A simple online calculator for calculating a gable roof at home will calculate the rafters, calculation truss system gable roof, calculation of the roof area, calculation of the lathing, calculation roofing sheet, calculation of the material for the roof.
House with a gable roof. The device of the gable roof of the house
The most economical financially among pitched roofs a gable roof is considered, a builder who does not have special education and skills can make a gable roof with his own hands. The design of the roof of the house will depend on the purpose of using the second floor area as an attic or as a living space - an attic. Different purpose - a different section of the rafters, their processing and roof insulation.
gable mansard roof requires the presence of chimneys in the attic rooms, toilet bathrooms should be located above non-residential areas, excluding the kitchen area. Also needs to be handled wooden bars antiseptic and fire retardant solutions, protecting against mold and fire. Consequently, the cost of a gable roof increases if the second floor of the house is made residential.
Gable roof drawing
The program for calculating the roof will draw a drawing gable roof Houses
calculate the exact amount building materials for rafter system and roofing. Start calculating your roof now!
The online gable roof calculator will help you calculate the angles of the rafters, required amount lathing, the maximum load on the roof, as well as the materials necessary for the construction of a roof of this type with a given size. You can calculate the roof from such popular roofing materials, like slate, ondulin, ceramic, cement-sand and shingles, metal tile and other materials.
The calculations take into account the parameters given in the TCP 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 sloping 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 premises and aesthetic appearance.
The rafters in the construction of a gable roof 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 arranged under such a roof, which is illuminated with the help of 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
At this angle, the rafters and roof slope are inclined. It is understood that the construction of a symmetrical gable roof is planned. In addition to calculating the angle, the calculator will inform you how the angle corresponds to 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
The total area of the roof (including overhangs of a given length). Determines the amount of roofing and insulation materials that will be needed for the work.
Approximate weight of roofing material
The total weight of the roofing material required to completely cover the roof area.
Number of rolls of overlapping insulation material
Total insulating material in rolls, which is required for roof insulation. The calculations are based on rolls 15 meters long and 1 meter wide.
The maximum load on the truss 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 total 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 build a roof with a given pitch.
Minimum section of rafters, Weight and Volume of timber for rafters
The table shows the recommended cross-sectional dimensions of the rafters (according to GOST 24454-80 Lumber conifers). To determine compliance, the type of roofing material, the area and shape of the roof structure, as well as the loads exerted on the roof are taken into account. The adjacent columns show the total weight and volume of these rafters for the entire roof.
Number of rows of battens
The total number of rows of sheathing for the entire roof. To determine the number of rows of crates for one slope, it is enough to divide the resulting value by two.
Uniform spacing between boards
To install the battens evenly without overspending, use the value shown here.
Number of batten boards in standard length
To batten the entire roof, you will need the number of boards indicated here. When calculating, 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 the cost of the crate.
Approximate weight of batten boards
Estimated total weight of the boards of the crate. The calculations use average density and moisture values for softwood.
Take advantage online calculator gable roof to calculate the amount of lathing, the angle of inclination of the truss system, the load (wind and snow) on the roof. Our free calculator will help you calculate the required amount of material for a given type of roof.
Specify roofing material:
Select a material from the list -- Slate (wavy 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) Bituminous (soft, flexible) tiles (15 kg/m2) From galvanized sheet (6.5 kg/m2) Sheet steel (8 kg/m2) Ceramic tiles (50 kg/m2) Cement-sand tiles (70 kg/m2) Metal tiles, corrugated board (5 kg/m2) Keramoplast (5.5 kg/m2) Seam roof (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 weight 1 square meter coverage (? kg/m2)
kg/m2
Enter roof parameters:
Base Width A (cm)
Base length D (cm)
Lift height B (cm)
Length of side overhangs C (cm)
Front and rear overhang length E (cm)
Rafter:
Rafter pitch (cm)
Type of wood for rafters (cm)
Working section of the side rafter (optional) (cm)
Lathing calculation:
Purlin board width (cm)
Lathing board thickness (cm)
Distance between decking 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 building ridge
5 m from 5 m to 10 m from 10 m
Terrain type
Open area Closed area Urban areas
Calculation results
Roof pitch: 0 degrees.
Tilt angle suitable for this material.
The angle of inclination for this material is desirable to increase!
It is desirable to reduce the angle of inclination for this material!
Roof surface area: 0 m2.
Approximate weight of roofing material: 0 kg.
Number of rolls of insulation material with 10% overlap (1×15 m): 0 rolls.
Rafter:
Load on the truss system: 0 kg/m2.
Rafter Length: 0 cm
Number of rafters: 0 pcs
Lathing:
Number of rows of lathing (for the entire roof): 0 rows.
Uniform distance between the boards of the crate: 0 cm
The number of boards of the crate with a standard length of 6 meters: 0 pcs
Volume of boards of an obreshetka: 0 m 3 .
Approximate weight of the boards of the crate: 0 kg.
About the calculator
An online gable roof calculator, also called a gable roof, will help you calculate desired angle the slope of the slopes, determine the cross section and number of rafters, the volume of materials per crate, the consumption of insulating materials, and at the same time take into account existing norms for wind and snow loads. You do not have to perform unnecessary additional calculations, because this calculator contains most of the existing roofing materials. You can easily calculate the consumption and weight of such common materials as bituminous tiles, cement-sand and ceramic tiles, metal tiles, bituminous and asbestos-cement slate, ondulin and others. If you are using non-standard material, or want to get more accurate calculations, you can specify the mass of your own roofing material by selecting the appropriate item in the drop-down list of materials.
Note!
The calculator makes calculations in accordance with the current SNiP "Loads and Impacts" and TCP 45-5.05-146-2009.
Gable roof (there are variants of the name "gable roof" and " gable roof”) - the most common type of roof, in which there are two inclined slopes from the ridge to the outer walls of the structure. The popularity of this type of roofs is explained by their moderate cost, ease of construction, good performance and attractive appearance.
In this design, the rafters of different slopes lean against each other in pairs and are sheathed with sheathing boards. The end of a building with a gable roof has a triangular shape and is called a pediment (the name “tongue” is also found). Usually under the slopes of the roof there is an attic space, naturally lit by small window openings located at the top of the gables.
When filling in the fields of the calculator, you can find additional information located under the sign.
For any questions or ideas regarding this calculator, you can write to us using the form at the bottom of the page. We will be glad to hear your opinion.
Additional information about the calculation results
Roof slope
At this angle, the slope and rafters are inclined to the base of the roof. Roofing materials have individual limiting roof slope angles, so some materials may have an angle outside allowable norms. Whether your angle satisfies the selected material or not - you will find out in the calculation results. In any case, it is always possible to adjust the height of the roof (B) or the width of the base (A), or choose a different roofing material.
Roof surface area
The area of the entire roof surface, including overhangs. To determine the area of one slope, it is enough to divide the resulting value by two.
Approximate weight of roofing material
The weight of the selected roofing material based on the total roof area (including overhangs).
Number of rolls of insulating material
The amount of insulation material required to build a roof. The quantity indicated in rolls is required for the entire roof area. The standard roll is taken as a basis - 15 meters long, 1 meter wide. The calculation also takes into account an overlap of 10% at the joints.
Load on the truss system
The maximum weight per rafter system. Wind and snow loads, the angle of inclination of the roof, as well as the weight of the entire structure are taken into account.
Rafter length
The full length of the rafters from the roof ridge to the edge of the slope.
Number of rafters
The total number of rafters required for a rafter system at a given pitch.
Minimum cross-section of rafters / Weight of rafters / Timber volume
- The first column shows the sections of the rafters according to GOST 24454-80 Softwood lumber. Here are the sections that can be used in the construction of a given structure. The calculator proceeds from the total loads that can affect the structure of a given roof and selects the cross-sections that satisfy them.
- The second column shows the total weight of all rafters with the specified section, if they are used to build a given roof.
- The third column shows the total volume of this timber in cubic meters. This amount will be useful to you when calculating the cost.
Number of rows of battens
The number of rows of lathing that will be needed for the entire roof with the given parameters. To calculate the number of rows of crates of one slope, you need to divide the resulting value by two.
Uniform spacing between boards
The volume of boards of the crate
The total volume of the crate for a given roof. This value will help you calculate the cost of lumber.
The roof in the 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 mass of the structure itself, the weight of the snow cover, and so on.
The roof is located on the truss system. This is the name of the frame structure on which the roof is fixed. She accepts everything external loads, distributing them among the supporting structures.
The rafter system includes the following elements:
- Mauerlat;
- Struts and braces;
- Side and ridge runs;
- Rafter legs.
A truss truss is a construction that includes all of the listed elements with the exception of the Mauerlat.
Calculation of the loads of a gable roof
Permanent loads
The first type is called such loads that always act on the roof (in any season, time of day, and so on). These include the weight of the roofing cake 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. Professionals for this task use computer programs, as well as special calculators.
The calculation of a gable roof is based on the calculation of the loads on the rafter legs. First of all, you need to determine the weight of the roofing pie. 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 approximate, high precision is not required here. Typically, builders calculate the weight per square meter of the roof. And then this figure is multiplied by the total area of \u200b\u200bthe roof.
The roofing pie consists of ondulin, a waterproofing layer (in this case, polymer-bitumen-based insulation), a thermal insulation layer (the weight of basalt wool will be calculated) and a crate (the thickness of the boards is 25 mm). We calculate the weight of each element separately, and then add all the values.
Calculation of the roof of a gable roof:
- A square meter of roofing material weighs 3.5 kg.
- A square meter of waterproofing layer weighs 5 kg.
- A square meter of insulation weighs 10 kg.
- A square meter of the crate 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 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 area of \u200b\u200bthe roof, then it will be possible to calculate a gable roof.
Variable roof loads
Variables are called such loads that act on the roof not constantly, but seasonally. A prime example is the snow in winter time. Snow masses settle on the roof, creating an additional impact. But in the spring they melt, respectively, the pressure decreases.
Variable loads 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 gable roof rafters. When calculating, you need to take into account many different factors affecting the roof of the building.
Now let's take a closer look at snow loads. When calculating given parameter you need to use a special card. There is marked the mass of snow cover in different regions of the country.
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 slope of the roof: the stronger the slope, the lower the correction factor. And there is important nuance- for roofs with a slope of 60 o it is not taken into account at all. After all, 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 by the following formula:
Where x is the correction factor. It depends on the location of the building and its height. And W o - the parameter selected on the map.
Calculation of the dimensions of the truss system
When the calculation of all types of loads is over, you can proceed to the calculation of the dimensions of the truss system. The execution of the work will differ depending on which roof structure is planned.
In this case, a double slope 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 railing;
- Rafter length.
There is a special table of cross-sections of rafter legs, in which you can find out this indicator based on the criteria described above.
The length of the rafters in the gable roof
When calculating manually, basic knowledge of geometry will be required, 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.
Consider a specific example:
It is required to calculate the length of the gable roof rafters 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. Substitute all the data in the formula.
L = 6 / 2 / cos 45 ≈ 6 / 2 / 0.707 ≈ 4.24 meters.
To the value obtained, you need to add the length of the visor. 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 way task execution. There are special computer programs designed to automate this process. The easiest way is to use Arkon. It's completely free program, which even a person who is poorly versed in computers can easily understand.
It is enough just to specify the input parameters based on the size of the house. The program will independently perform calculations and show the required section, as well as the length of the gable roof rafters.
How to calculate the length of the gable roof rafters: 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, and the area of \u200b\u200bthe roof, and much more, including calculating the length of the rafters. And how to make the last calculations will be discussed in this article. 
What type of roof
How to calculate the length of the rafters? This question will interest everyone who builds a house on their own. But to answer it, you should first find out many other parameters. First of all, it is worth deciding on the type of roof, because the length of the slope and rafters will depend on this. The most common option is considered to be a two-slope design. But here there are several options, namely:
You can consider even more complex structures, for example, multi-level ones. Such roofs will look very attractive. But to make a calculation, and especially to build a truss system, in this case, without the help of professionals, it will be almost impossible. Therefore, in most cases, they are limited to the three options listed above for a gable roof.
System type
The calculation of the length of the gable roof rafters will also depend on the system used. Here, experts distinguish the following two main varieties:
- Hanging system. This is the easiest option. In this case, the rafter legs rest only on the Mauerlat. The upper part of them simply connects to each other. Such a system is used if the width of the house is small. In this case, the length of the rafters should not exceed six meters. The hanging option is undesirable to use with an asymmetric gable roof.
- The layered system is a more durable truss system. It is used in the event that an axial load-bearing wall passes in the middle of the house. In this case, supports and a ridge run are installed, on which the upper part of the rafter legs is attached.
You can also use a combined version. 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 parameters of the system. If you have this option, then it is better to entrust everything to calculate to a specialist. In this case it will less mistakes, which means that 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 gable roof rafters. Before you calculate everything, you need to know a lot more information, namely:
In addition, when calculating the length of the rafters, you should find out what overhangs should be. Not one 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 owners of the house to decide. According to existing building codes, this parameter should be in the range from 50 to 60 centimeters. You should not do less, otherwise the walls and foundation may suffer. Sometimes overhangs make more than one meter. In this case, a small canopy is obtained along the wall, which can be used for rest or storage. 
Making calculations
And how is the length of the rafters calculated? If the roof has a symmetrical shape, then it is not difficult to calculate this parameter. For this, the formula of the Pythagorean theorem is used, namely: C is equal to the square root of A squared plus B squared, where:
- C is the desired length of the rafter;
- A is the height at which the ridge is located (from the base of the roof);
- B is half the width of the house.
At the same time, using this formula, you can calculate the dyne 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.
And how to make a calculation if the roof is asymmetric? In this case, the slopes will be different. But here you can use the Pythagorean theorem. You can calculate the rafters on the roof using the same formula, only first find out the value of the parameter "B" (in the first case it is equal to half the width of the house). If the roof is asymmetric, 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 parameter "B". 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 the 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 make it correctly and accurately. The strength of the entire truss system will depend on this. If you cannot calculate the length of the rafters to an integer, 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, asymmetric, broken) and the type of truss system (hanging, layered). Basic nuances and calculations.
The roof is not only the protection of the house from the external environment, but also a certain decorative element that gives the building a finished look. That is why developers are building today the most unusual roofs with complex structures truss systems.
The rafter system is the most important element in the arrangement of any roof. It accounts for the weight of the coating and precipitation. Therefore, the correct implementation of such a system, taking into account all the rules of building 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 is the rafter system
Any construction of this kind is carried out in the form of interconnected elements that strictly correspond to the calculations made earlier. This system includes the following elements:
- sloping legs, which are also called rafters;
- stops, sprengels and other fasteners that give the structure the necessary rigidity;
- vertical type racks;
- conjurers.
Note! It is necessary to take special responsibility when calculating the length of the rafters - any, albeit insignificant, error can lead to deformation of the roof geometry and, accordingly, its collapse.
If you do not understand the features of the roof structure, then it is better to contact qualified specialists. For self-calculation, use special calculators and tables - this will help you avoid mistakes.
Varieties of the truss system
Varieties of the truss system
Rafter systems are divided into two groups depending on the material used:
- wooden structures;
- metal structures.
There are also reinforced concrete truss systems, but they are used mainly in industrial buildings. In any case, whether the rafters are metal, wooden or concrete, they must be firmly attached to the walls of the house.
Often for the construction of rafters in country houses use wood, mainly coniferous species. Compared to metal, wood is easier to handle and install. Moreover, even if an error occurs in the calculations, then wooden details easy to replace.
Before proceeding with the calculations, first measure the width of the house. The fact is that although small slanted legs do not need additional extension, 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, the rafters are divided into:
In construction country houses inclined rafters are more often used, but often builders combine both. As already mentioned, extensions may be required sloping legs. It depends on the roofing material used in the construction. So, slate or ceramic tiles, due to their large weight, can only be installed on a rafter system of increased strength.
Types of gable truss 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 pick up a beam with b O large section (there is another way to strengthen - by splicing the boards).
And now - directly to the calculations.
What to consider when calculating rafters
First, let's define the fundamentals.
- The type and shape of the roof directly affect functional features rafter system. The fact is that the 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 ones) need additional stabilization elements - crossbars, sleepers, struts, etc.
- Very important in the calculations and future loads on the structure, mainly snow and wind. For example, in the 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, then the wind load will increase. In a word, it is necessary to determine the very “golden mean”, but not to the detriment of attractiveness. Very often only true masters can solve such a problem.
- Another important point in the calculation is the coating material. Many of these materials need certain conditions. So, flexible tile placed exclusively on a solid surface (in last resort- frequent crate). Ceramic tiles need a reinforced frame.
- Size and area - these are the main indicators that affect 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.
Note! distance between bearing walls called a run. With an increase in the run, the number of changes in the design increases, in particular, the number of stabilizing and reinforcing elements.
How to calculate the rafters on the roof
Now, having familiarized yourself with the starting points, you can take paper, a ruler and a pencil and proceed with the calculations.
First stage. Roofing cake weight
First, determine how much the roof itself will weigh. This is very important, because the truss system must withstand this weight for a long time. It is very easy to calculate: find out the weight per square meter of each of the layers, summarize the data obtained and add a correction of 10%.
Here is an example of such calculations.
- A square meter of the crate weighs 15 kg.
- The roofing will be, say, ondulin with a weight of 3.5 kg.
- Square meter bituminous waterproofing weighs another 6 kg.
- Weight of 10 cm layer mineral wool is approximately 10 kg per square meter.
Let's see what happens.
We add correction 10%, it turns out 37.95 kg. It is this figure that is an indicator of the weight of the roofing pie.
Note! In most cases, this weight does not exceed 50 kg, but experienced specialists are sure that the calculations should be based on 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 in enough in large numbers. Use a special formula to determine this load:
S in this case, this is the load of snow that you need to calculate;
µ - correction depending on the slope slope;
At flat roof, the slope of which does not exceed 25°, the correction will be equal to one; if the slope of the ramp is greater than 25°, but does not exceed 60°, then the correction will be 0.7. If a very steep roof is being built, then snow loads for it can not be calculated at all.
Sᶢ is the weight per square meter of snow cover. This indicator depends on the climatic features of a particular region, you can find out about it in SNiP.
Let's say the slope of the roof will be 25 °, and the mass of snow will be 200 kgf / m².
Use the formula below to calculate the wind load on the rafters.
Wᵒ in this case, it is a standard indicator that you must determine from the table (it all depends on which 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 the pitch and length of the rafters
The choice of section and length of the rafter leg
To calculate the length of the rafters, you can remember the geometry at school, namely the famous Pythagorean theorem. After all, the truss structure is, in fact, a right triangle and it is very simple to measure its diagonal. But do not forget to take into account when calculating:
- the strength of the bars;
- the possibility of deformation - what load the system can withstand without breaking.
Note! According to GOST, rafters should not bend more than 1/250 of their length. For example, if the length of the rafters is 5m, then multiply this number by 0.004 - so you get ultimate 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 running meter timber;
- there may be non-through cracks, but their length should not exceed half of the total length;
- wood must be treated with an antiseptic, flame retardant and biological protection agent.
In addition, when buying bars, pay attention to:
- manufacturer;
- date of manufacture;
- product name, standard;
- quality of execution of individual parts;
- dimensions and humidity of products;
- tree species.
Special computer programs
Judging by everything that has been said above, for calculating rafters, you need to have not only a sufficient stock of knowledge, but also drawing and drawing skills. Of course, not all of us can boast of all this.
Fortunately, today there are many computer utilities designed to facilitate calculations. There are professional ones among them, such as, for example, AutoCAD, but you can find more simple options. So, in the Arkon program, you can easily create various projects, as well as visually see how the future roof will look like.
Note! In such utilities there is also a calculation calculator, which was mentioned earlier. With its help, you can calculate the length, pitch and cross section of the 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 will not replace a full-fledged drafting.
As a conclusion
One of milestones roof construction is the calculation of the truss 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 - Installation of rafters
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Learn how to calculate the rafters on the roof! What data is needed for calculations, step by step guide, tables, photo + video.
The gable roof is formed on the basis of a frame that combines the elementary nature of the device and unsurpassed reliability. But the backbone of the roof in two rectangular slopes can boast of these advantages only in the case of a careful selection of rafter legs.
Parameters of the gable roof truss system
Calculations should be started if you understand that the rafter system gable roof- This is a complex of triangles, the most rigid frame elements. They are assembled from boards, the size of which plays a special role.
Rafter length
The formula will help determine the length of durable boards for the truss systema²+b²=c², derived by Pythagoras.
The length of the rafter can be found by knowing the width of the house and the height of the roof
The parameter "a" denotes the height and is self-selected. It depends on whether the under-roof space will be residential, and also has certain recommendations if an attic is planned.
Behind the letter "b" is the width of the building, divided in two. And "c" represents the hypotenuse of the triangle, that is, the length of the rafter legs.
Let's say that the width of half of the house is three meters, and it was decided to make the roof two meters high. In this case, the length of the rafter legs will reach 3.6 m (c=√a²+b²=4+√9=√13≈3.6).
To the figure obtained from the Pythagorean formula, 60–70 cm should be added. Extra centimeters will be needed to take the rafter leg out of the wall and make the necessary cuts.
The six-meter rafter is the longest, therefore it is suitable as a rafter leg
The maximum length of a beam used as a rafter leg is 6 m. If required durable board of greater length, then they resort to the method of fusion - nailing a segment from another beam to the rafter leg.
Section of rafter legs
For various elements of the rafter system, there are standard sizes:
- 10x10 or 15x15 cm - for Mauerlat timber;
- 10x15 or 10x20 cm - for the rafter leg;
- 5x15 or 5x20 cm - for running and brace;
- 10x10 or 10x15 cm - for the rack;
- 5x10 or 5x15 cm - for lying down;
- 2x10, 2.5x15 cm - for purlins.
The thickness of each part of the supporting structure of the roof is determined by the load that it will experience.
A beam with a section of 10x20 cm is ideal for creating a rafter leg
The section of the rafter legs of a gable roof is affected by:
The pitch of the rafters affects the cross section of the rafter legs most significantly. Increasing the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafter legs.
Table: cross-section of rafters depending on length and pitch
Variable impact on the truss system
The pressure on the rafter legs is constant and variable.
From time to time and with varying intensity, wind, snow and precipitation affect the supporting structure of the roof. In general, the slope of the roof is comparable to a sail, which can break under the pressure of natural phenomena.
The wind tends to overturn or raise the roof, so it is important to make all the calculations correctly.
The variable wind load on the rafters is determined by the formula W \u003d Wo × k x c, where W is the wind load indicator, Wo is the value of the wind load characteristic of a certain section of Russia, k is a correction factor determined by the height of the structure and the nature of the terrain, and c is the aerodynamic coefficient.
The aerodynamic coefficient can range from -1.8 to +0.8. A minus value is typical for a rising roof, and a positive value is for a roof that is being pressed by the wind. At simplified calculation with a focus on improving strength, the aerodynamic coefficient is considered equal to 0.8.
Calculation of wind pressure on the roof is based on the location of the house
The standard value of wind pressure is recognized from map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient that takes into account the change in wind pressure with height is also standardized.
Table: standard value of wind pressure
Table: value of coefficient k
The wind load is not only affected by the terrain. The housing area is of great importance. Behind the wall of tall buildings, the house is almost in no danger, but in open space the wind can become a serious enemy for it.
The snow load on the rafter system is calculated by the formula S = Sg × µ, that is, the weight snow mass per 1 m² is multiplied by a correction factor, the value of which reflects the degree of slope of the roof.
The weight of the snow layer is indicated in the SNiP "Truss Systems" and is determined by the type of area where the building was built.
Snow load on the roof depends on where the house is located
The correction factor, if the roof slopes heel less than 25 °, is equal to one. And in the case of a roof slope of 25–60 °, this figure decreases to 0.7.
When the roof is tilted more than 60 degrees, the snow load is discounted. Still, snow rolls down from a steep roof quickly, not having time to negative impact on the rafters.
Permanent loads
Loads that act continuously are considered to be the weight of the roofing pie, including the lathing, insulation, films and finishing materials for arranging the attic.
Roofing cake creates constant pressure on the rafters
The weight of a roof is the sum of the weights of all the materials used in the construction of the roof. On average, it is 40–45 kg / sq.m. According to the rules, 1 m² of the truss system should not account for more than 50 kg of the weight of roofing materials.
So that there is no doubt about the strength of the rafter system, 10% should be added to the calculation of the load on the rafter legs.
Table: weight of roofing materials per 1 m²
| Type of roof finish | Weight in kg per 1 m² |
| Rolled bitumen-polymer sheet | 4–8 |
| Bitumen-polymer soft tile | 7–8 |
| Ondulin | 3–4 |
| metal tile | 4–6 |
| Decking, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tiles | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| slate roof | 40–50 |
| Copper | 8 |
| green roof | 80–150 |
| Draft flooring | 18–20 |
| crate | 8–10 |
| The truss system itself | 15–20 |
Number of bars
How many rafters will be needed to equip the frame of a gable roof is set by dividing the width of the roof by a step between the bars and adding one to the resulting value. It indicates an additional rafter that will need to be placed on the edge of the roof.
Suppose it is decided to leave 60 cm between the rafters, and the length of the roof is 6 m (600 cm). It turns out that 11 rafters are needed (taking into account the additional timber).
The gable roof truss system is a construction of a certain number of rafters
The step of the beams of the supporting structure of the roof
To determine the distance between the beams of the supporting structure of the roof, you should pay close attention to such points as:
- weight of roofing materials;
- the length and thickness of the beam - the future rafter leg;
- degree of slope of the roof;
- level of wind and snow loads.
After 90-100 cm, it is customary to place the rafters in the case of choosing a light roofing material
A step of 60–120 cm is considered normal for rafter legs. The choice in favor of 60 or 80 cm is made in the case of the construction of a roof inclined by 45˚. The same small step should be, if desired, to cover wooden frame roofs with heavy materials such as ceramic tiles, asbestos-cement slates and cement-sand tiles.
Table: rafter pitch depending on length and section
Formulas for calculating the truss system of a gable roof
The calculation of the truss system comes down to setting the pressure on each beam and determining the optimal section.
When calculating the truss system of a gable roof, they act as follows:
- According to the formula Qr \u003d AxQ, they find out what is the load per linear meter of each rafter leg. Qr is the distributed load per linear meter of the rafter leg, expressed in kg/m, A is the distance between the rafters in meters, and Q is the total load in kg/m².
- They proceed to the determination of the minimum cross-section of the beam-rafter. To do this, study the data of the table listed in GOST 24454–80 “Softwood lumber. Dimensions".
- Focusing on standard parameters, choose the width of the section. And the height of the section is calculated using the formula H ≥ 8.6 Lmax sqrt (Qr / (B Rbend)) if the roof slope α< 30°, или формулу H ≥ 9,5·Lmax·sqrt(Qr/(B·Rизг)), когда уклон крыши α >30°. H is the height of the section in cm, Lmax is the working section of the rafter leg of maximum length in meters, Qr is the distributed load per linear meter of the rafter leg in kg / m, B is the section width cm, Rbend is the resistance of wood to bending, kg / cm². If the material is made from pine or spruce, then Rizg can be equal to 140 kg / cm² (wood grade 1), 130 kg / cm² (grade 2) or 85 kg / cm² (grade 3). Sqrt is the square root.
- Check whether the deflection value complies with the standards. It should not be more than the figure that results from dividing L by 200. L is the length of the working area. The compliance of the deflection value with the L / 200 ratio is feasible only if the inequality 3.125 Qr (Lmax)³ / (B H³) ≤ 1 is true. Qr indicates the distributed load per linear meter of the rafter leg (kg / m), Lmax is the working section of the rafter leg maximum length (m), B is the width of the section (cm), and H is the height of the section (cm).
- When the above inequality is violated, the indicators B and H increase.
Table: nominal dimensions of thickness and width of lumber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
An example of the calculation of the supporting structure
Assume that α (roof pitch) = 36°, A (rafter spacing) = 0.8 m, and Lmax (maximum rafter length) = 2.8 m. , which means that Rizg \u003d 140 kg / cm².
Cement-sand tiles were chosen for the roof covering, and therefore the weight of the roof is 50 kg/m². The total load (Q) experienced by each square meter is 303 kg/m². And for the construction of the truss system, bars 5 cm thick are used.
From this follow the following computational steps:
- Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
- H ≥ 9.5 Lmax sqrt(Qr/B Rbend).
- H ≥ 9.5 2.8 sqrt(242/5 140).
- 3.125 Qr (Lmax)³/B H³ ≤ 1.
- 3.125 242 (2.8)³ / 5 (17.5)³ = 0.61.
- H ≥ (approximate height of the rafter section).
Table standard sizes you need to find the height of the rafter section, close to 15.6 cm. A suitable parameter is 17.5 cm (with a section width of 5 cm).
This value is quite consistent with the deflection index in the regulatory documents, and this is proved by the inequality 3.125 Qr (Lmax)³ / B H³ ≤ 1. Substituting into it the values (3.125 242 (2.8)³ / 5 (17, 5)³), it turns out that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the truss system
The calculation of the gable roof truss system is a whole complex of calculations. In order for the bars to cope with the task assigned to them, the builder needs to accurately determine the length, quantity and cross section of the material, find out the load on it and find out what the step between the rafters should be.
