manufacture;
• product name, standard;
• quality of individual parts;
• size and humidity of products;
• wood species

Special computer programs

Judging by everything said above, to calculate rafters you need to have not only a sufficient supply of knowledge, but also drawing and drawing skills. Of course, not every one of us can boast of all this.

Fortunately, today there are many computer utilities designed to make calculations easier. There are professional ones among them, such as, for example, AutoCAD, but you can find more simple options. Thus, in the Arkon program you can easily create various projects, as well as clearly see what the future roof will look like.

Pay attention! Such utilities also have a calculation calculator, which was mentioned earlier. With its help, you can calculate the length, pitch and cross-section of rafters with extreme accuracy.

Such calculators are also available online, but all the data that can be obtained with their help is advisory in nature and does not replace a full-fledged drafting of the project.

As a conclusion

One of the most important stages roof construction is the calculation of the rafter system. Of course, it is better to entrust this matter to professionals, but preliminary measurements can be made on your own - this will help you understand the finished drawing.

Video - Installing rafters

Get the best by email

Find out how to calculate roof rafters! What data is needed for calculations, step by step guide, tables, photos + video.

One of the most important parts of a pitched roof is the rafter system, consisting of strong and reliable beams. It is the rafters that form the basis for the roof. It is important that the materials used can easily withstand not only the roof structure, but also the pressure of snow or ice masses in winter period, as well as wind loads throughout the year. In this regard, before proceeding with the installation of rafters, you should necessary calculations, taking into account all possible factors and nuances. Of course, you can order rafter calculations in various construction companies, however, such a service will cost a fairly decent amount, so the best option may become an independent calculation. So, how to calculate the roof truss system correctly? Of course, before moving on to the main question, it is worth studying the features of the rafters and the types of construction.

Features of the rafter system

To make a calculation of the rafter system, you should understand what it is. So, the rafters are the load-bearing structure of the roof, which takes on all external loads, in the form of snow drifts, heavy downpours or heavy winds. Its main elements are:

• vertical posts - necessary for maximum stability of the rafter system;
• rafter inclined legs - determine the slope of the roof slope and its general appearance;
• purlins - there are lateral and ridge varieties of purlins, the elements are necessary for fastening and supporting the rafter legs;
• tightening bolts, crossbars – fixing elements;
• struts - diagonal supporting beams that give stability to the rafters;
• ridge - an upper beam laid at the junction of two roof slopes;
• fillies - an element that allows you to increase the length of insufficiently long rafters in the case of installing a roof overhang;
• truss - a set of posts, braces, sheathing and other elements that form the basis of the roofing system.



When starting to calculate the rafters, you should calculate each individual element. It is also important to comply with the requirements for the rafter system; this will help you choose the right material, as well as create the most durable and long-lasting roof.

Basic requirements when choosing rafter material

Today, quite a few home owners prefer wood roofs. As a rule, the rafter system is made from coniferous trees. In this case, the timber should have a moisture content of no more than 20%. This is the so-called air-dried wood, which is characterized by the necessary strength and lightness. In addition to the humidity percentage, when choosing a tree, the following conditions should be observed:

• the presence of a minimum number of knots, cracks and other possible defects, for this you should choose grade 1 or 2 wood. When choosing grade 3 wood, you should pay attention that per 1 m of board or beam there are no more than 3-4 knots up to 3 cm high, and if there are cracks, their length and depth should be small;
• for load-bearing, capital elements, such as rafters, mauerlat, ridge, and so on, it is recommended to use timber with a thickness of more than 5 cm; it is optimal to use products with square or rectangular cross-section from 10 to 20 cm;
• when choosing coniferous boards, product lengths up to 6.5 m are allowed, and if used hardwoods, then the length of the lumber should not exceed 4.5 m. As a rule, hardwood is used for such structural parts as purlins and mauerlat. It is also worth giving preference to hard rocks.



Important! The entire constructed system must have rigidity and strength. That is, the finished structure must have a reliable fixation and be motionless. If at least one element does not meet this requirement, then there is a high probability that the roof may be destroyed by hurricane winds or heavy snowfall, and it will not matter how correctly the wooden roof rafters are calculated. In the most dire situation, not only the roof, but also the walls of the building will be destroyed. It is also worth keeping in mind that the rafter system should be made light, especially when wooden load-bearing walls are used. In order to be able to use strong and reliable beams, but not make the structure heavier, it is recommended to choose lumber with a low percentage of moisture, that is, about 10-15%. Also, don’t forget about processing wooden elements antiseptics, fire retardants, water repellents and other protective drugs. Before getting down to the question of how to correctly calculate the rafter system, you should get an idea of ​​the types of rafters.

Video on the topic:


Types of rafters

The specific type of rafters depends on the type of roof and when calculating the rafter system, this should be taken into account. For example, the roof can be gable or hipped, and accordingly the rafters will be calculated differently. At the same time, the presence structural elements and the principle of their installation remains practically unchanged. Today it is customary to distinguish 2 main types of rafter systems.

1. Layered rafters - in this case, the rafter legs rest on the walls of the building, and their middle is supported by an intermediate support. Mounted similar system if necessary, if you make spans longer than 5-7 m. Each additional support can increase the span length by 3-4 m.
2. – are installed when the distance between the external walls on which the rafter system is installed is no more than 6.5 m.

Having chosen a specific type of roof, as well as a type of rafter system, you can proceed to the implementation of all necessary calculations, that is, calculating the cross-section of the rafters, the load, the length and height of the beams, and so on.

Calculation of the load on the rafters

When calculating the roof rafters yourself, it is recommended to take increased parameters, so that you can have a certain margin of safety for the roof. Of course, this will increase the consumption of building materials, however, home safety issues should still be given top priority. So, the first step is to take into account all possible loads that will affect the roof structure. In particular, such loads include snow and wind loads. Also, when calculating the load on the rafter system, it is worth taking into account quite a few features. Including factors such as:

• weight of roofing material;
• sheathing weight;
• weight of insulation, hydro- and vapor barrier;
• weight of the rafter system.

Only by calculating each point can you calculate the rafter system. For example, the formula for calculating snow load would look like this:

S = Scalc·μ,
where S is the desired parameter, Scalc. is the value of the weight of snow per 1 sq.m, which should be taken from the SNiPs in force in a certain territory, and μ is a coefficient calculated from the angle of inclination of the roof. To calculate the wind load, you can also use the formula:

Wm = Wo·k·c,
where Wo is the standard parameter of wind pressure, determined according to SNiPs in force in the region, k is the coefficient of wind pressure, depending on the height of the roof above the ground, and c is the aerodynamic coefficient, which depends on the shape of the roof. Knowing all the initial values, making calculations is not difficult. However, today it is not at all necessary to carry out all the necessary measurements and calculations manually. After all, special programs have been created for these purposes, for example, a program for calculating a rafter system or a program for calculating rafters and trusses. Such programs include:

• Stropila;
• AutoCAD;
• Arkon;
• Online calculation services (construction calculators).

What is the operating principle of such software? It is quite simple, you need to enter all the parameters from SNiPs or the building plan into the appropriate windows or lines, then click the “calculate” button and the program will display the result. As a rule, these resources include all the necessary calculations, that is, wind and snow loads, as well as calculation of the total load, calculation of the distributed load, calculation of the rafter system, and so on. The programs also contain maps with wind pressure and snow weight in all regions. Even untrained users will be able to make calculations in such applications, and all parameters will be the most accurate. In addition, it should be borne in mind that certain parameters are constant and can be found in the instructions for building materials or on the Internet.

Type of roofing and its weight

Depending on what roofing material you plan to use, the load on the rafter systems also changes. Almost all types of coatings have a fixed weight, making calculations quite easy. Let's consider the weight of the main varieties roofing coverings, which are provided by manufacturers during manufacture.

As for the mass of the rough flooring, rafter system and sheathing, these values ​​are considered to be standard. In particular, the rough roofing structure will have a weight of 18-20 kg/sq.m, wooden sheathing– 8-10 kg/sq.m and rafters – 15-20 kg/sq.m. By summing up all the values, you can easily find the desired load parameter on the rafter system.

Calculation of rafters

Once the load has been determined, you can move on to a point such as calculating the rafter system. It is necessary to determine the load on each rafter leg in order to understand what cross-section the rafters should have, their strength and how much timber will be required for the rafters in each specific case. The formula for calculating the load on each rafter leg is as follows:

Qr=A·Q,
where Qr is the desired value, measured in kg/m, A –, measured in meters and Q is the total load acting on 1 sq.m of roof, measured in kg/sq.m (this is the value that was found in the calculations made by previously). You can also calculate the load in automated mode, using programs. Various applications allow you to calculate the cross-section of rafters, their number, height and many other parameters. Important! When calculating the rafter system, you should always round the parameters up, as this allows you to increase the strength of the roof structure.

Making the necessary calculations yourself is not at all difficult. Of course, if knowledge in this matter is not enough, you can always turn to specialists. However, a huge variety of automated programs can help you cope with the calculation of the rafter system without much hassle. It is important to remember that not only the strength and reliability of the roof, but also the safety of the residents of the house depends on the correctness of the calculations.