No house can be built without a roof, and no roof can be built without a supporting structure. Any construction begins with design and calculations. Consider how the calculation of rafters is performed.
Such calculations are extremely important. It is unacceptable to build truss systems "by eye" or "approximately". It is necessary to take into account all the loads that will have an effect on the roof. They are divided into:
- Permanent. This is the own weight of the coating, waterproofing, battens and other components of the "pie". If it is planned to install any equipment on the roof, then its weight must also be taken into account.
- Variables. This type of load includes the mass of precipitation falling on the roof and other influences that do not constantly affect the roof.
- Special.In seismically hazardous areas or in areas where hurricane winds regularly occur, it is necessary to lay an additional margin of safety.
How to calculate the weight of a roofing pie?
First of all, you need to calculate how much it will weigh itself house roof.
This is a necessary calculation - the rafters must withstand this constant load for a long time.
It is not difficult to make a calculation, you need to calculate the mass of one square meter of each of the layers of the "pie" of the roof. Then the weight of each layer is added up, and the result is multiplied by a correction factor of 1.1.
Calculation example. Take for example a roof covered with ondulin. The roof consists of the following layers:
- roof lathing, assembled from planks 2.5 cm thick. The weight of a square meter of this layer is 15 kg.
- Insulation (mineral wool) 10 cm thick, weight per square meter of insulation 10 kg.
- Waterproofing - polymer-bitumen material. The weight of the waterproofing layer is 5 kg.
- Ondulin. The weight per square meter of this roofing material is 3 kg.
We add the obtained values \u200b\u200b- 15 + 10 + 5 + 3 \u003d 33 kg.
We multiply by the correction factor 33 × 1.1 \u003d 34.1 kg. This value is the weight of the roofing pie.
In most cases, during the construction of residential buildings, the load does not reach 50 kg per square meter.
Advice! Experienced builders recommend relying on this figure, although it is clearly overpriced for most roofing.But on the other hand, if in a few decades the owners of the house want to change the roof, then they will not have to change all the rafters - the calculation was made with a solid margin.
Thus, the load from the weight of the roofing "pie" is 50 × 1.1 = 55 kg / sq. meter
How to calculate snow loads?
Snow load is a rather serious impact on roof structures, since a lot of snow can accumulate on the roof.
To calculate this parameter, you can use the formula:
S=Sgxµ.
In this formula:
- S is the snow load,
- Sg is the weight of snow cover that covers a square meter of horizontal surface. This value varies depending on the location of the house. You can find this coefficient in snip - truss systems.
- µ is a correction factor, the value of which depends on the angle of the roof. So for flat roofs that have an angle of inclination of 25 degrees or less, the value of the coefficient is 1.0. For roofs with an inclination angle of more than 25 and less than 60 degrees, the coefficient is 0.7. For a roof with steep slopes, snow loads can be ignored.
Calculation example. For example, it is necessary to calculate the snow load for the roof of a house under construction in the Moscow region, and the slope angle is 30 degrees.
The Moscow region is located in the III snow region, for which the calculated value of the mass of snow per square meter of horizontal surface is 180 kgf/sq. m.
180 x 0.7 = 126 kgf / sq. m.
This is the estimated snow load on the roof.
How to calculate wind loads?
To calculate the load on the rafters, the formula is used:
W = W x k
- Wo is a normative indicator, which is determined according to the tables, depending on the region of the country.
- k is a correction factor that will allow you to determine the change in wind load depending on the type of terrain and the height of the building.
| The height of the house, measured in meters | A | B |
| 20 | 1,25 | 0,85 |
| 10 | 1 | 0,65 |
| 5 | 0,75 | 0,85 |
A - these are open areas: steppes, coast of the sea or lake;
B - areas evenly covered with obstacles, for example, urban development or a forest area.
Calculation example. Calculate the wind load for a house with a height of 5 meters, located in a wooded area in the Moscow region.
The Moscow region is located in the I wind region, the standard value of the wind load in this region is 23 kgf/sq. m.
The correction factor in our example will be 0.5
23 x 0.5 = 11.5 kgf / sq. m.
This is the value of the wind load.
How to calculate the sections of rafters and other roof elements?
To calculate the length of the rafters, you need to know what roofing material is planned to be used, as well as what attic floors are made of (wooden beams or reinforced concrete slabs).
Standard rafters that go on sale have a length of 4.5 and 6 meters. But, if necessary, the length of the rafters can be changed.
The cross section of the beam, which goes to the manufacture of rafters, depends on the following factors:
- Rafter length;
- Rafter installation step;
- Estimated load values.
The data in the presented table are advisory, they cannot be called a full replacement for the calculations. Therefore, the calculation of the truss truss is a necessity to determine the bearing capacity of the roof. .
These tables are given in accordance with the atmospheric loads typical for the Moscow region.
| The step with which established rafter (cm) | Rafter length (meters) | ||||||
| 3,0 | 3,5 | 4,0 | 4,5 | 5,0 | 5,5 | 6,0 | |
| 215 | 100x150 | 100x175 | 100x200 | 100x200 | 100x250 | 100x250 | — |
| 175 | 75x150 | 75x200 | 75x200 | 100x200 | 100x200 | 100x200 | 100x250 |
| 140 | 75x125 | 75x175 | 75x200 | 75x200 | 75x200 | 100x200 | 100x200 |
| 110 | 75x150 | 75x150 | 75x175 | 75x175 | 75x200 | 75x200 | 100x200 |
| 90 | 50x150 | 50x175 | 50x200 | 75x175 | 75x175 | 75x250 | 75x200 |
| 60 | 40x150 | 40x175 | 50x150 | 50x150 | 50x175 | 50x200 | 50x200 |
Bar sections for the manufacture of other roofing elements:
- Mauerlat - 100x100, 100x150, 150x150;
- For valleys and the manufacture of diagonal legs - 100x200;
- Runs - 100x100, 100x150, 100x200;
- Puffs - 50x150;
- Crossbars - 100x150, 100x200;
- Struts - 100x100, 150x150;
- Hemming boards - 25x100.
Having decided on the cross section and length, as well as on the spacing of the rafters, it is easy to calculate the number of rafters, focusing on the length of the walls of the house.
When designing, in addition to the strength calculation, the designer must perform a deflection calculation.
That is, you need to not only ensure that the rafters do not break under the load, but also find out how much the beams can sag.
For example, the calculation of a wooden truss truss for the construction of a mansard roof must be performed so that the deflection does not exceed 1/250 of the length of the section on which pressure is applied.
Thus, if rafters with a length of 5 meters are used, then the maximum allowable deflection can reach 20 mm. This value seems quite insignificant, however, if it is exceeded, the deformation of the roof will be visually noticeable.
material quality requirements
If wooden rafters are calculated, then in addition to parameters such as length and cross section, the quality of the building material must also be taken into account.
Do-it-yourself roof rafters made from hardwood and coniferous wood.
The main requirements for the material are set out in GOST 2695-83 and GOST 8486-86. Among them:
- It allows the presence of knots in the amount of not more than three per meter section, the size of the knots should not exceed 30 mm.
- The presence of non-through cracks not exceeding ½ of the length is allowed;
- The moisture content of lumber should not exceed 18% when measured with a moisture meter.
When purchasing the material from which it is planned to mount the truss systems, the snip prescribes the verification of the quality document, which indicates:
- Manufacturer information;
- Standard number and product name;
- Product size, humidity and type of wood used;
- The number of individual items in the package;
- Release date of this batch.
Since wood is a natural material, it requires pre-installation preparation. This preparation is planned at the stage when the truss system is being designed - the snip provides for protective and constructive measures.
Protective measures include:
- Treatment of wood with antiseptics to prevent premature decay;
- Treatment of wood with flame retardant impregnations to protect against fire;
- Treatment with bioprotective compounds to protect against insect pests
Constructive activities include:
- Installation of waterproofing pads at the point of contact between brick and wooden structures;
- Creation of a waterproofing layer under the roofing material and vapor barrier - from the side of the premises in front of the insulation layer;
- Roof space ventilation equipment.
If all the requirements of the technology are met, the truss system of a wooden house will acquire higher strength qualities, and the roof structure will last a long time without requiring repairs.
Programs for the design and calculation of truss systems
As can be seen from the above, to calculate roof building systems pretty hard. You need to have a sufficient supply of theoretical knowledge, to have the skills of drawing and drafting. Naturally, not every person has such professional skills.
Fortunately, today the task of designing is greatly facilitated, since there are very convenient computer programs that allow you to develop designs for various building elements.
Of course, some programs are designed for professionals (for example, AutoCAD, 3D Max, etc.). It is quite difficult for an inexperienced person to deal with this software.
But there are also simpler options. For example, in the Arkon program, it is very easy to create a variety of draft designs in order to visually see how this or that roof will look like.
There is also a handy calculator for calculating rafters, which allows you to efficiently and quickly make calculations. The Arkon program is great for professionals, but can also be used by private users.
On the network you can also find a rafter calculation calculator that works online. However, the calculations made on it are only advisory values and cannot replace the development of a full-fledged project.
conclusions
Performing calculations during design is an important step in creating a roof. Its implementation must be entrusted to professionals. But preliminary calculations can be carried out independently, this will help to better understand the finished project.
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