Roof device and installation: rafter system

Recommendation points

• What loads does the roof experience during operation
• What are the rafters
• Basic information about roof trusses
• Lathing: what it is and what its parameters depend on
• What are attics and attic floors
• About eaves and dormers

This article provides basic information about rafters, trusses, lathing, attics, attics, eaves and dormers. The information will be useful to clients of construction organizations specializing in the installation of roofs.

The roof is not only a spectacular roofing material that reliably protects the entire structure from precipitation. The skeleton of the entire structure is a rafter system: the service life of the entire roof depends on its strength and reliability. Therefore, at this stage of construction, quality control of work performance should be strengthened. But in order to know what to look for, you need to familiarize yourself with the properties of the material and study the installation rules.

What loads does the roof experience during operation

The rafter system is a structure assembled from individual elements, designed to evenly distribute and transfer the load from the roof to the walls and partitions. Rafters are steel, wood or combined: the choice depends on the design solution. But the main requirement that is imposed on them is general: they must withstand the loads that atmospheric phenomena and the weight of roofing materials exert on them:

1. Snow. Unlike rain and icicles, which the supporting structures must cope with, it exerts serious pressure on the roof: depending on the characteristics of the local climate, the mass of snow per 1 m² the roof surface can be from 100 to 170 kg. For areas with snowy winters, the roof is built with slopes, the angle of inclination of which is 30 degrees or more: the snow will not linger on them for a long time. At an angle of inclination of more than 60 degrees, the load exerted by the snow is not taken into account.
2. Wind. Strong gusts can tear off sections of the waterproofing coating, but this is not the biggest danger that threatens the roof. Air currents that bypass the building at high speed create an area of ​​reduced pressure above the roof: the pulling force can damage it. To prevent this, the roofing device at the base must be made as tight as possible, the fastening of roofing materials to the base must be reliable.
3. Temperature fluctuations. They can also hurt. Compression and expansion of structural elements occurring over the years does not have the best effect on the condition of materials. In order to reduce the detrimental effect of these processes, they try to select roofing materials that have an approximately equal coefficient of thermal expansion.
4. Water vapor. Accumulating under the roof, it is absorbed into all structural elements, gradually making them unusable. This problem is partially solved with the help of a vapor barrier film that provides adsorption and removal of steam..
5. Insects and microorganisms can completely destroy the rafter system in a short period of time. To combat these pests, special impregnations are used..

All of the above factors have a periodic effect on the rafters (snow, strong wind) and constant (the weight of structural elements), but a combination of several at once is possible. Therefore, the correct calculation and installation of load-bearing elements is very important..

What are the rafters

We will talk about the most common systems: complex architectural structures are created according to a different principle. There is a classification of rafters according to the principle of “work” and device. They are of two types: the first includes layered rafters, the second – hanging.

Roof rafters are used most often: rafters of this type can be seen in houses with internal load-bearing walls or pillars. They are beams installed at an angle, one end of which is attached to the ridge girder (upper horizontal beam), and the other rests on the Mauerlat (beam laid on a load-bearing wall). The weight of the roof presses on the ridge girder, and from it the load passes to the system of struts and struts. The uprights are vertical bars, and the struts are set at an angle. They, in turn, press on the inner pillars and walls of the building..

This design can be used if the distance between the supports or the width of the spans does not exceed 16 m (data obtained as a result of the calculation). For long distances (when the length of the rafter leg exceeds 8 m), intermediate supports must be installed. It is not always possible to increase the cross-section of a bar or log in order to provide the necessary rigidity: the building may not withstand the large weight of the rafter structure.

With a span of no more than 6 m, hanging rafters can be used: they can be installed in the absence of internal supports. As in the case of the layered structure, the hanging rafters rest on the lower part of the Mauerlat, and the upper part is connected in half-wood or with a slotted spike. For additional fastening of the upper ends, you can use brackets (of course, provided that wooden beams or logs are used to make rafters). The rigidity of the structure can be given using a crossbar – transverse tie.

Rafters can be made not only from wooden logs, beams or planks: metal profiles are also a suitable material. Due to their special shape, they have sufficient rigidity, and they can be connected by welding.

The distance between the rafters depends on the climate in the area, the type of roof and the features of the sheathing device: it can range from 300 mm to 1 m.

Basic information about roof trusses

The best solution in many cases will be a roofing device based on trusses. They are used in houses with an open plan, without internal supports. The shape of trusses is most often a triangle (this is the most rigid geometric figure), assembled from wooden, metal parts or cast from concrete. At the same time, in the middle of the triangle, stands and struts are installed, designed for uniform distribution of the load, and its internal angles correspond to the angles of inclination of the roof slopes. Roof trusses are placed at regular intervals (within 1–2 m) and fastened to each other with horizontal jumpers: this is how the roof is formed.

In addition to the triangular shape, roof trusses can be polygonal or trapezoidal: outwardly, their design resembles trusses for building bridges. The first option assumes the presence of a horizontal middle part and two fractures, and the second is used with a minimum slope of the slopes. This roof looks the same as a flat one, but in fact there are slopes. Roof trusses of these types are well combined with new roofing technologies based on roll materials that do not require (unlike other roofing materials) large slope angles. Polygonal and trapezoidal trusses do not require large production costs, since they are created from a relatively small amount of building materials: the struts and struts in their structure are short. But there are also disadvantages, the main of which is the impossibility of arranging an attic. This forces the use of such systems mainly for the construction of industrial buildings..

Lathing: what it is and what its parameters depend on

On top of the rafter legs, beams are attached, designed for laying roofing materials: they are nailed to wooden rafters with long nails. These beams are collectively referred to as sheathing. In addition to the bearing function, the lathing plays the role of stiffeners and at the same time evenly distributes the load on the rafters.

The step of the lathing is selected depending on the roofing material used: for example, each type of metal tile has a profile that differs from the profile of the same material of another type. Their stiffeners are located at different distances. The same applies to any type of tile (ceramic, polymer, cement-sand). For laying sheet roofing materials, a sparse lathing with a step of 30-40 cm is used.

But some materials, such as flat slate, mica, euroruberoid, bitumen shingles, require a continuous sheathing. Such a crate can be created from moisture-resistant plywood or OSB sheets: this is due to the fact that the installation of flexible tiles and euroruberoid is carried out by gluing and tightness is possible only on condition of a monolithic base. In addition, solid sheathing is required on the ridge, valleys, eaves and ribs..

What are attics and attic floors

Angled slopes form an attic space. The owner of the house must determine how it will be used. Utilities can be laid there (ventilation and air conditioning system), a warehouse or even a living room equipped. Depending on the purpose, the attic can be arranged with or without insulation. But the residential attic, called the attic, is necessarily insulated: mineral or glass wool, polyurethane foam plates are laid between the roof and the crate.

Skylights should be designed to provide the largest usable area under the roof. For this, a break in the cross section is provided: with its help, a high ceiling of the attic is formed due to the flatter upper part of the slopes and a steeper lower one. It should be noted that the attic floor must withstand significant loads, which means that it must be built according to the same principle as the floors of the lower floors..

About eaves and dormers

Eaves overhangs are necessary to protect the outer walls from rain and melted snow: they simply remove moisture for a certain distance. In addition, they protect the attic from blowing and sweeping snow. Therefore, the rafters must necessarily extend beyond the outer walls: the minimum overhang is at least 0.5 m, the maximum is up to 2 m. The choice depends on the amount of precipitation that falls per year in a given area, as well as the structural features of the roof of the house. In the event that the rafters do not go beyond the walls, the overhang is formed using special “filly” boards that are nailed to the lower edge of the rafters.

Dormer windows are essential to provide natural lighting and ventilation in the attic. They can also be a kind of roof decoration: they are framed with carved details and embossing, stylistic painting.