The pitched roof device is based on rafters - a load-bearing structure that holds the protective roofing. This is a critical part of the overall frame, which in some configurations also acts as the attic body. In order for the truss system to fully comply with the tasks of operating the house, you should carefully approach its design, calculation and installation.
Original rafter design
A pitched roof cannot be formed by concrete elements due to the heavy load they can exert on 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 roofing base.
In a sense, a truss frame can be compared to a crate. The basis for the load-bearing beams are brick, stone or wooden walls, on which the load from the entire roof with roofing is transferred. The original design of the truss system includes transom parts (beams), girders, racks with supports, struts and others.items.
Mauerlats occupy a special place in the frame - by the way, the way they are executed determines whether the system belongs to a traditional or modern model. If in classical projects the Mauerlats opened outward, today architects and designers come up with various options for their disguise in the roofing “pie”. We can say that this is the most critical part of the structure, which bears the load from the entire frame. Mauerlat is a massive beam that directly connects the facade wall and rafters. And now it is worth considering other elements that form the frame of a pitched roof.
Components of the system
So, the Mauerlats 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 slope lines. The next most important element can be called the rafter leg. These are beams that are attached at an angle to the Mauerlats and connected in crowns, forming a triangular structure. As a rule, a row crate is arranged, serving as the base for the power elements of the truss system (puffs, corners, hardware, etc.). Fastening equipment in the future will ensure a reliable connection between the completed crate and roofing.
The rafter leg is attached to the Mauerlats in different ways with the expectation of snow and wind loads, but this fixation alone is not enough. Therefore, intermediate nodes of the truss system are introduced into the frame, which protrudestructural fasteners. This group includes girders, tie beams and metal pipes. In the first two cases, we are talking about wooden bars that run across or along the entire line of the rafter legs, combining them into one power structure. As for metal pipes, they are not fastened from the outside with anchors, as is the case with beams, but are inserted and penetrate the elements of the crate through special holes.
Types of rafter system
It has already been noted that in the lower region the rafter legs lie down and are fixed to the Mauerlats, 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 supports, and as they rise, they are supported by racks, struts and girders included in the crate.
In the upper part of the device of the rafter system of the layered type provides for the presence of a ridge. Two roof slopes will rest on it. The advantages of this configuration include the possibility of uniform distribution of loads over the entire area with tolerances for increasing the length of the rafter legs. In particular, if there are two runs, the crate can reach 12-15 m.
An alternative variant of the layered system involves a hanging arrangement of the truss frame. The fundamental difference of this scheme is the rejection of intermediate bearing support elements - for example, purlins and crossbars. The entire load is transferred tomauerlats and skate. What makes up for the support of the hanging truss system? Firstly, its implementation is allowed only in houses with walls, the distance between which does not exceed 6.5 m, which in itself causes weight reduction. Secondly, the roof will have not two, but four slopes that will converge in one central ridge, and this also increases the bearing potential of the system.
System design
Initially, you should create a design sketch 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 size of individual elements. For example, the distance from the ceiling at the level of the muaerlat to the ridge should be at least 2.5 m. The supporting structure can be represented in the form of a square or rectangle. The slopes should be taken away from the center - again, the project of the truss system for a private house should provide for two or four slopes, which will also determine the configuration of the frame placement. The main thing is to maintain the symmetry of opposite slopes.
Next, the optimal dimensions 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 sum them up and display the point with the highest peak. The ridge will be located in this zone - along the entire line along the roof, or in the form of a small segment in the center, as is the case with a four-slope system.
Calculationload-bearing structures
Often, wind and snow loads are taken into account when choosing the type of roofing "pie", which is not entirely correct - the power qualities of a roof with an external coating directly depend on the truss structure. First you need to calculate the slope. The sharper the angle of the crate, the less precipitation and, most importantly, the snow mass will linger on the roof. For the upper ridge rafters, it will be about 15-30 °, and for the lower ones - at least 60 °.
Wind loads can also vary depending on the slope angle. In this case, the geometry and plane of the roof will be important. So, a steep slope will be thrown by the wind, and a flattened slope will lift it from the leeward side. The horizontal direction of air flows should be taken into account. The optimal configuration of the truss system allows them to be broken with pressure removed to the foundation and tangentially from the roof overhang.
In addition to climatic factors, the own load from the mass of the rafter and roof structure is also important, which is calculated in kg per 1 m2. Specific data are formed on the basis of a separate calculation of the mass for the following items:
- Weight of lumber used for beams, poles, girders, battens, etc.
- Weight of insulation materials.
- Weight of substrates and roofing.
- Weight of fasteners, fittings and connecting rods.
The roof will make the greatest contribution to the load. The lightest will be the truss system for corrugated board, on which, on average,lays down 4-5 kg/m2. For shingles and slates, the average values are 10-12 kg / m2. And a special approach should be taken in calculating the pressure on roofs with ceramic and cement tiles. This is a heavy roof, exerting a load at the level of 30-40 kg / m2. As a rule, the total pressure force of the roof decking does not exceed 50 kg per 1 m2, so this value can be used as a standard value in the calculation. Even better, leave a small margin in case of force majeure.
What material is used in the construction?
To implement a high-quality and durable bearing system, lumber of suitable characteristics should be used. Optimally suited pine, spruce and other conifers, characterized by flexibility, strength and ease of processing. And only the highest grade can be used. If, in order to save money, it was decided to give preference to the 2nd and 3rd grades, then you will have to perform self-treatment of the material with antiseptics.
Special attention is paid to the structure. Mistakenly, many believe that the most durable is a solid timber. It is for the roof that experts recommend glued segments that do not undergo deformation and endure dynamic loads. Don't forget about humidity. From wood with a moisture content of up to 20%, the roof truss system is made, insulated from the inside with gypsum boards. Increasing the coefficient to 23-25% means that some of the elements will have to be located outdoors. The worst option would belumber from freshly cut solid wood, the moisture content of which is 30%.
Laying Mauerlat and ceilings
The base of the truss frame in the form of a Mauerlat is installed along the perimeter on the walls. A layer of waterproofing or roofing material is placed between the last brickwork and the timber. At the stage of the Mauerlat device, the installation of the truss system is carried out using anchors, studs or wire. It is desirable that the fixing elements and the fastening scheme itself allow for the possibility of being embedded in the wall masonry.
Then the transverse beams are laid from the same timber. 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 with metal corners or self-tapping screws. At this stage, specific normative parameters for distances and sizes should be maintained. So, the construction of the truss system in the bearing part should be carried out with a distance between the beams in the range of 50 to 100 cm. The optimal step is 60-70 cm.
Installation of rafters
At this stage, a beam for the side rafters and adjacent structural elements is prepared and mounted. For fastening to the Mauerlat, connecting grooves are cut in the beams and, if necessary, holes are made. In addition to integrating into the niche of the beam, the roofer must ensure that the metal staples are fastened with a mounting stapler.
The rafter legs from the Mauerlat are heading towards the ridge line. First you need to determine its center. In this area, a temporary rail is nailed on which the end will be held. The fastening of the beams at the top point is carried out with the help of metal plates and large-format hardware - from nails to brackets. Do not ignore the use of grooved joints. The combined installation of the truss system with locks, corners and bolts will increase the reliability of the structure.
When the frame is assembled, you can lay the resulting gables with bricks, boards or metal profiles. At the same time, do not forget about the ventilation holes and windows.
Isolation Operations
Even if the attic under the rafter 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, it is also necessary to provide for a counter-lattice, an air gap for ventilation, a film to prevent moisture penetration, etc. The lathing is formed by slats and timber on the supporting beams. If the installation of the truss system is implemented using metal pipes, then it is best to continue the rear superstructure of the crate over the entire area of \u200b\u200bthe slope from them. This will add reliability and durability to the structure. The only thing that should be provided as an addition is the treatment of metal surfaces with protective anti-corrosion compounds.
First of all, a waterproofing agent is attached to the rafters. You can use a universal membrane hydrovapor barrier, laying it tightly,not overlap. The joints must be glued with construction tape for sealing. The insulation layer is mounted from the inside. It can be expanded polystyrene, mineral wool or felt materials. It is important to emphasize that the device of the truss system should be maximally focused on ensuring 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 truss structure with a ridge and intermediate supporting elements is convenient for both operation and maintenance. This configuration does not limit the owner of the house in the possibilities of functional expansion of the roof. For example, air ducts, snow catchers, windows with hatches, etc. can be integrated into it. The main thing is that the roof truss system constantly meets the basic technical and operational requirements, coping with the loads. And for this it is necessary to periodically revise the rafter legs, supports and crossbars. The structure of wood must be periodically treated with moisture-proof, biological and refractory impregnations. The choice of specific compositions will depend on the type of lumber, the same moisture coefficient and operating conditions. Special attention is paid to fasteners. Metal elements, for example, must be replaced at the slightest sign of corrosion. For them, special protectivefunds.
