The rafter system serves as the main roofing structure. In the course of construction, snow and wind loads are taken into account, which the rafters must withstand. Roof systems are single-pitch, i.e. rafters in this case are support beams with fastening on the walls of a building.
Types of roof truss systems.
If the height of the walls is the same, then a flat view of the roof comes out, and at different heights, inclined ramps are created. For example, double-sloped roofs, that is, connections of rafters, which form two slopes, which is carried out taking into account a certain angle.
A type of double-sided truss system is a broken roof, which is used for the construction of mansard roofs in which the fracture helps to obtain a large free area. Constructions of semi-hinged (three-slope) roofs include a triangular roof slope and two trapezoidal. Their installation is a difficult task.
Hip (four-pitched) truss systems are considered more stable, they ideally conserve heat. They are made of 2 rays, having the form of a trapezoid and two hips in each of the ends. This type includes hipped roof. At production of mansard rafter systems their features are considered.
Figure 1. Scheme of roofing systems of rafters of mansard roofs: 1 - run, 2 - rafters, slant, 3 - stand , 4 - a support of overlapping of a penthouse (beam), 5 - a support of overlapping of the floor (beam).
The attic is a special room that is intended for housing . It is located on the roof, allowing you to plan the growth of the used space for the building without significant financial costs. With all this, the high strength of the roof rafters system must meet safety standards.
If the corners of the skylights of the mansard system of rafters are different from each other, then its installation is associated with more favorable conditions. Two types of rafters of mansard roofs are distinguished:
The following elements are distinguished in the scheme of roof systems of roof rafters of mansard roofs: 1 - run; 2 - rafters are mounted; 3 - stand; 4 - attic overlap support (beam); 5 - support floor overlap (beam). This is shown in fig. 1. The use of nasled mansard rafters is advisable in buildings with an intermediate bearing wall. The installation of the naslon system is possible only taking into account the distance between the two load-bearing walls of less than 7 m.
Hanging mansard rafters are used if there are no internal walls. Standard attic structures include rafters, tightening, that is, the lower belt, and the power plate acts as a supporting element. Rafters are fastened with brackets and wire, mounted in the walls, taking into account the level of stress that may arise.
Tools with materials that must be prepared before work include:
Figure 2 Table snow area.
The types of mansard roofing systems differ:
The calculation of the angles of a roofing inclined house ramp is of fundamental importance. It is carried out taking into account the following points: weather features of the terrain, construction and finishing materials used, architectural preferences of the owner of the building.
Figure 3. Map of snow load.
With an increased angle of inclination of the roof slope, there is a reduction in the size of the free space in the room. However, with a small angle it will be more difficult to clean the roofing system from pollution and precipitation, which will lower the reliability of the roof. The right choice of roofing material is very important.
Stability of the roof depends on the quality of installation of the truss system. The basic rules for installing roof systems should be followed. When building rafters, it is necessary to use wooden beams with a section of 10x10 cm. It is required to apply waterproofing materials: roofing sheets or roofing felt.
The frame structure is laid on reinforced concrete foundations. Fastening of the bearing knot is carried out with the help of a metal bracket and wire strapping. Often decoupling is often the method of connection for wooden construction. The moisture level of the tree used (best of all coniferous) should not exceed 15%.
A disinfectant atmosphere should be created. The use of damp bars can cause damage to the entire roof system, so they require drying before use.When marking the location of the supports calculate the distance so that the placement of the supports was strictly vertical in the same plane. A plumb line should be used for proper calculation.
In order to secure the fixing of the parts, stretching or bracing is necessary. For upholstery uprights, that is, the foundations of the walls of the attic, you can use suitable materials, such as drywall or plywood, used for double-sided plating. Between them the heat insulator is laid.
Figure 4. Table of the wind region.
Before conducting research to determine the degree of wind or snow loads, it is necessary to determine the type of terrain using a special map. The correct calculation of the truss system is related to the given loads, therefore, the corresponding formulas (1) and (2) are used:
S = Sg * µ B, (1).
In formula (1), Sg is an indicator determining the weight of snow cover per 1 square meter. land area in a horizontal position. This indicator is taken into account according to the table. The values corresponding to numbers I - VIII are determined by the snow load map shown in Fig. 2.
The value of µ is a coefficient that indicates the transition to snow loads on a piece of land from the total weight of the snow cover. Due to the coefficient µ, the angles of inclination of the mansard roof slopes are determined:
When the slope of the roof slope exceeds 60 °, the value of the coefficient µ is not taken into account under the conditions of general calculations of the snow load.
W = Wo * k, (2).
Figure 5. Table of the terrain type.
The value of Wo in formula (2) is normative for determining the wind load, which is taken into account in the formula according to the tabular data for the calculations (Fig. 4). The coefficient k depends on the climatic conditions of the region, takes into account all changes in wind pressure in height. The coefficient is determined taking into account tabular values (Fig. 5).
The letter "A" in the table indicates the presence of open coasts, water bodies, deserts, forest-steppe areas, etc. The letter "B" is associated with the designation of the presence of urban areas, forests or other areas that have irregularities with different barriers with a height of more than 10 m.
When installing rafters of mansard roof, the top bar is first laid and fixed with brackets, self-tapping screws or nails.
Its cross section can be 10 * 10 cm or 15 * 15 cm. The use of the first beam is associated with the creation of a subrafter pit.
At the next stage, the installation proceeds with the installation of a power plate that will not allow a sudden overturning of the roof due to a strong gust of wind. Mauerlat must transfer not only the load on the roofing system, but also on the walls of the house. It is mounted with the help of bars, for the quality of the device which is necessary to take into account the size of the cross section 10X15 cm or it is important to use boards with a thickness of 5 cm or more.
The board or cant is laid in a horizontal position by calculating the size of the wall surface. Under each board, installation of waterproofing is required so that moisture cannot get into the elements of the power plate. The next step is associated with the installation of the construction of truss legs, which can be done with your own hands or purchased in finished form.
When calculating the pitch of the rafter legs, which are then applied to the mauerlat with a pencil, you can use a measuring tape. Installation of the last rafters is made to the frontal surface, taking into account that the rafters by their top, as well as the line along the edge of the front, are at the same level. Based on 1 meter of the roof, it is possible to make only one mistake.
Special attention during installation of the rafter system should be given to the following points:
After the last rafters have been installed, a level is drawn between them, which is necessary for fastening other rafters, which are connected simultaneously with the installation of a strong ridge support. Its use is necessary if the size of the length of the roof is more than 7 m, and the roof frame has the greatest weight.
Didn't find the answer in the article? More information on the topic:
How to calculate the floor lag
Calculation of lag for the floor: features.Dependence of the pitch lag on the thickness of the rough flooring. Installation of support columns. Advantages and disadvantages of the floor on the logs.
Calculation of lumber in one cube
Calculation of lumber in a cube according to the mathematical formula for calculating the volume. How to determine the number of boards or timber on the final table of values for edged and unedged lumber.