The erection of the roof of the house is the final stage of construction. This is a responsible event, on which depends both the longevity of the operation of the building and the safety of the people living here. One cannot do without a correct calculation in this matter.
A mandatory element of the entire system is the rafter leg. It will have constant and temporary loads. Therefore, before installing the roof yourself, you need to familiarize yourself with all the details of this process. There are certain rules and regulations that govern the arrangement of the rafter system.
general characteristics
The roof truss system is the main supporting structure. It has the main burden. To ensure the reliability of the system, each rafter leg is fixed rigidly. This allows the roof to withstand gusts of wind, snow drifts and other influences.
The material for such structural elements is most often wood. It is easier to adjust, and the cost of work is significantly reduced. Particular attention should be paid to special antiseptic and fire-retardant treatments.
The angle of the rafters is equal to the slope of the roof slopes. The bottom of the rafters rests on the Mauerlat. This allows you to evenly distribute the load. The top of these elements rests on a bar under the ridge or intermediate rails.
Rafter legs are sometimes also called a diagonal or oblique foot, and also simply a rafter.
Load
The rafter leg is subjected to various loads. These are permanent and temporary varieties. The first group includes the total weight that the rafter system has, as well as other roofing materials (vapor barrier, waterproofing, roofing material, slate). Also here include elements of the interior decoration of the room or attic.
Temporary loads can occur due to a variety of reasons. They can be calculated only approximately. In this case, take into account the maximum possible precipitation that can be observed in this climate zone. Also, gusts of wind can affect the roof. If in this area they are long and have great strength, this feature should be taken into account in the calculations.
It is necessary to take into account the weight of people who will perform the construction or maintenance of the roof in the future, as well as the total value of their tool. Take into account the load that will be provided by additional equipment (chimney, antenna, aerator, ventilation, etc.). If additional structures are installed on the roof, their weight is also taken into account.
Calculation Rules
To perform the calculation of the rafter legs, after assessing the load, you should determine the type of material, as well as the type of the roofing system itself. It can be suspended or layered. In the first case, the rafters have two points, but of which there are extreme supports. This creates a horizontal bursting force. Rafter legs perform work on compression and bending. To reduce this force, they are connected by stretching.
The generic type of construction is applicable in buildings with an average bearing wall or intermediate supports, the ends of which are adjacent to the external walls. The rafter leg at the same time performs the function only on bending. In this case, the total cost of the material is reduced. But such a system is suitable only for those buildings in which the supports are separated from each other no further than 6.5 m.
When performing the calculation, the angle of inclination of the roof is also taken into account. At this stage, all additional elements of the system are also considered.
Length
The length of the rafter leg is also calculated in accordance with all existing operating conditions of the roof. Initially, you should determine the size of the building itself. This takes into account all the ledges, porches, attics, which contains the construction plan.
Next, the shape and angle of the roof are considered. Next, based on the data obtained, choose the length of the rafter leg. Usually this figure does not exceed 6 m. This is due to the standardization of sawn timber for sale. To purchase longer varieties of products, you have to pay a decent amount of money.
If necessary, these structural elements are increased. The need for long rafter legs sometimes arises with the arrangement of hip, half-hip or diagonal rafters.
Section
After calculating the length, it is required to determine the cross section of the rafter leg. Its type is influenced by several factors. All operating conditions specific to the roof will matter in this process. The level of loads, the slope of the roof, the type of roof must be taken into account when choosing a section. An important factor is the total area of the building, its configuration. The more factors will affect the operation of the building, the wider the cross-section of the rafters should be chosen.
Also, do not forget to take into account the features of the climate zone. It is important to immediately plan what material is used during the construction of rafters. Most often it is wood. She must be of good quality. Humidity of wood for rafters should be 20-22%.
Today, there are many special programs that allow you to calculate the cross section of the rafter leg. To prevent mistakes, you should use them. But it is more correct to entrust this work to a professional. The most popular section size is 50 by 150 mm.
Laying step
The rafter leg, the dimensions of which were calculated in accordance with the building rules, requires proper installation. To do this, you need to calculate the step of laying these structural elements.
In this case, the load that the above factors have on the roof plays an important role. Correctly choosing the parameters of the rafters using special tables, you can calculate the installation step. With a standard cross-sectional size (50 by 150 mm) and a length of 3 m, the distance between the rafters will be 1 m.
If the load level is high, this figure is best reduced. This is especially true of slate roofing. In the case of using rafters with a standard cross section, but with a length of 4-4.5 m, the pitch between the structural elements is reduced to 60 cm.
The slope of the roof is also important in the calculations. For example, with a slope of 45 degrees, the step of the rafters can reach 120-140 cm. But in the case of a large amount of precipitation in this area, this figure decreases to 60-80 cm.
Installation Basics
The rafter leg assembly consists of several elements. These include deadbolts, struts and struts. Particularly important is their participation in a system of small width. If this is not done, the rafter structure may sag over time. Therefore, in such cases, a special lattice is equipped.
The rafters are known to rest on Mauerlat. It is placed over the entire width of the building or laid only under the bottom of the structural elements. If the rafter system is wooden, the Mauerlat is made of logs or timber. This is the top of the log house. But in brick buildings, the Mauerlat is installed flush with the inner surface of the walls. A layer of waterproofing is laid between them.
Installation of a conventional roof
The rafter leg and Mauerlat are very important elements of the whole structure. The quality of the roof depends on the correctness of their installation. They can be attached in two ways. Mounting can be rigid or sliding. The correct variety is selected in accordance with the type of rafter system (layered, hanging).
Rigid type of fastening provides design immobility. At the same time, they make cuts on the rafters. Structural elements are fixed with metal brackets, angles, wire or long nails.
A sliding joint is also called a hinge. If it is necessary to ensure freedom of subsidence of wooden elements on the log house, this type of installation is preferable. They make a gash on the rafter's leg and fasten it with a Mauerlat with two nails. They drive in diagonally. Another nail is driven from above.
Hip Roof Mounting
When arranging a hip roof, the rafter leg is often longer than 6 m. Extension is required here. Two rafters pair. In this case, it is required to perform their amplification. This procedure involves the arrangement of struts. These are vertical racks, which are installed in the design no more than 2 pieces.
These reinforcements are supported by a wood lining. It is mounted for overlapping or tightening. Diagonal rafter elements are always longer than ordinary ones. They are subjected to a load that is 1.5 times more than usual. Therefore, the amplification procedure in this case is simply irreplaceable.
Having become acquainted with such an element as a rafter leg, every novice builder will be able to delve deeper into the process of erecting a roof. If you comply with all building codes and rules, you can achieve high quality construction.