If you adhere to proper nutrition, then avocados are probably in ...
Before proceeding with the construction of the roof, of course it is desirable that it be designed for strength. Immediately after the publication of the last article ““, questions began to come to my mail regarding the choice of the section of rafters and floor beams.
Yes, understanding this issue in the vastness of our beloved Internet is really quite difficult. There is a lot of information on this subject, but as always, it is so scattered and sometimes even contradictory that it is easy for an inexperienced person who in his life may not even have come across such a subject as "Sopromat" (lucky for someone), it is easy to get confused in these wilds.
I, in turn, will now try to compile a step-by-step algorithm that will help you independently calculate the truss system of your future roof and finally get rid of constant doubts - what if it doesn’t stand up, but suddenly it falls apart. I must say right away that I will not delve into the terms and various formulas. Well, why? There are so many useful and interesting things in the world that you can fill your head with. We just need to build a roof and forget about it.
The whole calculation will be described using the example of a gable roof, which I wrote about in
So Step #1:
We determine the snow load on the roof. To do this, we need a map of the snow loads of the Russian Federation. To enlarge the picture, click on it with the mouse. Below I will give a link where you can download it to your computer.
Using this map, we determine the number of the snow region in which we are building a house and from the following table we select the snow load corresponding to this region (S, kg / m²):
If your city is located on the border of regions, choose a higher load value. It is not necessary to correct the resulting figure depending on the angle of inclination of the slopes of our roof. The program that we will use will do it itself.
Let's say in our example we are building a house in the suburbs. Moscow is in the 3rd snow region. The load for it is 180 kg / m².
Step #2:
Determine the wind load on the roof. To do this, we need a map of the wind loads of the Russian Federation. It can also be downloaded from the link below.
Using this map, we also select the corresponding region number and determine the value of the wind load for it (the values are shown in the lower left corner):
Here column A - open coasts of the seas, lakes and reservoirs, deserts, steppes, forest-steppes and tundras; column B - urban areas, forests and other areas evenly covered with obstacles. It should be taken into account that in some cases the type of terrain may differ in different directions (for example, a house stands on the outskirts of a settlement). Then select the values from column "A".
Let's go back to our example. Moscow is located in the 1st wind region. The height of our house is 6.5 meters. Suppose that it is being built in a settlement. Thus, we accept the value of the correction factor k=0.65. Those. the wind load in this case will be equal to: 32x0.65 \u003d 21 kg / m².
Step #3:
You need to download to your computer a calculation program made in the form of an Excel table. We will continue to work on it. Here is the download link: ". Also here are maps of snow and wind loads of the Russian Federation.
So, download and unpack the archive. We open the file "Calculation of the truss system", while we get to the first window - "Loads":
Here we need to change some values in the cells filled with blue. All calculations are done automatically. Let's continue with our example:
In the plate "Initial data" we change the angle of inclination to 36 ° (what angle you will have, write this, well, I think everyone understands this);
We change the pitch of the rafters to the one that we have chosen. In our case, this is 0.6 meters;
load roofs (load from the own weight of the roofing material) - we select this value from the table:
For our example, we choose a metal tile with a weight of 5 kg / m².
Snow. district - here we enter the sum of the values of snow and wind loads that we received earlier, i.e. 180+21=201 kg/m²;
Insulation (mans.) - we leave this value unchanged if we lay the insulation between the rafters. If we make a cold attic without insulation, we change the value to 0;
In the plate "Crate" enter the required dimensions of the crate. In our case, for a metal tile, we will change the crate step by 0.35 m and the width by 10 cm. We leave the height unchanged.
All other loads (from the own weight of the rafters and lathing) are automatically taken into account by the program. Now let's see what we got:
We see the inscription "The load-bearing capacity of the crate is ensured!" We don’t touch anything else in this window, we don’t even need to understand what the numbers are in other cells. If, for example, we choose a different rafter pitch (larger), it may turn out that the load-bearing capacity of the crate will not be ensured. Then it will be necessary to select other sizes of the crate, for example, increase its width, etc. In general, I think you will understand.
Step #4:
Sling.1» and go to the window for calculating rafters with two support points. Here, all the incoming data entered by us earlier is already substituted by the program automatically (this will be the case in all other windows).
In our example from the article “Do-it-yourself gable roof of a house”, the rafters have three points of support. But let's imagine that there are no intermediate racks and make a calculation:
We change the length of its horizontal projection on the rafter diagram (the cell is filled with blue). In our example, it is equal to 4.4 meters.
In the plate "Calculation of rafters" we change the value of the thickness of the rafter B (specified) to what we have chosen. We put 5 cm. This value must be greater than that indicated in the cell Tue (stable);
Now in the line Accept H"We need to enter the selected rafter width in centimeters. It must be greater than the values specified in the lines " Ntr., (dur.)" and " Ntr., (deflection)". If this condition is met, all the inscriptions at the bottom under the rafter scheme will look like “Condition met”. In the line " H, (by grade)” indicates the value that the program itself offers us to choose. We can take this figure, or we can take another. Usually we choose the sections available in the store.
So what we got is shown in the figure:
In our example, in order to comply with all strength conditions, it is necessary to choose rafters with a section of 5x20 cm. But the roof scheme shown by me in the last article has rafters with three support points. Therefore, to calculate it, we proceed to the next step.
Step #5:
Click on the tab at the bottom of the work screen Sling.2" or " Sling. 3″. This opens a window for calculating rafters with 3 support points. The choice of the tab we need is made depending on the location of the middle support (rack). If it is located to the right of the middle of the rafter, i.e. L/L1<2, то пользуемся вкладкой "Sling.2". If the rack is located to the left of the middle of the rafter, i.e. L/L1>2, then we use the tab "Sling.3". If the rack is exactly in the middle, you can use any tab, the results will be the same.
On the rafter diagram, we transfer the dimensions in the cells filled with blue (except for Ru);
According to the same principle as described above, we select the dimensions of the section of the rafters. For our example, I took the dimensions of 5x15 cm. Although it was possible and 5x10 cm. I just got used to working with such boards, and there will be more safety margin.
Now it is important: from the figure obtained during the calculation, we will need to write out the value of the vertical load acting on the rack (in our example (see figure above) it is equal to 343.40 kg) and the bending moment acting on the rack (Mop. = 78.57 hmmm). We will need these figures later when calculating racks and floor beams.
Next, if you go to the tab " Arch“, a window for calculating the rafter system, which is a ridge arch (two rafters and a puff), will open. I will not consider it, it will not work for our roof. We have too large a span between the supports and a small angle of inclination of the slopes. There you will get rafters with a cross section of the order of 10x25 cm, which is of course unacceptable for us. For smaller spans, this scheme can be used. I am sure whoever understood what I wrote above will deal with this calculation himself. If you still have questions, write in the comments. And we move on to the next step.
Step #6:
Go to the "Rack" tab. Well, everything is simple here.
The previously determined values of the vertical load on the rack and the bending moment are entered in the figure, respectively, in the cells “N=” and “M=”. They were recorded in kilograms, we enter them in tons, while the values \u200b\u200bare automatically rounded;
Also in the figure we change the height of the rack (in our example it is 167 cm) and set the dimensions of the section we have chosen. I chose a board 5x15 cm. At the bottom in the center we see the inscriptions “Central provided!” and "Off-center. secured." So everything is in order. The safety factors "Kz" are very large, so you can safely reduce the section of the racks. But we will leave it as is. The result of the calculation in the figure:
Step #7:
Go to tab "Beam". Floor beams are affected simultaneously by a distributed load and a concentrated load. We need to consider both. In our example, beams of the same section cover spans of different widths. Of course, we make a calculation for a wider span:
- in the plate "Distributed load" we indicate the step and span of the beams (we take 0.6 m and 4 m from the example, respectively);
— accept the values Load (normal)=350 kg/m² and Load (calc.)=450 kg/m². The values of these loads in accordance with SNiP are averaged and taken with a good margin of safety. They include the load from the own weight of the floors and the operational load (furniture, people, etc.);
- in the line " B, given» enter the width of the section of the beams that we have chosen (in our example it is 10 cm);
In the lines " H, strength" and " H, deflection» the minimum possible heights of the beam section will be indicated at which it will not break and its deflection will be acceptable. We are interested in the largest of these numbers. We take the height of the beam section based on it. In our example, a beam with a section of 10x20 cm is suitable:
So, if we did not have racks resting on floor beams, the calculation would be completed on this. But there are racks in our example. They then create a concentrated load, so we continue to fill in the plates "" and " Distribution + concentrator«:
In both plates we enter the dimensions of our spans (here I think everything is clear);
In the plate "" we change the values of Load. (norm.) and Load. (calc.) by the figure that we received above when calculating the rafters with three points of support - this is the vertical load on the rack (in our example 343.40 kg);
In both plates we enter the accepted width of the beam section (10 cm);
The height of the beam section is determined by the plate " Distribution + concentrator." . Again, we focus on a larger value. For our roof, we take 20 cm (see the figure above).
This completes the calculation of the truss system.
I almost forgot to say: the calculation program we use is applicable for truss systems made of pine (except Weymouth), spruce, European and Japanese larch. All wood used is 2nd grade. When using other wood, some changes will need to be made to the program. Since other types of wood are rarely used in our country, I will not describe now what needs to be changed.
The strength of the roof of the building depends on how correctly the calculation of the rafters is performed. In this design, all parameters are important: length, angle of inclination of the roof, section of the beams.
Factors to consider when calculating
The calculation of the section of the rafters and their length is carried out in several stages. At the first stage, the snow and wind load is calculated for the selected roof configuration, taking into account correction factors for the building height and slope angle.
Then the load from the weight of the roofing material, insulation and battens is added. 10% is added to the resulting total load for a margin of safety. The final value is used to calculate the rafters.
It is quite difficult to perform a competent calculation if you do not take into account the strength and frequency of the loads exerted on them.
Factors affecting the roof are divided into three groups:
- permanent loads;
- variable loads;
- special loads.
Constant loads act on structural elements without ceasing, regardless of the seasons.
These include the mass of the roof, waterproofing, lathing, vapor barrier, thermal insulation and all individual parts of the roof that have a constant weight and exert pressure on the truss system.
The mass of a shed or gable roof increases when massive devices and devices are installed on it - antennas, ventilation, snow retainers, etc.
A strong influence on the strength of the rafters of a single-pitched and gable roof is exerted by the weight of the snow layer, the wind blowing and the workers climbing the roof.
Such loads are called variable, since it has a periodic nature - strong pressure is replaced by its absence.
A special type includes loads that occur in regions where hurricanes or earthquakes often occur.
With this type of load, an additional margin of safety is taken into account during the design and construction of buildings.
Calculation for roof rafters is a rather difficult task, a non-specialist may not be able to cope with it.
Calculation of the load on the rafters
The wind load is calculated in a simplified way as follows: we multiply the regional indicator of the wind load by the correction factor. The regional indicator is taken from the SNiP according to the wind load map.
Correction factor for buildings in height:
- below five meters is taken in the range of 0.5 - 0.75;
- from five to ten meters - 0.65 - 1.0;
- from ten to twenty meters - 0.85 - 1.25.
A smaller value of the coefficient is used for built-up or wooded areas, where the force of the wind is pacified by obstacles, a larger value is taken for open areas.
In the event that the building is located on an open territory on at least one side, a larger range value is also applied.
The snow load is calculated in a similar way - the snow load index is multiplied by the correction factor.
The coefficient depends on the angle of the roof:
- a gentle slope with a slope of up to 25 degrees has a coefficient of 1.0;
- for a slope with an angle of inclination from 25 to 60 degrees, the coefficient is 0.7;
- if the slope angle exceeds 60 degrees, then the snow load is not taken into account.
The snow load indicator is indicated on the corresponding SNiP map similarly to the wind load map.
If the building is located close to the border of two regions, then the value for the region with the highest score is used.
The obtained values of wind and snow load are summed up. The final value obtained at this stage of the calculations is called the indicator of variable loads.
The calculation of permanent loads acting on the truss system depends on the type of roof chosen.
Permanent loads are calculated for the roofing "pie" by adding the weight of its components - lathing, insulation, waterproofing, roofing material.
Weight of the most common roofing materials:
- cement-sand tiles: 20 - 30 kg per square meter;
- slate: 10 - 14 kg per square meter;
- shingles: 6 - 8 kg per square meter;
- metal tile: 3.5 - 4.5 kg per square meter;
- ondulin: 3 kg per square meter.
From the above data it follows that the static load may vary depending on the selected type of roofing material used.
Adding the values of static and variable loads and adding 10% for a margin of safety, we get the final value, which will be used for further calculation of the rafters.
Calculation of the size and pitch of rafters for single-pitched and double-pitched roofs
For accurate calculation of the truss system, there are specialized programs and online calculators.
However, for a simple shed and gable roof, the necessary parameters can be calculated independently without their help.
It should be noted that the rafter should protrude outward beyond the edge of the wall by at least 60 cm. The standard length of the rafter is 6 m. If necessary, by calculating the length, it can be increased.
The calculation of the step for the rafters should take into account the distance between them in the range of 60 - 100 cm. The greater the load, the more often it is necessary to install the rafters.
The total number of rafters per roof slope is equal to the length of the slope divided by the pitch of the rafters, plus one rafter. Accordingly, for a gable roof, this number must be doubled.
The less often the calculated pitch of the rafters, the wider the rafter beam. For load-bearing structures of a gable or shed roof, this size should be at least 15 cm for large buildings, and for summer cottages (sheds, gazebos and baths) - 10 cm.
Then the number of rafters per slope is set. To do this, its length should be divided by the installation step. If the house is gable, then the resulting value should be doubled.
The choice of a suitable rafter section depends on the pitch of the rafters and their length:
| Rafter length, cm | Rafter pitch, cm | Rafter section, cm |
| Up to 600 | 140 | 10x20 |
| 100 | 8x20 | |
| Up to 400 | 180 | 9x18 |
| 140 | 8x18 | |
| 100 | 8x16 | |
| Up to 300 | 180 | 9x10 |
| 120 | 8x10 |
To reduce the deformation of rafters and beams during operation, it is best to use dry lumber for the rafter system.
When choosing beams for rafters, it is necessary to pay attention to the absence of cracks and knots.
In the most common case, for a gable roof of a one-story building covered with slate, it is advisable to use a wooden rafter with a section of 5x15 cm.
Varieties of roof structures
Before starting roofing work, you need to choose the best option for the roof structure. Each of them has its pros and cons.
Classification of truss systems:
- hanging;
- layered;
- hybrid.
If the roof has a standard width of 6 m (respectively, such a leg length of the rafter), then hanging systems will do. By fixing the ends to the roof ridge and the load-bearing wall, fastening is made.
In addition, a tightening is installed that prevents deformation of the pressure and stress of the truss structure. In addition, they take on the role of load-bearing beams.
Inclined systems are suitable for roofs of any width. The fixation of the bed in relation to the Mauerlat ensures the stability and reliability of the entire structure.
As a result, the pressure is smoothed out by the rack, and the stress is reduced. The advantages of a layered truss system are in a fairly simple installation, but the work will be expensive, since additional lumber will be required to equip the beds.
Hybrid structures are most suitable for multi-pitched roofs, in which transitions are accompanied by numerous repeating beams, reinforcements, beams, posts, bevels and other elements that ensure the stability of the system.
The construction of a hybrid structure is expensive and quite difficult, therefore, qualified specialists should be involved in the development of the project and construction.
Seemingly simple at first glance, the question of calculating the rafters (their cross section, length, pitch and other parameters) actually requires a thorough and responsible approach.
It is not enough just to estimate the distance from the top of the outer wall of the building to buy the right amount of roofing lumber, because in this calculation you will have to constantly adjust the work.
To avoid problems during construction, it is necessary to take into account many important parameters: from the thickness and length of the beams to the area of the future roof.
In addition, the terrain and climate of the region in which construction is being carried out is of great importance.
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. It accepts all external loads, distributing them over 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 snow in winter. 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 this 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 here there is an 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 a manual way of doing 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 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, there will be fewer errors, 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 designs of 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 geometry of the roof 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, wood is used, mainly coniferous species. Compared to metal, wood is easier to handle and install. Moreover, even if an error occurs during the calculations, then the wooden parts are 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 the construction of country houses, inclined rafters are more often used, but often builders combine both. As already mentioned, it may be necessary to build up the oblique 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 about 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 the functional features of the truss 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 tiles are laid exclusively on a solid surface (in extreme cases - a 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! The distance between the bearing walls is called the 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.
- A square meter of bituminous waterproofing weighs another 6 kg.
- The weight of a 10 cm layer of mineral wool is approximately 10 kg per square metre.
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 the snow load, because snow can accumulate on the roof in a fairly large amount. 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;
For a 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 5 m, then multiply this number by 0.004 - so you get the 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 of 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 simpler 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 project.
As a conclusion
One of the most important stages in the construction of the roof 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
Get the best in the mail
Learn how to calculate the rafters on the roof! What data is needed for calculations, a step-by-step guide, tables, photos + videos.
Rafters are the backbone of any roof. They bear the main load associated with the weight of the roof, wind and snow pressure. For long-term and trouble-free operation of the roof, it is important to make accurate calculations of these loads, determine the strength characteristics of the rafters, their cross section, length, quantity, as well as the amount of material required for the arrangement of the roof frame. All these calculations can be done independently.
Calculation of rafters using online programs
It is easiest to calculate the rafters using an online calculator. You set the initial data, and the program calculates the necessary parameters. Existing programs are different in their functionality. A number of them are complex in nature and calculate many parameters of the truss system, others are much simpler and involve the calculation of one or two indicators. Among the complex services, it is worth highlighting the Stroy-calc series of construction calculators for calculating the parameters of roof rafters with one, two slopes, an attic and hips.
The Stroy-calc calculator is used to calculate the parameters of roof rafters with one, two slopes, an attic and hips
The program also takes into account the roofing material, i.e., together with the calculation of the truss system, you can obtain data on the required amount of finishing coating from:
- ceramic tiles;
- cement-sand tiles;
- bituminous tiles;
- metal tiles;
- slate (asbestos-cement slabs);
- steel seam roof;
- bituminous slate.
In order to obtain the desired result, the following information is entered:
- roof characteristics: roofing material, base width, base length, rise height, overhang length;
- rafter characteristics: rafter pitch, type of wood for rafters;
- lathing characteristics: width, board thickness, distance between rows;
- snow load on the rafters: selection of the snow load region on the map.
The program contains drawings of types of roofs, which graphically show the data entry parameters. As a result, information is displayed on:
- roof - slope angle, surface area, approximate weight of the roofing material;
- rafters - length, minimum section, quantity, volume of timber for rafters, their approximate weight, layout (drawing);
- crate - the number of rows, the distance between the boards, the number of boards, their volume, approximate weight.
Online calculators, of course, cannot take into account the design features of rafters in all situations. To obtain accurate data for a specific roof option, all calculations must be done manually. We offer you methods for calculating the loads on the rafters (snow, wind, roofing cake), as well as determining the parameters of the rafters (section, length, quantity, pitch). Based on these data, it will also be possible to calculate the amount of wood needed to equip the truss system.
Calculation of the load on the rafters
The rafters hold up the roof. Therefore, loads are transferred to them both from external natural factors and from the weight of the roofing cake (battens, insulation, hydro and vapor barriers). The main external loads are associated with the effects of snow and wind.
Snow load
Snow load is determined by the formula: S =μ ∙ S g , where:
- S - the desired value of the load;
- μ - coefficient determined by the slope of the roof (the greater the slope, the lower this coefficient, since the snow will melt, so its pressure will be less);
- S g - the norm of snow pressure in a particular region of the country (kg / m 2), calculated from the results of long-term observations.
The angle of the roof is calculated from its main triangle
To determine the coefficient μ, it is necessary to know the angle of inclination of the slope. It often happens that the width and height of the roof are given, but the angle of inclination is unknown. In this case, it must be calculated by the formula tg α \u003d H / L, where H is the height of the ridge, L is half the width of the building (along the gable side), tg α is the tangent of the desired angle. Further, the value of the angle itself is taken from special tables.
Table: slope angle value according to its tangent
| tgα | α, deg |
| 0,27 | 15 |
| 0,36 | 20 |
| 0,47 | 25 |
| 0,58 | 30 |
| 0,70 | 35 |
| 0,84 | 40 |
| 1,0 | 45 |
| 1,2 | 50 |
| 1,4 | 55 |
| 1,73 | 60 |
| 2,14 | 65 |
Suppose the house is 8 m wide and 2.32 m high at the ridge. Then tg α = 2.32/4 = 0.58. According to the table, we find that α \u003d 30 o.
The coefficient μ is determined by the following method:
- at slope angles up to 25 о μ = 1;
- for angles from 25 to 60 about μ = 0.7;
- for steeper slopes μ = 0, i.e. the snow load is not taken into account.
Thus, for the considered structure μ = 0.7. The value of S g is selected based on the location of the region in which construction is being carried out on the map of snow loads.
The snow load map allows you to determine the pressure of snow on the roof in various regions of Russia
Having determined the number of the region on the map, the value of the standard snow load can be found from the corresponding table.
Table: normative snow load by region
| region number | I | II | III | IV | V | VI | VII | VIII |
| S g, kg / m 2 | 80 | 120 | 180 | 240 | 320 | 400 | 480 | 560 |
Let's assume that our house is located in the Moscow region. This is the third region in terms of snow load. S g here is 180 kg/m 2 . Then the total snow load on the roof of the house will be S = 0.7 ∙ 180 = 126 kg / m 2.
wind load
The wind load depends on the region of the country where the house is built, the height of the house, the characteristics of the terrain and the slope of the roof. It is calculated according to the formula: W m \u003d W about ∙ K ∙ C, where:
- W about - standard value of wind pressure;
- K - coefficient taking into account the change in wind pressure at altitude;
- C - aerodynamic coefficient, taking into account the shape of the roof (with gentle or steep slopes).
The normative value of wind pressure is determined from the map of wind loads.
The wind load map allows you to determine the wind pressure on the roof in various regions of Russia
Table: standard wind load by region
| region number | 1a | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| W o , kgf / m 2 | 24 | 32 | 42 | 53 | 67 | 84 | 100 | 120 |
According to the level of wind loads, the Moscow Region is in the first zone. Therefore, the standard value of wind pressure W about for our case is 32 kg/m 2 .
The value of K is determined from a special table. The higher the house and the more open area it is built, the greater the value of K.
Table: coefficient taking into account wind pressure at altitude
Let's take the average height of the house - from 5 to 10 m, and the area will be considered closed (this type corresponds to most areas where suburban construction is carried out). Hence, the coefficient K in our case will be equal to 0.65.
The aerodynamic coefficient can range from -1.8 to 0.8. A negative coefficient means that the wind is trying to raise the roof (usually with gentle slopes), a positive coefficient means that it is tilting (with steep slopes). For reliability, we take the maximum value of this coefficient equal to 0.8.
The wind affects the roofs with steep and gentle slopes in different ways.
Thus, the total wind load on the house we are considering will be equal to W m = 32 ∙ 0.65 ∙ 0.8 = 16.6 kg / m 2.
Roofing cake weight
The total weight per square meter of the roofing cake will be equal to the sum of the specific gravity of all its constituent elements:
- crates made of coniferous wood (8 - 12 kg);
- roofing (for example, we take corrugated board - 5 kg);
- waterproofing from a polymer membrane (1.4 - 2.0 kg);
- vapor barrier made of reinforced film (0.9 - 1.2 kg);
- insulation (mineral wool - 10 kg).
The weight of other types of roofing can be determined from a special table.
Table: weight of various types of roofing
For greater reliability, we take the maximum values of the weight of the components of the roofing cake: P \u003d 12 + 5 + 2 + 1.2 + 10 \u003d 30.2 kg / m 2. We add a margin of 10% in case of any additional structures or non-standard types of coating: P = 30.2 ∙ 1.1 = 33.2 kg / m 2.
Total load on the rafters
The total load on the rafters is calculated by the formula: Q \u003d S + W m + P, where:
Recall that the calculation is carried out for the Moscow region, the roofing is corrugated board, the angle of inclination of the roof is 30 °: Q = 126 + 16.6 + 33.2 = 175.8 kg / m 2. Thus, the total load per square meter of rafters is 175.8 kg. If the roof area is 100 m 2, then the total load is 17580 kg.
It is erroneous to believe that reducing the weight of the roofing significantly reduces the load on the rafters. Let's take cement-sand tiles (50 kg / m 2) as a coating. Then the weight of the roof will increase by 45 kg / m 2 and will not be 33.2, but 76.4 kg / m 2. In this case, Q \u003d 126 + 16.6 + 76.4 \u003d 219 kg / m 2. It turns out that with an increase in the mass of the roofing by 10 times (from 5 to 50 kg / m 2), the total load increased by only 25%, which can be considered a not so significant increase.
Calculation of rafter parameters
Knowing the magnitude of the loads on the roof, we can calculate the specific parameters of the material required for the installation of the truss system: section, length, quantity and pitch.
Selection of the cross section of the rafters
The cross section of the rafters is calculated by the formula: H \u003d K c ∙ L max ∙ √Q r / (B ∙ R izg), where:
- K c - coefficient equal to 8.6 at an angle of inclination less than 30 about, and 9.5 at a greater slope;
- L max - the largest span of the rafter;
- B is the thickness of the rafter section in meters;
- R bend - bending resistance of the material (kg / cm 2).
The meaning of the formula is that the required section size increases along with an increase in the largest span of the rafter and the load on its linear meter and decreases with an increase in the thickness of the rafter and the resistance of wood to bending.
Let's calculate all the elements of this formula. First of all, we determine the load per linear meter of the rafter. This is done according to the formula: Q r \u003d A ∙ Q, where:
- Q r - calculated value;
- A - the distance between the rafters in meters;
The logic of the calculation is quite simple: the less often the rafters are located and the smaller they are, the greater the load per linear meter will be.
We have already calculated the total load per 1 square meter of rafters. It is equal to 175.8 kg / m 2 for our example. Let us assume that A = 0.6 m. Then Q r = 0.6 ∙ 175.8 = 105.5 kg/m. This value will be required for further calculations.
Now let's determine the width of the sawn timber section according to GOST 24454–80 "Softwood lumber". We look at what sections the wood is sawn - these are standard values.
Table: determination of standard board width values depending on its thickness
| Board thickness - section width, mm | Board width - section height, mm | ||||||||
| 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 | ||||||||
Decide on the thickness of the board (B). Let it correspond to the most commonly used edged lumber - 50 mm or 0.05 m.
Next, we need to know the largest span of the rafter (L max). To do this, you need to turn to the project and find a drawing of a truss truss, where all its dimensions will be indicated. Let us take in our case L max equal to 2.7 m.
The value of the largest rafter span (Lmax) is an important component for calculating its cross section and is determined from the drawing of the truss truss
The value of the resistance of the material to bending (R bend) depends on the type of wood. For the first grade, it is 140 kg / cm 2, the second - 130 kg / cm 2, the third - 85 kg / cm 2. Let's take the value for the second grade: it is not very different from the first, but the second grade of wood is cheaper.
We substitute all the obtained values \u200b\u200bin the above formula and get H \u003d 9.5 ∙ 2.7 ∙ √ (105.5) / (0.05x130) \u003d 103.4 mm. With a rafter thickness of 50 mm, there is no standard width value of 103.4 mm, so we take the nearest larger value from the table above. It will be 125 mm. Thus, a sufficient cross-section of lumber with a rafter pitch of 0.6 m, a maximum span of 2.7 m and a roof load of 175.8 kg / m 2 is 50x125 mm.
- mauerlat - 100x100, 100x150, 150x150;
- rafter legs and valleys - 100x200;
- crossbars - 100x150, 100x200;
- racks - 100x100, 150x150.
These are sections with a margin. If you want to save material, you can use the above method.
Video: calculation of loads on rafters and their cross section
Rafter length
In the manufacture of rafters, in addition to the section, their length is also important. It depends, in particular, on the slope with which the roof will be built. The pitch of the roof usually varies between 20 and 45 degrees, but varies depending on the roofing material used, since not every roofing material can be used with any roof pitch.
Influence of the type of roofing material on the angle of the roof slope
Permissible roof slope angles for roofing materials:
- roll coatings - flat and low-slope roofs (up to 22 o);
- bituminous roofing and folded metal sheets - any slope;
- fiber cement sheets, corrugated board - from 4.5 o;
- metal tile, bituminous, ceramic tile, slate - from 22 o;
- high-profile piece tile, slate - from 25 about.
Permissible roof slope angles are determined by the roofing material used.
Despite the fact that the permissible roof slope angles can be very small, we still recommend making them large to reduce the snow load. For corrugated board, they can range from 20 o, metal tiles - 25 o, slate - 35 o, seam roof - 18 - 35 o.
The length of the rafters of different types of roofs is considered differently. We will show how this is done for a single-pitched and gable roof.
Calculation of the length of the rafters of shed roofs
The length of the rafter leg is calculated according to the formula L c \u003d L bc / sin A, where L bc is the amount by which the wall must be raised, and A is the angle of the roof slope. To understand the meaning of the formula for calculating L c, recall that the sine of the angle of a right triangle is equal to the ratio of the opposite leg to the hypotenuse. Thus, sin A \u003d L bc / L c. The value of L bc can be calculated by applying the formula: L bc \u003d L cd ∙ tg A, where L cd is the length of the wall of the house.
All formulas for calculating the truss system of a pitched roof are taken from a right triangle, which is the projection of the under-roof space onto the gable
It is easiest to find the values \u200b\u200bof tg A and sin A using the table.
Table: determination of the values of trigonometric functions by the angle of the roof slope
| Roof slope angle, degrees | tg A | sin A | cos A |
| 5 | 0,09 | 0,09 | 1,00 |
| 10 | 0,18 | 0,17 | 0,98 |
| 15 | 0,27 | 0,26 | 0,97 |
| 20 | 0,36 | 0,34 | 0,94 |
| 25 | 0,47 | 0,42 | 0,91 |
| 30 | 0,58 | 0,50 | 0,87 |
| 35 | 0,70 | 0,57 | 0,82 |
| 40 | 0,84 | 0,64 | 0,77 |
| 45 | 1,00 | 0,71 | 0,71 |
| 50 | 1,19 | 0,77 | 0,64 |
| 55 | 1,43 | 0,82 | 0,57 |
| 60 | 1,73 | 0,87 | 0,50 |
Consider an example.
- Let's take the length of the wall of the house, equal to 6 m, and the angle of inclination of the roof of 30 o.
- Then the height of the wall rise L bc = 6 ∙ tg 30 o = 6 ∙ 0.58 = 3.48 m.
- The length of the rafter leg L c \u003d 3.48 / sin 30 o \u003d 3.48 / 0.5 \u003d 6.96 m.
Calculation of the length of the gable roof rafters
A gable roof can be represented as an isosceles triangle formed by two slopes and a transverse ceiling beam.
The graphical representation of a gable roof in the form of an isosceles triangle allows you to determine the length of the rafter leg in two different ways
The length of the rafter leg (a) can be determined in two different ways.
- If the width of the house b and the angle of inclination of the roof A are known. Then a \u003d b / (2 ∙ cos A). Let's say that the width of the house is 8 m, and the angle A is 35 o. Then a \u003d 8 / (2 ∙ cos 35 o) \u003d 8 / (2 ∙ 0.82) \u003d 4.88. We add 0.5 m to the overhangs and get the length of the rafter leg equal to 5.38 m.
- If the width of the roof b and its height in the ridge h are known. In this case a = √b 2 + h 2 . Let us assume that the height of the ridge is 2.79 m. Then a = √4 2 +2.79 2 = √16 + 7.78 = √23.78 = 4.88. We add 0.5 m to the overhang and as a result we have the same 5.38 m.
It must be borne in mind that the standard length of sawn wood is 6 meters. With a longer length, they will either need to be spliced or made to order, which, of course, will be more expensive.
Video: rafter calculation
Rafter step calculation
Pitch is the distance between adjacent rafters. It determines how many rafters we need for the roof. The step size is usually set equal to from 60 cm to 1 m. To calculate a specific step size, you must:
- Select an approximate step.
- Determine the length of the slope. Usually this value is set by the project.
- Divide the length of the slope by the approximately selected step size. If a fractional number is obtained, then the result is rounded up and 1 is added (this adjustment is necessary because there must be rafters along both slope boundaries).
- Divide the slope length by the number obtained in the previous paragraph.
For clarity, we will show the calculation process using a specific example.
Suppose that the approximate step is 1 m, and the length of the ramp is 12 m.
- We divide the length of the slope by the approximately selected step size: 12 / 1 \u003d 12.
- We add 1 to the resulting number, we get 13.
- We divide the length of the slope by the resulting number: 12/13 \u003d 0.92 m.
It must be understood that the value obtained is the distance between the centers of the rafter logs.
The step between the rafters can also be determined from the table for a given cross section and the length of the rafter leg.
Table: calculation of the pitch of the rafters depending on the length of the rafter leg and the section of the beam
| Rafter pitch, m | Rafter leg length in meters | ||||||
| 3,0 | 3,5 | 4,0 | 4,5 | 5,0 | 5,5 | 6,0 | |
| 0,6 | 40x150 | 40x175 | 50x150 | 50x150 | 50x175 | 50x200 | 50x200 |
| 0,9 | 50x150 | 50x175 | 50x200 | 75x175 | 75x175 | 75x200 | 75x200 |
| 1,1 | 75x125 | 75x150 | 75x175 | 75x175 | 75x200 | 75x200 | 75x200 |
| 1,4 | 75x150 | 75x175 | 75x200 | 75x200 | 75x200 | 100x200 | 100x200 |
| 1,75 | 75x150 | 75x200 | 75x200 | 100x200 | 100x200 | 100x250 | 100x250 |
| 2,15 | 100x150 | 100x175 | 100x200 | 100x200 | 100x250 | 100x250 | - |
According to the same table, you can determine the permissible cross-section of the rafter, knowing the size of the step and its length. So, with a step of 0.9 m and a length of 5 m, we get a section of 75x175 mm.
With the thickness of the beam of the rafter legs more than usual, the distance between the rafters can also be made larger.
Table: calculation of the pitch of rafters from thick beams and logs
| Distance between the rafters m | The greatest length of the rafter leg, m | ||||||
| 3,2 | 3,7 | 4,4 | 5,2 | 5,9 | 6,6 | ||
| 1,2 | timber | 9x11 | 9x14 | 9x17 | 9x19 | 9x20 | 9x20 |
| log | 11 | 14 | 17 | 19 | 20 | 20 | |
| 1,6 | timber | 9x11 | 9x17 | 9x19 | 9x20 | 11x21 | 13x24 |
| log | 11 | 17 | 19 | 20 | 21 | 24 | |
| 1,8 | timber | 10x15 | 10x18 | 10x19 | 12x22 | - | - |
| log | 15 | 18 | 19 | 22 | - | - | |
| 2,2 | timber | 10x17 | 10x19 | 12x22 | - | - | - |
| log | 17 | 19 | 22 | - | - | - | |
Calculation of the number of rafters
- Depending on the load on the rafter system, we select the section of the rafter leg.
- We calculate the length of the rafter.
- According to the table, we select the step of the rafters.
- We divide the width of the roof by the pitch of the rafters and get their number.
For example, we calculate the number of rafters for a gable roof 10 m wide with a rafter leg length of 4 m and its cross section of 50x150 mm.
- We set the step equal to 0.6 m.
- We divide 10 m by 0.6 m, we get 16.6.
- Add one rafter to the edge of the roof and round up. We get 18 rafters per slope.
Calculation of the amount of wood required for the manufacture of rafters
For the construction of rafters, coniferous wood is most often used. Knowing how many rafters are required for the roof and how much wood is contained in one bar, we calculate the required amount of wood. Suppose that we have made a complete calculation of the truss system and received that 18 units of timber with a size of 150x150 mm are needed. Let's look at the table below.
Table: the amount of timber in a cubic meter of lumber
| The size timber, mm | Number of beams 6 m long 1 m 3 lumber, pcs. | The volume of one bar 6 m long, m 3 |
| 100x100 | 16,6 | 0,06 |
| 100x150 | 11,1 | 0,09 |
| 100x200 | 8,3 | 0,12 |
| 150x150 | 7,4 | 0,135 |
| 150x200 | 5,5 | 0,18 |
| 150x300 | 3,7 | 0,27 |
| 200x200 | 4,1 | 0,24 |
The volume of one bar 150 x 150 mm is 0.135 m 3. This means that the volume of lumber for 18 rafters will be 0.135 m 3 ∙ 18 = 2.43 m 3.
Video: material calculation for gable roof rafters
Greetings! My name is Michael. I am 59 years old. Higher education. I work as a seller - consultant in a company for the manufacture and installation of metal-plastic structures. I love my work and I hope that my experience and knowledge will be of interest to you.
We offer a professional free calculation of the gable roof truss system using an online calculator website, 3D visualization and detailed drawings. Detailed calculations of the roof and roofing, all materials, lathing, rafters, mauerlat. Try calculating a gable roof right now!
Our online calculator truss system will calculate a gable roof:
- calculation of the length of the gable roof rafters
- number of rafters and pitch
- calculation of the area of \u200b\u200ba gable roof and the angle of inclination
- roof sheathing calculation
- the number of sheet roofing materials (for example, corrugated board, metal tiles, slate)
- parameters of vapor barrier and insulation
To form the calculation of the gable roof calculator, you need to measure and enter the following dimensions in the appropriate boxes:
The cross-section (thickness x width) and the pitch of the rafters depend on the angle of the roof, its type, the length of the rafter leg, the maximum withstand main loads, as well as on the type and weight of the roof covering (and even to some extent on the width of the insulation). If you do not know where to get the standard parameters of rafters and battens, our article "" will help you.
The calculator performs the calculation of materials for the roof, starting from the dimensions of the roofing sheet you entered and from the calculated value of the roof area. We advise you to buy the amount of roofing materials for the roof, boards and timber for the truss system with a small margin, it is always better to take the leftovers to a hardware store than to pay a lot of money for the delivery of a missing pair of boards.
Be careful! Depending on how accurate the values you enter, the online calculator will be able to calculate the gable roof so reliably.
Simplify your calculations and save time, the program will draw rafter plangable roof and displays the results of the calculation of the gable roof according to the data you entered in the form of a drawing of the gable roof in different viewing angles, and its interactive 3d model.
On the tab " 3 D- View» you can better see your future gable roof in 3D view. In our opinion, visualization in construction is a very necessary feature.
If you have a gable roof with different slopes in your project, you should calculate twice using the calculator - for each slope separately.
