The basis of the wooden roof of the house is the frame that holds the roofing. It contains a large number of elements, each of which is necessary and intended for a specific function. One of them is a support for the rafters, sliding, which serves to prevent deformation of the roof during shrinkage of a wooden house when its geometry changes.
Shrinkage of a wooden house
In one year, the log house can become lower by 15%. The average shrinkage is 8%. At the same time, it can occur unevenly, which negatively affects the condition of the rafter system. The resizing of the wooden structure takes place all year round. In the rainy summer season, the wood swells, and in the winter the material dries out. In addition, the walls gain and lose moisture in different ways, depending on the light and wind rose. If there is support on the central load-bearing wall, the deformations of it and the external walls differ significantly, which also affects the uneven changes in the geometric dimensions of the house. In the heating period, the walls, especially the central one, decrease in size from drying out.
In the absence of a central wall, even greater problems may arise. If the rafter system is not assembled as a truss, its rigidity is much lower. As a result, not only vertical, but also horizontal loads act on the walls. Under the influence of pressure, they can bend out, and the house in the plan will take the form of a barrel. In this case, the roof is deformed and sags in the ridge. "Pyatistenka" allows you to get rid of this problem or significantly reduce it.
Screw Jack Application
To compensate for the precipitation and seasonal movements of the house, screw jacks can be installed. They should be located on all vertical supports in the house. Tightening the jacks with wrenches compensates for the movements of the log structures.
The purpose of the use of sliding supports
It is not always advisable to rigidly fasten the rafter legs to the Mauerlat or to the floor beams with nails, brackets, etc. If the box of the house can shrink, it is necessary to create the ability of the rafters to move relative to the supports. Previously, this was done with the help of forged wire, which created a connection with a certain degree of freedom of the parts of the structure.
Since then, the technology has changed, and more advanced devices, such as a sliding rafter support, have been used. It is attached to the walls as follows:
- to the upper rim of the log house;
- to the upper harness of the frame structure;
- to floor beams;
- to Mauerlat.
Why are the supports made sliding?
The sliding support makes it possible to compensate for the distortions of the roof frame during its operation. When the wood dries, the geometric dimensions of the structures change. At the same time, the width of the house decreases, but due to sliding along the beam of the overlap, the sagging or bursting of the roof, leading to subsequent leaks, does not occur. The reason for the ingress of precipitation inward may be different, but in no case should one underestimate the influence of deformations from shrinkage.
Advantages of sliding supports
- Creating a reliable connection of the base with the roof.
- Ensuring the mutual movement of wooden structures with shrinkage of the base, preserving the strength and tightness of the roof.
- Simplicity of the device, reliability and durability.
- Low cost and the ability to do it yourself from improvised materials. At the same time, steel plates must be at least 2 mm thick and protected from above with an anti-corrosion coating.
Design features of rafters
A pair of rafters of a gable roof is made according to one sample. The connection in the ridge is done end-to-end, with a gap or an overlap. The rafters are interconnected by a metal plate with bolts and nuts. The main design feature is the creation of sliding mounts in the lower supports. Rafters are placed on the Mauerlat on top or with an insert for fixing the structure in a vertical plane.
The support for the rafters is sliding, serving as a fastening element, is made of two parts:
- strap fixed on the rafter leg;
- corner with a loop attached to the supporting beam (Mauerlat or support beam).
There is an even simpler version of how a sliding support for the rafters can be made (photo below). For this, a corner with a slot is used. It is mounted on the rafters along its axis, and in the slot with a gap is the head of a bolt screwed into the body of the Mauerlat.
This support for the rafters (sliding - Kucis 120x40x40) costs only about 25 rubles apiece and is easier to install than others.
Devices are made of galvanized steel plates. They are mounted on one or two sides of the rafters. The two-sided option is preferable for roofs with high wind and snow loads and low mass of the roof structure. Fasteners are used with anti-corrosion coating, so as not to spoil the wood. The stroke length depends on the expected displacements and ranges from 6 to 16 cm.
Open and closed supports
Open-type sliding supports for rafters ("glides"), consisting of two parts, can be made by yourself, but you should use a template. All parts must be sized. An open connection consists of a curved plate (mounting plate) and a corner with holes for mounting (at least five).
In a closed connection, the mounting plate is inserted into the corner holder with the possibility of movement, but cannot be removed from it and is not separable. The assembled design allows you to perform installation more efficiently. If there is an effort to detach the rafters from the Mauerlat under the influence of wind loads, the loop can be unbent, and the sliding support of the rafters closed has greater strength.
Important! It is advisable to produce a rafter system with the possibility of sliding relative to the base only for a symmetrical gable roof. This is its advantage, because it increases reliability at low cost. Supports can be used for a pitched roof.
Sliding support for rafters: dimensions
The support includes two components - a guide plate and a corner. The size of the permissible movement of the rafter leg depends on the size of the product.
The popular sliding support for Kucis rafters of open and closed type has a plate width of 40 mm, a support height of 90 mm, a length of up to 160 mm. The metal for the plates is taken 2-2.5 mm thick. They are profiled by cold stamping of low carbon steel, which has good ductility. Corrosion protection is a hot dip zinc coating. When purchasing uncoated products, they should be painted with paint so that the material does not rust.
The corner is attached to the support beam. If its surface is round, a flat area is prepared for installation, grinding wood on a log to the size of the L-shaped part of the part. The photo below shows a similar sliding support for the rafters closed (Paz). Both types of products are stamped and inexpensive.
The rails are attached to the rafters in the extreme position to ensure maximum movement. Since when the roof shrinks, the angle of inclination relative to the ridge changes, in the upper part of the rafters it is necessary to make an articulated bolt connection. Special perforated plates can be used here.
Mounting
The support for the rafters sliding during the installation of the roof frame is used as follows.
- Rafters are made of boards with a width of 150-200 mm and a thickness of 50 mm. With insufficient length, the rafter leg is lengthened, overlapping with a chaotic arrangement of fastening elements to prevent cracking.
- A run is installed from above - a supporting beam located horizontally and perpendicular to the sliding rafters.
- In the lower part, the rafters are fixed on the cuts made in the Mauerlat. In this case, their jamming is not allowed. But at the same time, the cuts should keep the boards from lateral distortions. The presence of Mauerlat evenly distributes the load on the entire building. In its absence, the local pressure on the walls from the rafters becomes greater.
- Fittings of suitable sizes are selected depending on the range of displacement of the rafters.
- A platform is being prepared under a corner on a support log.
- The corner is attached perpendicular to the log to ensure free movement of the plate.
- The guide is inserted into the loop of the corner and is attached to the rafter along its axis. Installation is done in the lowest position, since the wall from the bottom will be compressed and reduced in size. In this case, the rafter leg will fall down.
- Rafter boards are connected from above by articulation.
Conditions guaranteeing the strength, reliability and durability of the roof
- Compliance with the necessary mechanical properties and dimensions of the sliding supports for rafters.
- The location of the guide support is parallel to the axis of the rafters, and the corner is perpendicular to ensure reliable sliding.
- The cross section of the boards under the rafters should not be less than 150x50 mm.
- The roof frame must be treated with an antiseptic and a flame retardant.
- Compliance with the correct proportions of the roof frame.
- Production of the entire rafter system according to the template.
- Creating a hinge ridge.
- Raising the rafters should be done alternately in the lower and upper parts.
Sliding support for rafters: application for other purposes
Compensation of shrinkage of wood is used not only on roof frames. It is used wherever it is necessary to create the ability to move parts of wooden structures, for example, when erecting partitions in wooden houses. If it is necessary to finish the wall in the bathroom with ceramic tiles, it is simply impossible to sheathe it with sheet material.
It is necessary to install a "floating" frame fixed on top of the sliding supports, and then fasten to it drywall, on which the tile will be glued. Moreover, the sewing is not done right up to the ceiling. When the walls are deformed, the frame will remain motionless, and the lining will not collapse.
Conclusion
Sliding supports for rafters in wooden houses are necessary to prevent deformation of the roof during shrinkage of the house. It is especially active in the first years after construction (about 8%). Correct installation of rafters will ensure constant tightness and strength of the roof.