High-quality, durable and strong roof construction is the most important stage in building a house. This article will talk about what a rafter system is and how to calculate the length of the rafters and its other elements.
The service life of the roof and its reliability and safety are directly related to how well and competently the calculation of the truss system and its construction are performed.
Rafters are the main link of the supporting roof structure, the calculation of the strength of the rafters is carried out in order to ensure that they can withstand various loads:
- Snow cover pressure;
- Impact of gusts of wind;
- The weight of the roof and the material laid on it;
- The weight of people performing various work on the roof and their tools and equipment, etc.
In this regard, the implementation of various calculations, such as the calculation of the truss structure, the legs of the rafters, their length, etc. is a particularly important step in roof design.
When performing such calculations, one should take into account both the materials used in the construction of the roof and the features of the area in which the construction is being carried out: the average thickness of the snow cover in winter and the values of wind loads.
The main elements of the truss system
The rafter system includes the following main elements:
- The legs of the rafters, located at a slope;
- Vertical racks;
- Struts (also inclined);
- Beams for tying the lower parts of the rafters, located horizontally, etc.
It is important to perceive the rafter system as a complex of interconnected elements, the importance of which cannot be underestimated.
When performing the calculation of truss structures, one should not make the slightest mistake, as well as when designing fasteners or the location of rafter legs.
This is due to the fact that any slightest mistake that has crept into the calculation of the truss truss, the legs of the rafters or another element of the supporting structure can lead to the collapse of the roof under the influence of the wind flow.
To avoid this, calculations should be entrusted to professionals who have the required knowledge and skills and experience in performing such calculations.
To date, there are a number of tables for calculating the rafter system, designed to help with the independent design of the roof.
In addition, to facilitate the task, you can use the calculator for calculating the truss system, which allows you to minimize the likelihood of a miscalculation.
Features of the construction of truss systems
The rigidity of the roof frame is provided by the fastening of the rafters to each other. In addition, the frame must be firmly connected to the frame of the house under construction in order to prevent the roof from being torn off by strong winds.
The most widely used in the construction of cottages, summer cottages and residential country houses are wooden rafter systems.
They differ in both fairly simple manufacture and simple installation. In addition, if necessary, the wooden elements of the rafter system are quite easy to process, giving them the desired shape, taking into account various shortcomings and errors in the construction of the walls of the house.
Metal rafters are much more difficult to fit on the spot, and reinforced concrete structures are practically impossible to process, so both of these materials are used in the construction of truss structures of buildings of fairly large size.
Depending on what material is used in the construction of the house, the rafter support is also chosen:
- In the case of cobbled houses, the rafters rest on the upper crowns;
- In frame wooden houses, the fulcrum of the legs of the rafters is the upper trim;
- The supports of the rafters of brick and stone houses are located on the supporting bars.
Important: The main advantage of roof truss structures is the ability to equip almost any configuration of the attic space.The main disadvantage is that the connections between the rafters in many cases prevent the installation of vertical windows.
Varieties of truss systems
Depending on the material used, there are several types of truss structures:
- Wooden;
- wood-metal;
- Reinforced concrete;
- Metal.
It is important to remember that with an increase in the weight of the roofing material, the power of the rafter system should be increased and the distance between them (rafter pitch) should be reduced.
This rule primarily applies to the structures of truss systems used in the construction of roofs covered with cement-sand material or ceramic tiles. The most popular are metal and wooden truss structures.
In addition, truss structures are classified according to the method of construction.
There are two types of roof truss structures:
- Sloped, most widely used;
- Hanging.
Most often, the rafter system has the shape of a triangle as a geometric figure that has the greatest rigidity and efficiency.
The composition of complex roof trusses usually includes the following main structural elements:
- Legs of rafters (beams) - 1;
- Crossbars - 5;
- Struts - 3;
- Racks - 2;
- Puffs, etc.
Depending on the complexity of the design, other elements of the rafter system can be used.
Sloped rafters are more common as they cost significantly less than hanging rafters with an easier installation procedure for the rafter system. In addition, the combination of hanging and inclined rafters in one design is quite often used.
An example of the calculation of the rafter system
Let's give an example of the calculation of the truss system. It is important to bear in mind that this calculation includes a number of interrelated calculations:
- The first calculation based on the strength of the rafters. This checks whether the rafter beam can break under a given load.
- Second calculation roof structure evaluates the likelihood of deformation of the rafters: the load that the beams can withstand is calculated, and how much they are able to bend without breaking.
Important: for any roof configuration, the deflection of the rafters should not exceed 1/250 of the total length of the site.
Specific calculation of rafter legs:
- Let's say that the length of the rafter is 6000 mm.
- Multiplying this value by 0.004, we get 24 mm, which may seem like a rather small value. However, even its minimum excess will lead to deformation of the rafter structure. A significant excess of the value will cause a strong deflection of the roof.
When calculating wooden rafters two main indicators of roof construction are used:
- Its stability and strength. The values of the indicators used can be found in specially compiled tables developed by specialists;
- Roof deflection. In this case, the values required for the calculation can be obtained from SNiP “Wooden structures. Design standards”, which includes, among other things, various indicators, including the elasticity of wood.
To obtain the most accurate result, it is necessary to find out the value of the design resistance index.
Also, when performing calculations, it is important to know the values of such characteristics and parameters as the angle of inclination of the designed roof and its various sizes, as well as the indicators of the material used for mounting the rafter system.
An example of calculating the cross section of a rafter leg using tables:
- Let's say that the length of the rafter is 300 cm;
- In this case, there are two options for the section of the board: height - 18 cm and thickness - 4 cm, or height - 16 cm and thickness - 6 cm;
- The cross section of the beam in this case will be 10x12 centimeters;
In the same way, you can choose standard rafter leg cross-sections for beams of various lengths.
In addition, we should not forget that at the moment the Internet offers a wide selection of special calculator programs designed to facilitate the calculations of various elements of the rafter system and minimize the risk of error and inattention in the calculation.
Using such programs is quite simple: just enter the required values and the program will display all the necessary calculation results.
Fulfilling rafter calculation and its various elements on your own, you should either consult with qualified specialists, or use programs designed for this.
The most important thing is to remember that any inaccuracy made in the calculation can lead to damage or collapse of the entire roof of the constructed house.
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