How to calculate the parameters of the gable roof of a private house? You can use an online calculator. But what if there is no way to use the rafter calculator? If you wish, you can calculate on paper the main parameters of the construction of the roof. I will tell you how to perform calculations in accordance with the loads acting on the truss system.
Factors affecting the truss system
| Illustrations | Calculation options |
 | The weight of the snow. Despite the slope of the slopes, a large amount of snow accumulates on the roof surface, as shown in the photo. The mass of snow cover affects the roof pie, the rafters and the load-bearing walls. |
 | wind pressure. Depending on the angle of inclination, the wind affects the roof.
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 | Roofing material weight. A pie is a multilayer structure, which, depending on the number of structural elements, has one or another mass. This means that when doing calculations with your own hands, you need to find the optimal ratio of the parameters of the pie and the material from which the load-bearing walls are built. |
 | Rafter weight. The stronger the rafters, the heavier they are and their price is higher, and vice versa, reducing the strength of the rafters will lead to the fact that the system will be lighter.
Our task, in the calculations, is to choose those parameters of the rafters that will correspond to the mechanical load from the roofing material. |
Calculation of the maximum weight of snow
The value of the maximum snow severity can be calculated by the formula S=µ·Sg, where:
- S is the amount of snow load (in kg / m 2);
- µ - coefficient of slope of the roof (depends on the angle of inclination of the rafters α);
- Sg - standard weight of snow (in kg / m 2).
In order to make calculations according to the proposed formula, we will determine the dependence of the conditional value µ on the angle of inclination α.
In the diagram you can see the ratio of the angle of inclination of the slope and the geometric parameters of the truss truss, which is formed by diagonal and horizontal beams.
Table 1 offers the already calculated results of dividing such quantities as the height of the roof to the ridge and half the puff - the beam forming the ceiling.
An angle of inclination (α) of 30° or less corresponds to a factor (µ) of 1. If the angle is equal to or greater than 60°, then µ is 0. If 60°>α>30°, then the value of µ can be calculated using the formula: µ = 0.033 (60-α).
Parameters of standard snow load in kg/m²:
I - 80;
II - 120;
III - 180;
IV - 240;
V - 320;
VI - 400;
VII - 480;
VIII - 560.
After the slope coefficient of the rafters and the parameters of the normative snow severity are known, we return to the formula S = µ·Sg, insert the available parameters and calculate the rafters taking into account the impact of the precipitation layer.
Calculation of the maximum allowable wind pressure
The importance of calculating the effects of wind is due to the following points:
- If the angle of inclination α is greater than 30°, the windage of the structure increases. Because of this, one of the slopes or the gable has additional pressure, which negatively affects the state of the structure.
- If the angle of inclination α is less than 30°, when the air flow goes around the roof, an aerodynamic lifting force and a turbulence zone under the overhangs are formed.
The calculation of the permissible load of the air flow is carried out according to the formula Wo K C = Wm, where:
- Wm is the maximum allowable impact of the air flow;
- Wo is the conditional effect of the air flow (determined from Table 2 and from the wind pressure map);
- K is the coefficient of change in the effect of air flow with height (shown in Table 3 in relation to the height of the building);
- C is the drag coefficient.
The aerodynamic drag coefficient C, in accordance with the configuration of the roof and the building, can have a value of <1.8 (the wind lifts the roof), >0.8 (the wind presses on one of the slopes). Let's simplify the calculation in the direction of increasing strength and assume that the value of the coefficient C is 0.8.
Now that all the coefficients are known, it remains to insert them into the formula Wo·K·C = Wm and calculate the maximum allowable value of the impact of the air flow Wm.
Calculation of the mass of the roof
When buying roof coverings, you can find out the weight from the seller or on the packaging. But in order to calculate in advance which material is suitable, you can use the table. To calculate, you need to calculate the area of \u200b\u200bthe roof slopes and multiply by the proposed values.
In addition to the mass of the coating, the load-bearing walls bear the weight of the rafters themselves, the boards of the lathing, counter-lattices, etc. The average values of the severity of the elements of the truss system can be found in the proposed table.
The weight values are given on the basis of kilograms per square meter, on the basis that the distance between the boards of the crate is standard 50-60 cm. To calculate the mass of the structure, we find out the area of \u200b\u200bthe slopes and multiply by the proposed values.
It is desirable to round up the results of calculations so that the resulting value provides the greatest strength of the truss system.
Summing up
Now you know what factors take into account the calculation of the roof truss system, and therefore you can calculate the necessary values on your own, without using the online calculation calculator. More useful information can be found by watching the video in this article. Ask questions of interest in the comments.
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