Do-it-yourself rafters: calculation of loads, installation stages. Load from the weight of the roof Calculation of the load per 1 m of the rafter system

Load Combination

Now, knowing the coefficients, let's finally find out the value of all the calculated loads.
Snow load: S calculated = 198 kg/m2 1.4 = 277.2 kg/m2
Wind load: Wcalc = 27.56 kg/m2 1.4 = 38.58 kg/m2
Constant load: Gcalc = 52.22 kg/m2 1.1 = 57.44 kg/m2

For complete information, I’ll tell you that it’s not correct to simply sum up the resulting loads - the result will be higher.

Sometimes many loads act on the design structure at once. For example, constant load from the structure, payload from people, payload from furniture, snow load, wind load and others. But the chance that all their maximum values ​​will be in effect at the same time is close to zero. Therefore, temporary loads are further divided into short-term and long-term, and their own combination coefficients are introduced for them. Somewhere 0.9, and somewhere 0.3. And when summing up these loads, they are simply multiplied by these coefficients.

But in our case, we don’t have many loads, and we will sum them up without combination coefficients (it won’t get any worse).

u = 277.2 + 38.58 + 57.44 = 373.22 kg/m2

Those. one rafter leg with a cargo area No. 1 equal to 5,769 m2(considered above) will bear the load

Q = 373.22 kg/m2 5.769 m2 = 2,153 kg

And the linear distributed load along the length of the rafter leg L=6.410m(calculated above) will be equal to:

q = 2,153 kg / 6.410 m = 335.88 kg/m

Calculation of the rafter system

Calculation of the strength of the rafter leg will be based on the following formula:

M / W ≤ Rben

Where M– maximum bending moment
W– moment of resistance of the cross section to bending
Rizg– calculated bending resistance (1st grade of wood – 14 MPa, 2nd grade – 13 MPa, 3rd grade – 8.5 MPa)

Moment of resistance of a rectangular section:

W = b h h /6

Where b– section width of the rafter leg
h– height of the section of the rafter leg

If you wonder what the height h is 1.5 times larger than width b

W = b (1.5 b) (1.5 b) / 6 = 0.375 b b b
M / 0.375 b b b ≤ Rben
b ≥ root 3 (M / Rben / 0.375)

If you wonder what the height h is 2 times larger than width b, then in the end we will have the following formula.

W = b (2 b) (2 b) / 6 = 0.667 b b b
M / 0.667 b b b ≤ Rben
b ≥ root 3 (M / Rben / 0.667)

The initial data is grade 1 pine, and the geometry and loads are the same as in the examples above.

We calculate the maximum bending moment on our calculator by entering the values ​​​​calculated above or using the formula M=q·L1·L1/8(less accurate):

L1 = 5189 mm – main span
L2 = 1221 mm – right console

The result will be the maximum bending moment M=1008.7 kg m

Let's convert our moment from kg*m to N*mm.

M = 1008.7 kg*m 10 1000 = 10,087,000 N*mm

If we set the ratio h/b=2, we would get the following:

b ≥ root 3 (M / Rben / 0.667)
b ≥ 3rd root (10087000 / 14 / 0.667) ≥ 102.61 mm

We take b = 125 mm, and the height h will then be 2·125=250 mm. We accept h = 250 mm.

The resulting section of the rafter leg is 125x250 mm

Conclusion

So, in Tomsk, for a roof at an angle of 35 degrees with a rafter pitch of 900 mm made of grade I pine, a height to the ridge of 7 m with corrugated sheeting as a roofing material, rafters with a cross section of 125x200 mm are suitable.

P.S. This article was written over several days and more work will be done on it, so the author will be very grateful if you share this article with your friends and colleagues and write a comment.

The online gable roof calculator will help you calculate the angles of the rafters, the required amount of sheathing, the maximum load on the roof, as well as the materials required to build a roof of this type with given dimensions. You can calculate the roof from such popular roofing materials as slate, ondulin, ceramic, cement-sand and bitumen tiles, metal tiles and other materials.

The calculations take into account the parameters given in TKP 45-5.05-146-2009 and SNiP “Loads and Impacts”.

A gable roof (also known as a gable or gable roof) is a type of roof that has two inclined slopes that run from the ridge to the outer walls of the building. This is the most common type of roof today. This is explained by its practicality, low construction costs, effective protection of premises and aesthetic appearance.

The rafters in a gable roof structure rest on each other, connecting in pairs. On the end side, gable roofs have the shape of a triangle; such ends are called gables or gables. Usually, an attic is installed under such a roof, which is illuminated using small windows on the gables (attic windows).

When entering data into the calculator, be sure to check the additional information marked with the icon.

At the bottom of this page you can leave feedback, ask your own question to the developers, or suggest an idea to improve this calculator.

Explanation of calculation results

Roof slope

The rafters and roof slope are inclined at this angle. It is understood that it is planned to build a symmetrical gable roof. In addition to calculating the angle, the calculator will inform you how the angle complies with the standards for the roofing material you have chosen. If you need to change the angle, then you need to change the width of the base or the height of the roof, or choose a different (lighter) roofing material.

Roof surface area

Total roof area (including overhangs of a given length). Determines the amount of roofing and insulating materials that will be needed for the work.

Approximate weight of roofing material

The total weight of roofing material required to completely cover the roof area.

Number of rolls of overlapping insulation material

The total amount of insulating material in rolls that will be required to insulate the roof. The calculations are based on rolls 15 meters long and 1 meter wide.

The maximum load on the rafter system. The calculations take into account the weight of the entire roofing system, the shape of the roof, as well as the wind and snow loads of the region you specify.

Rafter length

The full length of the rafters from the beginning of the slope to the ridge of the roof.

Number of rafters

The total number of rafters required to construct a roof at a given pitch.

Minimum section of rafters, Weight and Volume of timber for rafters

The table shows the recommended dimensions of rafter sections (according to GOST 24454-80 Softwood lumber). To determine compliance, the type of roofing material, the area and shape of the roof structure, and the loads placed on the roof are taken into account. The adjacent columns display the total weight and volume of these rafters for the entire roof.

Number of rows of sheathing

The total number of rows of sheathing for the entire roof. To determine the number of rows of sheathing for one slope, it is enough to divide the resulting value by two.

Uniform spacing between boards

To install the sheathing evenly and avoid unnecessary overspending, use the value indicated here.

Number of sheathing boards standard length

To sheath the entire roof, you will need the number of boards indicated here. For calculations, a standard 6-meter board length is used.

The volume of boards of the crate

The volume of boards in cubic meters will help you calculate the cost of sheathing.

Approximate weight of sheathing boards

Estimated total weight of sheathing boards. The calculations use average values ​​of density and moisture content for coniferous wood.

A pitched roof has a system of inclined planes (slopes). The design of the rafter system is selected and calculated, taking into account the presence of supports for it, the type of covering, the size and shape of the building being covered. A special calculation will help you select the required rafter size and ensure the strength of the roof.

Types of gable roof rafter systems

The design of the rafter system is selected based on the number of supports for it and the distance between them.

Layered rafters rest on the external load-bearing walls of buildings and on additional internal supports, if the distance between the main supports exceeds 4.5 m. The rafter leg from below rests on a support beam (mauerlat), which transfers the weight from the roof to the wall of the building. The upper end is connected to the ridge purlin and the other rafter leg.

1, 2 - hanging rafter system. 3, 4 - layered rafter system. a - rafters, b - tightening, c - crossbar, d - purlin, e - mauerlat, f - strut, g - stand.

The hanging type of rafter systems is tightened at the level of the lower support nodes or above them and does not require intermediate supports. The distance between external load-bearing supports should not exceed 6.5 m. This version of the truss structure can be classified as triangular trusses. The distance in plan between them is assumed to be 1.3-1.8 m.

Coating composition

Roof

Eternite roofs are flat or corrugated sheets of asbestos cement. This is a cheap type of roofing that is quite easy to install. Recently, studies have shown its harmful effects on human health.

Slate roofs also include slate roofs. They are constructed from natural material with a layered structure of slate. Euroslate and ondulin are descendants of ordinary slate. They are compressed fiberglass or cellulose, which are impregnated with bitumen.

Metal roofing is often used in the construction of residential buildings. It reliably protects the house from atmospheric influences, is lightweight and is not labor-intensive to install. This type of roofing includes corrugated sheeting, galvanized steel, and aluzinc.

Roll roofing is a soft type of roofing. They are waterproof, resistant to environmental influences and easy to install. These include the following types:

• roofing felt (rubemast, steklomast, euroroofing felt, roofing felt, etc.);
• bitumen-polymer (stekloizol, steklokrom, linokrom, etc.);
• membrane roofs (PVC, thermoplastic membranes, synthetic rubber films, etc.).

If earlier tiled roofs were only ceramic, today there are: cement-sand, bitumen and metal tiles.

Wooden roofs are rarely used due to the difficulty of construction. They come in shingle, pancake, shingle, ploughshare, and plank varieties.

Light-transmitting roofs are made of polymer materials and glass. These include cellular polycarbonate, corrugated polyvinyl chloride, triplex, polyester, etc.

Lathing

The roof deck or sheathing is the foundation for the roof. It is made from boards or bars. When installing a metal, wooden or tiled roof, the sheathing beam is taken with the following cross-section:

• 50x50 mm with a distance between rafters of 1.0-1.1 m;
• 50x60(h) mm with rafter pitch - 1.2-1.3 m;
• 60x60 mm with a step of 1.4-1.5 m.

For other types, you can use 2.5 cm thick boards. A double deck of boards is installed under the roll roofing. The working bottom layer is laid perpendicular to the direction of the rafters with gaps. The top one is laid at an angle of 45° to the underlying layer. The width of the boards for it is taken to be no more than 8 cm, and the thickness is 2 cm.

rafters

Wooden rafters are made of logs, sawn on one edge, from sawn timber (beams, boards laid on edge). For layered rafters, a round cross-section of logs is better suited. Their diameter is 12-20 cm. The advantages of using logs compared to boards or timber are as follows:

• saving wood (to withstand the same loads, a round section requires a smaller diameter of the source material);
• higher fire resistance limit;
• less consumption of metal fasteners;
• higher rigidity and durability.

Calculation of a layered rafter leg

A step of 1.0-1.5 m is allowed between the rafter legs. Their cross-section is determined by calculation, based on the strength and rigidity of the structure. To do this, the calculated constant load on the rafters is determined, which includes the calculation of constant loads per linear meter of roofing and snow load.

Scheme of load distribution along the rafter leg: α - roof inclination angle, q - total constant loads, q

The initial data for calculation are:

• installation step of rafter legs;
• the angle of the roof;
• roof width and height.

The choice of parameters, as well as the selection of most coefficients, depends on the material of the roofing covering and the detailed composition of the roofing pie.

For sloping roofs, permanent loads are calculated using the formula:

The rafter leg is also calculated for rigidity (deflection). The standard load is used here:

• α is the angle of inclination of the roof;
• n, n c - reliability coefficients for snow loads - 1.4, roof loads - 1.1;
• g is the weight of 1 m2, which is absorbed by the rafter leg (roofing, sheathing, rafters);
• a - step of rafter legs (along the axis).

• S g is the weight of snow per 1 m2, which depends on the climatic region;
• c e - coefficient of snow drift due to the influence of wind and other atmospheric influences, depends on the mode of operation of the roof;
• c t is the thermal coefficient.

Coefficients c e and c t are adopted in accordance with the requirements of SP 20.13330.2011 section 10 “Snow loads” in accordance with 10.5 and 10.6. For a private house with a pitched roof with a roof slope of over 20°, the coefficients c e and c t are equal to one, therefore, the snow cover formula is:

µ is a coefficient that depends on the angle of inclination of the roof and is determined in accordance with Appendix D of SP 20.13330.2011:

• for roofs with a slope of less than 30° µ = 1;
• for roofs with an angle of inclination over 60° µ = 0;
• in other cases for an inclination angle of 30°<α<60° µ = 0,033 х (60°-α).

The weight of snow cover by region can be clarified in SP 20.13330.2011 “Loads and Impacts”, where the region number is also determined according to the map of Appendix Zh.

Snow cover weight S g

Since the rafter leg is subject to bending from the influence of loads on it, it is tested for strength as a bending element, according to the formula:

M< m и R и W нт

• M – design bending moment;
• R and is the calculated bending resistance of wood;
• m and is a coefficient reflecting working conditions;
• W nt is the moment of resistance of a given section;
• R and = 130 kg/cm 2 - for pine and spruce;
• m and is equal to 1.0 - for sections with a height of up to 15 cm and 1.15 - for sections with a height of more than 15 cm.

The moment of resistance and moment of inertia for the rafter material are calculated individually. Based on the data obtained, the required size of the structural elements of the rafters is selected.

The proposed calculation is approximate and requires additions in the form of the maximum permissible length of supporting elements, placement of spacer or support beams and racks.

Example #1

Let's consider a tiled ceramic roof on a gable roof in the Moscow region (III climatic region).

Tilt angle 27°; cos α = 0.89; rafter spacing along the axis - 1.3 m; The design span of the rafters is 4.4 m. The lathing is made from 50x60 mm timber.

Roof weight per 1 m2:

• roof weight - 45 kg;
• rafter leg weight - 10 kg.

Total: g n = 62 kg/m2

• q = (1.1 x 62 x 0.89 + 1.4 x 126 x 0.89 2) x 1.3 = 260 kg/m.

• q n = (62 x 0.89 + 126 x 0.89 2) x 1.3 = 201 kg/m
• M = 0.125 x q x l 2 = 0.125 x 2.60 x 440 2 = 62,920 kg∙cm

Moment of resistance:

Moment of inertia (I), which is necessary from the condition of possible deflection f = 1/150 l; E = 100,000 kg/cm2; qn = 201 kg.

Using specially developed tables, you can determine the diameter of the logs for the rafters.

Log diameter (cm) depending on W and J (for logs trimmed to one edge).

According to the table above, we determine the diameter of the log - 18 cm.

Example No. 2

Let's take all the data from the previous example, but for an ondulin roof. It is necessary to calculate the cross-section of the rafter leg made of timber.

Tilt angle 27°; cos α =0.89; rafter spacing along the axis - 1.3 m; The design span of the rafters is 4.4 m. The lathing is made from 50x60 mm timber.

Roof weight per 1 m2:

• ondulin roof weight - 3.4 kg;
• sheathing - 0.05 x 0.06 x 100 x 550/25 = 7 kg;
• rafter leg weight - 10 kg.

Total: gn = 20.4 kg/m2

• q = (1.1 x 20.4 x 0.89 + 1.4 x 126 x 0.89 2) x 1.3 = 207.6 kg/m.

• qn = (20.4 x 0.89 + 126 x 0.89 2) x 1.3 = 153.3 kg/m
• M = 0.125 x q x l 2 = 0.125 x 2.08 x 440 2 = 50,336 kg∙cm

Moment of resistance:

Moment of inertia (I), which is necessary from the condition of possible deflection f = 1/150 l; E = 100,000 kg/cm2; qn = 153.3 kg.

We accept timber with a height of 15cm. For timber with a height of more than 14 cm, Ri = 150 kg/cm 2. That's why:

Using the table, we determine the cross-sectional size of the timber for the rafters.

Width (b) and height (h) of the beam depending on W and J.

We accept a beam with a cross section of 10x15 cm for the rafter leg.

The above formulas can be used to calculate other roof coverings. In this case, the load on the rafter leg is calculated based on the selected option. Formulas may change:

• rafter length;
• rafter pitch;
• roof pitch angle;
• snow load, which is selected according to the construction region;
• sheathing weight.

The connection between the rafter legs and the purlin must be reliable. This ensures that there is no destructive thrust on the walls of the building. Wooden structures must be inspected from time to time, therefore, when constructing layered rafters, the distance from the mark of the top of the attic floor to the bottom mark of the mauerlat is taken to be at least 400 mm.

Gable roofs are still a tradition in private housing construction today. Proper roof construction is a strong, durable and beautiful home.

It is recommended to calculate rafters as accurately as possible, based on the characteristics of the construction site, the external load on the rafter system, the size and configuration of the structure, and the characteristics of the material for constructing the roof.

Types of loads on rafters

The construction of a pitched roof requires the creation of a strong frame - the supporting structure of the roof. At the design stage, it is necessary to calculate the rafters in order to determine the length and cross-section of the elements that take on the main loads (constant and variable).

Constant loads include the weight of the roofing pie itself, which consists of an external covering, sheathing, waterproofing layer, heat insulator, vapor barrier and internal lining of the attic or attic space. This type of load also includes the weight of equipment or other objects that are planned to be placed on the roof or fixed to the rafters from the inside.

Variable loads mean the impact of wind and precipitation, as well as the weight of the person repairing or cleaning the roof. This category also includes special loads, including seismic ones - their presence places increased demands on the reliability of the roof.

Calculation of the weight of the roofing pie

Before calculating the cross-section of the rafter leg of a single-pitch, gable or hip roof, it is important to determine the weight of the roofing pie. This requires a calculation, the formula of which is extremely simple: the weight of one square meter of each layer of the roofing system is summed up, and the resulting result is multiplied by 1.1 - a correction factor that allows you to increase the reliability of the structure by 10%.

Thus, the standard roof weight calculation looks like this:: (weight of 1 m2 of sheathing + weight of 1 m2 of roofing + weight of 1 m2 of waterproofing + weight of 1 m2 of insulation) × 1.1 = weight of the roofing cake, taking into account the correction factor. When using most popular roofing materials (except for the heaviest), this load on the rafters does not exceed 50 kg/m2.

When developing a project for a pitched or gable roof, it is enough to focus on the weight of the roofing pie equal to 55 kg/m2. This approach will allow you to build a roof frame with a margin of strength and subsequently change the type of roofing without recalculating the rafter system.

Snow and wind loads

For many regions of Russia, the issue of snow loads on rafters is relevant - the rafter leg is required to withstand the weight of accumulated snow without deforming. The smaller the angle of inclination of the roof (usually this applies to a lean-to structure), the higher the snow load. The construction of an almost flat pitched roof requires the use of large cross-section rafters and a minimum installation step. In this case, you should regularly clean the pitched roof, the angle of inclination of which does not exceed 25°.

The formula S = Sg × µ allows you to calculate the snow load (S). Wherein:

• Sg – reference value of the weight of snow cover on 1 square meter of horizontal surface (selected from the table in SNiP “Rafter systems” depending on the region of construction);
• µ is a correction factor, the value of which is determined by the angle of inclination of the roof.

The coefficient µ is equal to:

• 1.0 – slope angle up to 25°;
• 0.7 – slope angle from 25 to 60°.

For roofs with slopes whose slope angle exceeds 60°, snow loads are not taken into account in the calculations.

To calculate the wind load (W), the formula W = Wo × k is used, where:

• Wo – reference value of wind load characteristic of a particular region (selected from the table);
• k is a correction factor, the value of which depends on the height of the structure and the type of terrain.

A – open area (field, steppe, coast);

B – urban development, forest.

The relationship between the cross section and the length of the rafters

Calculating the length of the rafters is quite simple, if you take into account that almost the entire roof is a system of triangles (it doesn’t matter whether we are talking about a single-pitched, gable or complex roof). Knowing the length of the walls of the building, the angle of inclination of the slope or the height of the ridge, using the Pythagorean theorem the length of the rafter leg from the edge of the wall to the ridge is calculated. To the obtained value you need to add the amount of the eaves overhang (if the rafters protrude beyond the edge of the wall). In some cases, the eaves overhang is formed by installing fillies - boards for extending the rafter leg. The length of the fillies is added to the length of the rafters when calculating the roof area - this will allow you to determine the exact amount of materials for installing the roofing pie.

To determine which board or timber section is suitable for the construction of a particular pitched, gable or hip roof, you can use the table of standards, which shows the correspondence between such parameters as the thickness of the lumber, the length of the rafter leg and the installation pitch of the rafters.

The cross-sectional parameters of the rafters vary from 40×150 mm to 100×250 mm. The length of the rafter leg depends on the angle of inclination of the slope and the length of the span between opposite walls. As the angle of inclination of the slope increases, the length of the rafter increases, which requires the use of lumber of a larger cross-section to ensure the necessary structural strength. At the same time, the snow load on the roof is reduced, and the rafter installation step can be made less frequent. At the same time, a decrease in the pitch of the rafters leads to an increase in the total load on the rafter leg.

When performing calculations, it is necessary to take into account all factors in order to achieve the required strength of the roof frame, including taking into account the characteristics of wood (density, degree of humidity, quality) when constructing wooden structures, and the thickness of metal elements when constructing metal roof frames.

The supporting structure of the roof must have a high degree of rigidity - it is necessary to prevent deflection of the rafters under loads. Deflection occurs if errors were made when calculating the cross-section of the roof elements and the installation pitch of the rafters. If the deflection of the rafters was detected after the roof was installed, additional elements (struts) can be used to stiffen the structure. If the length of the rafter leg of a lean-to, gable or hip roof exceeds 4.5 meters, without installing struts, a deflection can form regardless of the cross-section of the wooden rafter legs. This should be taken into account when calculating the length of the rafters.

The basic principles of calculation are based on the fact that the choice of beam thickness depends on the total load on the roof. An increase in the thickness of the rafters leads to an increase in the strength of the roof and eliminates deflection, but at the same time the total weight of the rafter system increases significantly, that is, the loads on the building structures and foundation increase. Rafters on residential buildings are installed in increments of 60 - 100 cm, the specific value depends on:

• design load;
• rafter sections;
• characteristics of roofing material;
• slope angle;
• width of the insulating material.

The calculation of the number of rafter legs is directly related to the step of their installation. Initially, a suitable installation step is selected, then the length of the wall should be divided by this value, add one to the result and round the number. By dividing the length of the wall by the result obtained, you can get the required gap between the rafters.

When determining the number of rafters on one slope, it is important to remember that the distance between the axes of the rafter legs is taken into account.

Metal rafter structures

In private housing construction, the use of metal rafter systems is less common, since a metal frame needs to be installed by welding - this leads to an increase in the complexity and volume of work. You can order the construction to be manufactured in production, but its installation will require the use of special equipment. Designing a metal roof frame requires accurate calculations and compliance with the dimensions of all elements, since it is not possible to adjust the part directly during installation.

There are no complaints about the strength of metal rafter systems: the use of metal profiles makes it possible to eliminate rafter deflection even when covering large spans without installing additional elements for strength and rigidity. Metal rafters can span spans of more than 10 meters without forming a deflection under design loads.

When calculating a metal rafter system, you should take into account the weight of the material, the loads on building structures and the foundation. The strength parameters of metal rafters and their high resistance to deflection loads make it possible to significantly reduce the number of these elements compared to a wooden structure.

The calculation of the metal roof frame should be based on reference values ​​for the strength of elements (channels, angles, beams, etc.) depending on their shape and thickness. The size of the spans and the angle of inclination of the slopes should be taken into account.

The supporting structure for a metal rafter system (mauerlat) should be a metal beam securely fastened to the upper edge of the wall.

Calculation of rafters: length, load, cross-section and number of rafters per roof

How to calculate the loads on a truss structure

City dwellers often want to live in their own home. If you decide to build this house, when preparing its technical design, do not forget to first calculate the rafters, which determines the parameters of all load-bearing structures. Thanks to preliminary calculations, you will avoid errors in the design and after construction you will be able to live peacefully in your home without worrying about its integrity.

The roof truss system is the most important and most important element of the roof structure, which ensures its stability and strength.

Based on what factors should the calculation be made?

In order for the calculation of the rafter system to be carried out correctly, it is necessary to determine the intensity of the loads on the roof. Such loads are divided into several types:

Construction of the rafter system. In order for the frame to be strong, the wooden rafter legs rest firmly on the outer walls through the mauerlat (longitudinal beam).

1. Permanent in nature. This is a load that will constantly affect the rafter system; it includes the own weight of the roof, sheathing, waterproofing and vapor barrier, insulation and other elements that form a constant value with a stable fixed weight.
2. Variables. These are loads determined by climatic factors: wind and its intensity, the amount of snow and other precipitation. They only affect the rafters occasionally.
3. Special. This type of load takes into account extreme manifestations of climatic factors or their increased intensity. This type of load must be taken into account in areas where seismic activity, hurricanes or storm winds are likely.

Taking all these factors into account at the same time, especially if you are doing it for the first time, is quite difficult. After all, it is necessary not only to take into account the loads, but also the weight and strength that the rafter beam has, the method of fastening the boards to each other, and other quantities. Many people think that this work can be made easier by a rafter calculation program, but this is not entirely true. Such programs operate with already calculated data on the loads that the rafter system will have to withstand. Therefore, after carrying out your own calculations, you will get a feel for all the design features of the roof that you will build.

Calculation of permanent loads

Schemes of standard snow loads. If the roof slope is more than 60 degrees, the snow load is not taken into account in the calculation of the rafter system.

Before determining what the length of the rafters will be, you need to understand what to focus on. Therefore, it is right to start with something simple, that is, by determining the weight of the roof structure itself. To do this, you must calculate what the weight of one square meter will be. m of each layer. First you need to study the technical characteristics of the material that should be there; usually the required value is indicated there. After all the data has been received, add all the values ​​together and increase the result by 10%, thereby setting the safety margin of the rafter system. It is better to select materials so that per square meter. m of roof area did not account for more than 50 kg of weight.

Snow load calculation

To undertake further calculations of rafters, you should move on to calculating variable loads, and specifically snow loads, since many areas experience the long-term influence of snowy winters. And the weight of the snow acting on the roof should not break the beam used as a rafter leg.

This type of load is calculated using the formula: snow weight per 1 sq.m × correction factor = total snow load. The first value is an average value and varies depending on the regional location of the house. The correction factor must be taken from SNiP 2.01.07-85. This result should also be increased by 10%, thereby creating a safety margin.

Wind load calculation

Wind load diagram. They depend on the area where the house is located.

This indicator is very important for inclined structures, which are roof slopes. At small angles of inclination there is a danger of roof destruction, and at large angles the wind pressure is very high over the entire surface of the slope, so the height of the roof must be thought out as carefully as possible. The calculation formula looks like this: region indicator × coefficient = wind load. To determine the region's indicator, there is a table of values; the coefficient varies depending on the height of the house and the area around it (forest, steppe, high-rise buildings). You can find out the exact values ​​of these two quantities in the same SNiP, as they should be suitable for your project.

Calculation principle

Calculation of loads on rafter systems. Calculation of the truss structure and arrangement of elements is carried out by developing plans and roofing diagrams.

Having set out to correctly calculate the length of the rafter leg, realize that almost the entire roof is a system of triangles, regardless of the configuration of the trusses. Therefore, determining the length of the boards required for the structure will not be difficult. What section of beam to choose or the number of legs is another matter. A guide to the correctness of these calculations can be a table of standards, where you can see the correspondence between the length, cross-section and pitch of the legs.

For example, the cross-section of rafters for a pitched roof can vary from 40*150 mm to 100*250 mm. The shorter the installation step, the greater the length of the rafter leg, which means that the total load on it increases, and as a result, the cross-section of the rafters should be larger. Everything matters in these calculations: what kind of wood you use for the timber, how the wood was dried, where the structure is located, what loads it will be subjected to. Don't neglect any factors. A detailed example of rafter calculations can be found in SNiPs for building design.

What algorithm of actions to follow

Table of weights of roofing materials. The value of loads on rafter systems can vary significantly depending on the selected roofing covering.

Calculation of rafters: loads that need to be taken into account

Calculator for calculating the load on rafters to determine the optimal cross-section

For the manufacture of rafter legs, high-quality lumber of a certain cross-section is used. Its strength characteristics must be guaranteed to be sufficient so that the roof structure can withstand all the loads placed on it.

Calculator for calculating the load on rafters to determine their optimal cross-section

To determine this parameter, you will have to carry out some calculations. A calculator for calculating the load on rafters to determine the optimal cross-section of lumber for their manufacture can provide all possible assistance.

The necessary explanations for the calculations will be given below.

Algorithm for calculating the cross-section of rafter legs

The work will be built in two stages. First, using a calculator, the distributed load per 1 linear meter of rafter leg will be determined. Then, according to the attached table, it will be possible to select the optimal size of timber for the manufacture of rafters.

Step one - calculating the distributed load on the rafter legs

The calculation calculator will ask for the following values:

• Slope angle. This value is directly related to the levels of external loads on the roof - snow and wind.

With the steepness of the slope and, accordingly, with ridge height(ridge unit) the special calculator to which the link leads will help you figure it out.

• Type of planned roofing. Naturally, various coatings have their own mass, which determines the static load on the rafter system. The calculator already takes into account not only the weight characteristics of various coatings, but also the materials of sheathing and roof insulation.
• It is necessary to indicate the zone of your region according to the level of possible snow load. It is easy to determine from the diagram map below:

Schematic map for determining your zone by snow load level

• The zone is determined in a similar way by the level of wind pressure - for this there is its own schematic map.

Schematic map for determining the zone according to the degree of wind impact on the roof

• It is necessary to take into account the specific location of the building on the ground. To do this, you need to evaluate its “surroundings” and choose one of the three proposed zones, “A”, “B” or “C”.

There is a nuance to this. All natural or artificial wind barriers can be taken into account only if they are located at a distance from the house not exceeding 30×N, Where N– this is the height of the building along the ridge. For example, for a building 7 meters high, a circle with a radius of 210 meters is obtained. If the obstacles are further away, it will be considered open terrain.

• Finally, you will need to enter the height of the house in meters (at the ridge).
• The last window of the calculator is the step of installing rafter legs. The more often they are installed, the less will be the distributed load falling on each of them, but at the same time, their number naturally increases. You can “play” with the step value to track the dynamics of changes in the distributed load - this will make it possible to select the optimal value for further determining the cross-section of the rafters.

Step two - determining the cross-section of the rafter leg

So, there is a value of the distributed load per linear meter of the rafter leg. Surely it was calculated in advance and rafter length(if not, it is recommended to go to the appropriate calculator). With this data you can already enter the table to determine the cross-section of the beam.

There is one more nuance. If the rafters are too long, then to increase their rigidity, additional reinforcing elements of the system are often provided - racks (headstocks) or struts. They allow you to reduce the “free span” distance, that is, between adjacent support points. It is this value that will be needed to enter the table.

The illustration with arrows shows an example of determining the cross-section of a rafter for a distributed load of 75 kg/linear meter and with a distance between support points of 5 meters. On the left side of the table, you can take any of the proposed values, which seems more convenient: boards or beams with minimum sections: 40 × 200; 50×190; 60×180; 70×170; 80×160; 90×150; 100x140. In addition, you can use a log with a diameter of 140 mm.

Rafters are the main load-bearing elements of the roof structure

The durability and reliability of the entire roofing structure as a whole depend on their quality and correct calculation.

Calculator for calculating the load on rafters to determine the optimal cross-section - with the necessary explanations

Calculator for calculating the load on rafters to determine the optimal cross-section for design

When installing rafters, lumber of suitable sizes is used to withstand the applied roof loads. The cross-section of the elements must be determined taking into account all factors affecting the operational characteristics of the structure. By using our calculator you can make the calculation process much easier.

The thickness and width of the rafters corresponds to the expected load

Introduction to the calculation algorithm

All work can be divided into two main stages. In the first of them, using the presented program, the load per linear meter is calculated. Next, using a special table, the acceptable cross-section of the beam used as a rafter leg is determined.

Stage No. 1: obtaining the result in the form of a distributed load

The calculator fields require you to enter certain parameters.

Auxiliary map for determining the load created by snow cover

• The slope angle is indicated first of all in order to understand what kind of load will be exerted by external factors in the form of snow and wind. The optimal slope must be selected taking into account the roofing coating used and other characteristics.
• It is necessary to indicate the type of roofing material, because the weight of coatings can vary significantly. In this way, it is possible to find out the static load that will be exerted on the rafter legs. The presented program already contains weight indicators of various materials, and not only roofing ones.
• In a special field, you should also select a region zone corresponding to a certain snow load. To determine it, a special map is used.
• In the same way, the pressure exerted by the wind is recognized and entered. To do this, use the appropriate card.
• The location of the building must also be taken into account. You are asked to evaluate and mark one of the options. The building may be located in an open area, in wooded areas or in a dense urban area. When choosing an item, you need to take into account the most acceptable option. All artificial and natural wind barriers must be considered if they are within a certain distance. To determine in which zone the building is located, you should multiply 30 meters by its height (from the ground to the ridge). The result obtained will be the radius for drawing the circle. If the main obstacles are outside the circle, then the building is in an open area.
• The height of the building in meters must be indicated in a special field of the initial data. It is necessary to reflect the distance to the highest point, which is usually the ridge.
• The final point is the step of installing the rafters. With frequent installations, the distributed load decreases. If necessary, you can change the distance between them to look at the value of the force transmitted to each linear meter of the element.

A special map is presented to determine the load created by wind

Stage No. 2: determining the cross-section of the beams used for the rafter system

When the distributed load acting on each meter of the beam has been obtained, you can use the table to find out the appropriate dimensions for each specific case. The length of the rafter leg must also be determined. Having such data, you can refer to the table to help you select the section.

One more point needs to be taken into account. If the beams are relatively long, then special elements such as racks or struts are used to improve the strength properties. They make it possible to reduce the flight distance directly between reference points.

It is proposed to use a table to determine the cross-section of the rafters

If the load distributed between the rafters is 75 kg per meter of length, and the step between the support points is 5 meters, then after studying the table you can understand that certain sections are suitable for the work.

A little about choosing lumber

If you plan to build a residential building, then pine wood can be used for rafters. For baths where hot air rises, you can purchase lumber from larch or other moisture-resistant species. There should not be any cracks or excessively large knots on the surface of the beams.

The moisture content of the lumber used should be within 18-22 percent, otherwise deformation changes in the system are possible, which will certainly affect the durability of the structure. In addition, poorly dried beams quickly rot. Raw elements create installation difficulties. It is much more difficult to lift them to a height than dry ones, since a significant proportion of the weight is water.

Calculator for calculating the load on rafters to determine the optimal cross-section with explanations