Rafters are the main supporting structural element of any roof. There are many ways to install them. Very often, the roofs of houses are collected, for example, on layered expansion rafters. Their main feature is that they work during the operation of the roof and bending, and compression. That is, not only vertical, but also horizontal force is transmitted to the walls.
Design features
In fact, such rafters are a transitional option from non-piling overhead to hanging. They transfer vertical load through the ridge run and racks. But such rafters exert pressure on the walls due to the fact that they are attached to the Mauerlat or beams rigidly. With this method of fixation, horizontal stress inevitably arises in the roof frame. The layout of the expansion rafters, presented below, demonstrates the principle of their operation visually.
What to consider when designing a project
Before mounting the roof, of course, detailed installation plans for all of its elements, including rafters, should be developed. When designing a roof frame, you should first determine:
- with the angle of inclination of the rafters;
- their cross section;
- the type of lumber used;
- installation step;
- mounting methods.
What should be the angle of inclination
It is worth approaching the calculation of this parameter as responsibly as possible. The reliability and durability of such a design as a roof with rafters are directly dependent on how correctly the angle is chosen. It’s best, of course, to build a better roof over the building. In this case, it will be possible to equip a residential attic or attic in the house. In addition, suburban buildings with such roofs look more solid. However, steep roofs cost, of course, more expensive than flat roofs.
In addition to the features of the exploitation of the future roof and the architecture of the building, when choosing the angle of inclination of the slopes, factors such as:
- total weight of the roofing material used;
- possible snow load;
- wind load.
For each specific roofing material there is its own minimum acceptable indicator of the angle of inclination. So, for example, for a metal tile it is 12-14 degrees, for slate - 22 degrees, etc. When developing a project, this, of course, must be taken into account.
Possible wind and snow load on the roof is determined by special tables compiled for each specific region. In most regions of Russia, the optimal slope angle of slopes, taking into account these indicators, is 30-45 degrees.
What lumber to choose
For obvious reasons, the most important part of the roof frame is precisely the rafters. The design (both the nodes and the legs themselves) of them, as the main supporting element in this case, should be as stable and durable as possible. Therefore, the choice of lumber for them needs to be approached as responsibly as possible.
Most often, pine beams are used for roof rafters of country houses. Such wood contains a lot of resins, and therefore it is quite resistant to moisture. In addition, pine lumber has a sufficiently large margin of safety and is not too expensive.
How to make the calculation of layered rafters
The cross-section of the beam for the supporting elements of the roof frame is calculated taking into account the following factors:
- total weight of the roofing cake;
- step between rafters;
- their lengths.
Mounted rafters can be installed in increments of 50 to 120 cm. In regions where there is a lot of snow in winter, the distance between these elements is usually made smaller. The same applies to roofs trimmed with heavy roofing material, such as clay tiles. If there is not too much rainfall in the region during the cold season, you can save and install the rafters with a big step. In the European part of Russia, when using ondulin or a profiled sheet of roofing for roof sheathing, rafters are usually mounted at a distance of about 80-100 cm from each other.
As for the required length of the legs, it is quite easy to determine it. The design of the roof rafters, like any other, is such that in the frontal projection the roof of the house is usually an isosceles triangle. Therefore, in order to calculate the leg length, you just need to divide half the length of the span of the building by the cosine of the slope angle. To the resulting figure should be added about 40 cm to the overhangs.
Knowing the length of the rafter legs and determining the distance between them, you can easily find out what section they should have. For this you need to use special tables. Most often, a beam with a cross section of 150x150 cm is taken under the rafters. Under the Mauerlat, since it has a serious load during operation, they usually use a thicker material - 150x200 or 200x200 cm.
How to fix it below
Mauerlat to concrete and brick walls is usually fastened to anchors previously embedded in masonry or mortar. There are several ways to fix the rafters themselves:
- by cutting;
- using galvanized corners and bolts;
- using nails, clamps and wire.
The rafters are mounted when assembling the roof frame either directly on the Mauerlat, or on the beams that extend beyond the plane of the plane. In the latter case, fastening can be performed in two ways:
- On steep roofs (more than 35 degrees), the felling of the rafters into the beam is often made with a single tooth. In this case, a spike is cut out in the leg, and a special emphasis in the counterpart. Actually the fastening itself is carried out using a bolt.
- On flat roofs, to increase the supporting area, the connection is made by cutting with the use of a double tooth.
When attaching directly to the Mauerlat, only the landing nest is cut out in the rafters. In this case, rigid fixation is most often performed using special galvanized perforated corners. They are installed on the sides of the legs and bolted. To reduce the horizontal load on the walls from the roof, sometimes a beam is also stuffed on the rafters from below (for emphasis in Mauerlat). Such a fastener is usually additionally fixed with a wire rod. It is embedded in the wall at the stage of laying or pouring the reinforcing belt.
Sometimes wooden rafters are fixed on a Mauerlat and on a knot of three nails. This is also a pretty reliable way. In this case, one nail is hammered in the middle of the rafters, and the other two are at a slight angle on both sides of it. With this method of fastening, it is also advisable to additionally use a wire rod. Sometimes, instead of it, special clamps are used, grasping the rafters above the Mauerlat.
How to attach on top
Above, layered rafters can also be fixed in several ways. Sometimes they are mounted directly on the ridge run by the cutting method, but more often they are still paired over it. In the latter case, farms can be assembled using the “half-tree” method or using thick steel plates or bars.
Since the layered expansion rafters transfer to the walls including the horizontal load, on large spans, like hanging ones, sometimes they are additionally connected in pairs with a crossbar under the ridge.
Installation procedure
A layered system of rafters of a house of this variety is assembled as follows:
- A template is cut out from a long board, according to which the rafters are subsequently cut out. Its use allows you to make exactly the same elements, and therefore to assemble a neat roof.
- Extreme racks are installed (on the gables).
- In between, an inelastic reference cord is stretched horizontally strictly horizontally.
- Intermediate racks are attached to the floor beams .
- A ridge run is fixed on them. You can fix it on racks using the half-tree method or corners.
- Extreme rafter farms are mounted.
- With the selected step, intermediate ones are set.
After the base of the roof frame is assembled, proceed to the installation of insulation, waterproofing, lathing and roofing material.
Additional elements of rafters
Very often, the frame of this variety is installed on large houses with wide girders. In this case, in order to avoid the need to use a thick beam, and therefore unnecessary expenses, additional elements are usually included in the roof structure - struts or rafters. The first one is fixed at the junction of the rack and the bed with one end, and the second, at an angle of 45 degrees, is attached to the rafter itself. The result is an additional support that does not allow the leg to bend too much under severe loads. You need to cut the brace exactly at an angle of 45 degrees. The presence of gaps between him, the stance and the leg is not allowed.
The use of roof beams allows you to collect even more reliable roofs on houses with very wide spans. In this case, the ridge run is not installed. Instead, two rods are first mounted along the walls, on which racks are mounted. Beams are laid on them, reliably supporting the layered rafters. Mounting points in this case are performed in the same way as on the Mauerlat. That is, in the rafter, the landing nest is first selected, and then it is attached to the beam using the corners.
What you need to know
Use layered rafters spacers mainly only on houses with brick, block or monolithic walls. This is due primarily to the fact that in this case, the attachment to the Mauerlat is rigid. Rafters of this variety are not installed on wooden houses, because such buildings after construction are able to give strong shrinkage. As a result, a roof rigidly fixed on the upper rim can simply lead. On such houses, the rafters are fixed to the Mauerlat on special elements - “slides”, ensuring their mobility.
Conclusion
As you can see, it is not so difficult to calculate and install the layered expansion rafters yourself. However, no matter what method of attaching them to the Mauerlat, beams and girders is chosen, all work must be done carefully - using high-quality additional elements, without violating technology. Otherwise, the roof will turn out to be unreliable and will soon require repair or even reinstallation.