The device of a pitched roof is based on rafters - a power supporting structure that holds a protective roofing. This is a responsible part of the overall frame, which in some configurations also acts as the attic body. In order for the rafter system to fully comply with the tasks of operating the house, you should carefully approach its design, calculation and installation.
The original design of the rafters
The pitched roof cannot be formed by concrete elements due to the large load that they can give to the main frame and floors. Therefore, even today, the wooden skeleton with beams and supporting elements in a triangular configuration remains the optimal solution for the construction of the roof base.
In a sense, the rafter frame can be compared with the crate. The basis for the supporting beams are brick, stone or wooden walls, on which the load is transferred from the entire roof with a roofing. The original design of the rafter system includes girder parts (beams), girders, racks with supports, struts and other elements.
Mauerlat occupy a special place in the framework - by the way, the way they are executed determines whether the system belongs to a traditional or modern model. If in classic projects Mauerlat opened out, today architects with designers come up with various options for disguising them in a roofing “pie”. We can say that this is the most critical part of the structure, which accounts for the load from the entire frame. Mauerlat is a massive beam that directly connects the front wall and the rafters. Now it’s worth considering the other elements that make up the pitched roof frame.
System components
So, Mauerlat, located on top of the wall masonry, will act as a direct support base for the truss structure. As a rule, two beams are laid - along opposite lines of the ramp. The next most important element can be called the rafter leg. These are beams, which are inclined to be attached to the Mauerlat and connected in crowns, forming a triangular structure. As a rule, an in-line crate is arranged, which acts as the base for the power elements of the rafter system (puffs, corners, hardware, etc.). Fastening equipment in the future will provide a reliable connection between the made crate and the roofing.
The rafter leg is attached to the Mauerlat in different ways with the expectation of snow and wind loads, but this fixation alone is not enough. Therefore, intermediate nodes of the rafter system, which act as structural clamps, are introduced into the frame. This group includes girders, tightening beams and metal pipes. In the first two cases, we are talking about wooden bars that crosswise or along the entire line of the rafters, combining them into one power structure. As for metal pipes, they are not fixed from the outside with anchors, as is the case with beams, but they are inserted and penetrate the lathing elements through special holes.
Types of rafter system
It has already been noted that in the lower region the rafters lie down and are fixed to the Mauerlat, and in the upper region they pass into a triangular crown. This is not the only frame placement configuration, but it is the most common. In this case, layered rafters are used, in which the legs rest on the supports, and as they rise, they are supported by struts, struts and girders included in the crate.
In the upper part, the device of the rafter system of the lay type provides for the presence of a ridge. Two roof slopes will rest on it. The advantages of this configuration include the possibility of an even distribution of loads over the entire area with tolerances for increasing the length of the rafter legs. In particular, in the presence of two runs, the crate can reach 12-15 m.
An alternative version of the layered system involves a hanging arrangement of the rafter frame. The fundamental difference between this scheme is the rejection of intermediate supporting support elements - for example, from runs and crossbars. All load is transferred to Mauerlat and skate. What makes up for the support of the hanging rafter system? Firstly, its implementation is allowed only in houses with walls, the distance between which does not exceed 6.5 m, which in itself leads to weight loss. Secondly, the roof will have not two, but four slopes, which converge in one central ridge, and this also increases the bearing potential of the system.
System design
Initially, you should create a sketch of the design on paper. Today, design companies use software like SolidWorks and SCAD for this task. It is necessary to develop a project taking into account the construction standards of SNiP, which, in particular, impose restrictions on the dimensions of individual elements. For example, the distance from the ceiling at the level of the Mauerlats to the ridge should be at least 2.5 m. The supporting structure can be represented in the form of a square or rectangle. Slopes should be taken from the center - again, the design of the rafter system for a private house should include two or four slopes, which will determine the layout of the frame. The main thing is to withstand the symmetry of opposing slopes.
Next, the optimal sizes of the Mauerlat, rafter legs, floor blocks and racks are determined. To calculate the area, it is desirable to divide the overlap into several functional zones, then add them up and display the point with the highest peak. In this zone, the ridge will be located - along the entire line along the roof, or in the form of a small segment in the center, as is the case with the four-slope system.
Design calculation for loads
Often wind and snow loads are taken into account when choosing the type of roofing “cake”, which is not entirely correct - the power qualities of a roof with an external coating directly depend on the roof structure. First you need to calculate the slope. The sharper the crate angle, the less rainfall and, most importantly, the snow mass will linger on the roof. For the ridge upper rafters, it will be about 15-30 °, and for the lower rafters - at least 60 °.
Wind loads can also vary depending on the angle of the ramp. In this case, the geometry and plane of the roof will be important. So, a steep slope will be discarded by the wind, and a flattened one will lift from the leeward side. The horizontal direction of air flow should be taken into account. The optimal configuration of the rafter system allows you to break them up with pressure transfer to the foundation and tangentially from the roof overhang.
In addition to climatic factors, the own load of the rafter and roof structure mass, which is calculated in kg per 1 m2, is also important. Specific data are generated on the basis of a separate mass calculation for the following items:
- Weight of lumber used for beams, supports, crossbars, lathing, etc.
- Weight of insulation materials.
- The weight of the substrates and the roof.
- Weight of fasteners, hardware and connecting rods.
The roof will make the greatest contribution to the load. The most lightweight will be the rafter system for corrugated board, on which 4-5 kg / m2 falls on average. For bituminous tiles and slate, average values leave 10-12 kg / m2. And a special approach should be in calculating the pressure on roofs with ceramic and cement tiles. This is a heavy roof, exerting a load of 30-40 kg / m2. As a rule, the total pressure force of the roofing does not exceed 50 kg per 1 m2, therefore, when calculating, you can lay this value as a standard. Even better is to leave a small margin in case of force majeure.
What material is used in the construction?
To implement a high-quality and durable carrier system, lumber suitable for its characteristics should be used. Pine, spruce and other conifers, which are flexible, durable and easy to process, are ideal. And you can use only the highest grade. If, in order to save money, it was decided to give preference to the 2nd and 3rd grades, then you will have to perform independent processing of the material with antiseptics.
Particular attention is paid to the structure. Mistakenly, many believe that the most durable is a solid bar. It is for the roof that experts recommend glued segments that do not undergo deformation and withstand dynamic loads. Do not forget about humidity. From the wood with a humidity coefficient of up to 20%, the roof truss system is insulated with plasterboard plates insulated from the inside. Raising the coefficient to 23-25% means that some of the elements will have to be located outdoors. The most undesirable option will be lumber from a freshly cut solid wood, the moisture content of which is 30%.
Mauerlat and flooring
Along the perimeter on the walls, the base of the rafter frame in the form of a Mauerlat is installed. Between the last brickwork and the timber, a layer of waterproofing or roofing material is laid. At the stage of the Mauerlat device, the installation of the rafter system is carried out using anchors, studs or wire. It is desirable that the fixing elements and the mounting scheme itself allow for the possibility of being embedded in the wall masonry.
Then the transverse beams from the same beam are laid. Their task will be to connect two opposite walls, which will form the base of the attic floor. Docking of the Mauerlat with beams is carried out by metal corners or self-tapping screws. At this stage, specific regulatory parameters for distances and sizes should be maintained. So, the construction of the rafter system in the bearing part should be carried out with a distance between the beams in the range from 50 to 100 cm. The optimal step is 60-70 cm. At the same time, the beam spacing beyond the wall line should be left within 30-50 cm.
Installation of rafters
At this stage, a beam is prepared and mounted for side rafters and adjacent structural elements. For fastening to the Mauerlat, connecting grooves are cut out in the beams and holes are made if necessary. In addition to integration into the niche of the beam, the roofer must ensure that the metal staples are fastened with an assembly stapler.
Rafter legs from the Mauerlat go to the line of the ridge. First you need to determine its center. In this area, a temporary rail is nailed on which the end will be held. The beams are fastened at the upper point using metal plates and large-format hardware - from nails to brackets. Do not ignore the use of groove joints. The combined installation of the rafter system with locks, corners and bolts will increase the reliability of the structure.
When the frame is assembled, it is possible to lay the formed gables with brick, boards or metal profiles. At the same time, do not forget about the ventilation openings and windows.
Insulation Operations
Even if the attic space under the roof frame will not be used as an attic, the roof must be insulated and waterproofed. Even before the level of the lower layer of the roofing cake, a counter-lattice, an air gap for ventilation, a film to prevent moisture penetration, etc. should also be provided. The lath is formed by slats and beams on the supporting beams. If the installation of the rafter system is implemented using metal pipes, it is best to continue the rear superstructure of the crate over the entire area of the ramp from them. This will add reliability and durability. The only thing that should be provided as a supplement is the treatment of metal surfaces with protective anti-corrosion compounds.
First of all, a waterproofing device is attached to the rafters. You can use a universal membrane hydro-vapor barrier, laying it in a stretch, and not overlap. The joints are necessarily glued with construction tape for sealing. The insulation layer is mounted inside. It can be polystyrene foam, mineral wool or felt materials. It is important to emphasize that the device of the rafter system should be as focused as possible on providing fire protection. Therefore, the choice of natural and combustible synthetic insulators should be approached very carefully. In shape it is better to use thick plates. They are physically more reliable and, in addition to insulation, will provide decent sound insulation.
Conclusion
The classic rafter structure with a ridge and intermediate supporting elements is convenient both in operation and in maintenance. This configuration does not limit the owner of the house in the possibilities of functional expansion of the roof. For example, it is possible to integrate air ducts, snow blowers, windows with hatches, etc. The main thing is that the roof rafter system constantly meets the basic technical and operational requirements, coping with the loads. And for this, it is necessary to periodically audit the rafter legs, supports and crossbars. The structure of the wood should be periodically treated with moisture protective, biological and refractory impregnations. The choice of specific compounds will depend on the type of lumber, the same humidity coefficient and operating conditions. Special attention is paid to mounting hardware. Metal elements, for example, should be replaced at the slightest sign of corrosion. For them, special protective equipment is also selected.