Metal rafters - design, mounting and calculation
Rafters and lathing - roof frame, which bears the brunt of the roofing pie, thermal insulation, snow load, so they must have a high bearing capacity, durability. The idea of using metal rafters to make the construction of the roof more rigid, not new, but earlier it was used mainly to cut off industrial or economic structures. Now the truss frame and the sheathing of metal profiles are considered to be a real alternative to the wooden elements of the roof, if the length of the slope exceeds 10 meters.
Functions rafters
The roof truss structure consists of a set of interrelated elements that form trusses. The pitch, the size of the cross section between the rafters and other supports determines the calculation of the loads to which they are subjected during operation. The roof frame performs the following functions:
- Load distribution. The interconnected units, reinforced by corners, evenly distribute the weight of the roof, which can be up to 500-600 kg, taking into account the snow load. The larger the section of the rafters and the smaller the pitch between them, the greater the bearing capacity of the structure.
- Grading and shape. Rafters at an angle to the base of the roof, form an inclined plane of the slopes, so that on the surface of the roof does not accumulate snow and water.
- Formation of the basis for fixing the roofing material. Fastening the topcoat of the roofing pie occurs to the roof frame. The lathing acts as a basis for fixing the coating, distributing its weight evenly across the truss beams.
Note! What should be all the nodes, rafters and lathing of the roof structure determines the engineering calculation. To determine the required bearing capacity of the frame, it is necessary to calculate the total load to which it will be subjected. To do this, add the weight of the roofing material, insulation, waterproofing, maximum snow load with the weight of the truss system.
Types of roof systems
The most common material from which the lathing and rafters of the roof frame are made is wood. However, if the weight of the roofing material is large enough, and the length of the slope is more than 6 meters, the design is too massive. Builders have to reduce the step between the rafter legs, to increase their cross-section, because of which the roof nodes gain a lot of weight, increasing the load on the foundation. It is possible to unload the bearing walls and the base of the building, using more durable, but lightweight metal rafters. By the type of material used, the following types of roof systems are distinguished:
- Wood. Rafters and lathing of wood used for the construction of roofs, the length of the slope of which does not exceed 7-10 meters. The fastening of the frame elements between themselves occurs with the help of screws, nails or moving metal elements. The step between the legs is usually between 50-80 cm.
- Metallic. Metal roofing frames are made of steel metal profile with a zinc coating, which is not afraid of moisture. Rafters and lathing of this material are lightweight, durable, so the step between them can be increased to 1.5-2 meters. The fastening of the metal profile is performed by welding or fasteners. Metal roof nodes are used for the length of the slope from 10 meters.
- Combined. The rafter framework combining metal and wooden knots is called combined. The combination of the supporting elements of wood and galvanized steel allows you to make a cheaper structure with high bearing capacity, increasing the step between the rafters.
Note that the metal and wooden elements of the frame can not be interconnected without a lining of waterproofing material or treatment with an antiseptic preparation. Since the metal has a high thermal conductivity, its proximity to the tree leads to the formation of condensate and decay of the rafters.
Attachment methods
Metal rafters are assembled in trusses of a triangular, trapezoidal or arched shape. To the frame beams attach internal stiffeners, which form the corners, significantly increasing the carrying capacity of the frame. This system allows you to take a step between the rafters more, making a calculation on the support capabilities of each farm. The fastening of the metal elements of the roof frame is performed by one of the following methods:
- Fastening with welding. If you weld the details of the truss system using a welding machine, you can get a rigid structure with high strength and carrying capacity. If you correctly perform the calculation of the roof, you can lighten the frame and reduce the load on the foundation of the structure. The disadvantage of this method is that only professional can perform welding using special equipment.
- Fastening with bolts. Fixing the rafters with fasteners allows for less rigid fastening. This method of assembling the truss frame on the basis of metal profiles is used in private house construction, where the length of the slopes does not exceed 10 meters. Failure to weld speeds up the installation of the roof.
Skilled craftsmen make the calculation that the metal crate and rafters can withstand more weight than wood, so you can increase the step between them and reduce the thickness of the cross section of the elements. Moreover, the building stores sell finished truss trusses, the fastening of which was carried out by welding, suitable for overlapping buildings of standard width.
Benefits
Metal truss frame is used for the construction of roofs of any shape, any sloping with a slope of 1-2 degrees. Steel corners, pipes of round and rectangular section, and taverns are used as the material from which the rafters and the sheathing are made. In order to correctly choose the thickness of the frame elements and choose the step between them, the roof structure is calculated, taking into account the permanent and temporary loads that are transmitted to the truss beams during operation. Advantages of the truss system from this material are considered:
- Fire safety. In contrast to the wooden, metal beams of the frame is not the country of ignition, which increases the fire safety of the building.
- Ease of maintenance. The zinc plating coated with steel corners protects the roof frame from corrosion during its entire service life. They, unlike wood, do not require annual treatment with antiseptic.
- Waste-free. Although metal structures are quite expensive, it is considered cost-effective, since the fastening is performed by the welding method, and there is practically no waste.
- Long service life. If you correctly carry out the calculation of loads, then the roof frame made of metal will last more than 100 years, which exceeds the service life of even the most stable roofing.
Professional roofers believe that it is advisable to use welded steel structures for the manufacture of the roof truss frame with a slope length from 10-12 meters. The main task is to correctly calculate the loads, and then determine the step between the legs in accordance with the climatic characteristics and the properties of the roofing material.
disadvantages
Despite the obvious advantages, metal truss systems are not the most popular constructive solution in private housing construction. Even a big step between the rafters and the sparse crate at a high cost of metal cannot make the structure cheaper than wooden. The disadvantages of metal rafters consider:
- High thermal conductivity. The metal has a high coefficient of thermal conductivity, so the rafters and the lathing of the roof form cold bridges. From the point of view of energy efficiency, the metal profile frame is not the best solution.
- Problem of transportation and installation. When choosing a metal truss system, take into account what to transport, raise to a height and carry out the fastening of long and heavy elements more difficult than rafters of standard length lumber.
- The complexity of installation. For lifting and fixing farms, special equipment is used, the rent or purchase of which is expensive.
- Deformation under the action of high temperature. Although the metal is considered non-combustible materials, during a fire it is strongly deformed, which usually leads to the collapse of the roof.
To determine whether it makes sense to use more expensive trusses from metal profile, it is necessary to perform the calculation of the roof frame. If the length of the slope exceeds 10 meters, and the load is more than 450-600 kg, the installation of a metal frame is economically viable.
