Rafters are the main structural element of any roof. There are many ways to install them. Very often, the roofs of houses are assembled, for example, on layered spacer rafters. Their main feature is that they work during the operation of the roof both in bending and in compression. That is, not only vertical, but also horizontal force is transmitted to the walls.
In fact, such rafters are a transitional option from layered non-thrust to hanging. They transmit the vertical load through the ridge run and racks. Such rafters exert expansion pressure on the walls due to the fact that they are rigidly attached to the Mauerlat or beams. With this method of fixing, horizontal stress inevitably arises in the roof frame. The diagram of the spacer rafters, presented below, demonstrates the principle of their work clearly.
Things to consider when drafting a project
Before the installation of the roof, of course, detailedinstallation schemes for all its elements, including rafters. When drafting a roof frame, first of all, you should decide:
- with angled rafters;
- their section;
- type of lumber used;
- setting step;
- fastening methods.
What should be the angle of inclination
The calculation of this parameter should be approached as responsibly as possible. The reliability and durability of such a structure as a roof with layered rafters directly depends on how correctly the angle is chosen. It is best, of course, to arrange a steeper 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 more, of course, than flat roofs.
In addition to the features of the operation 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 roofing material used;
- possible snow load;
- wind load.
For each specific roofing material, there is its own minimum allowable slope angle. So, for example, for metal tiles it is 12-14 degrees, for slate - 22 degrees, etc. When developing a project, this, of course, must be taken into account.
The possible wind and snow load on the roof is determined by special tables compiled for each specificregion. In most regions of Russia, the optimal angle of inclination of the slopes, taking into account these indicators, is 30-45 degrees.
Which lumber to choose
For obvious reasons, the most important part of the roof frame is the rafters. The design (and nodes, and the legs themselves) as the main supporting element in this case, should be as stable and durable as possible. Therefore, the choice of lumber for them must be approached as responsibly as possible.
Most often, pine timber is used for the 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 fairly 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 roofing cake;
- step between rafters;
- their lengths.
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 finished with heavy roofing material, such as clay tiles. If there is not too much precipitation in the region during the cold season, you can save money and install rafters with a large step. In the European part of Russia, when using ondulin or profiled roofing for roofingrafter sheets 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 simple to determine it. The design of the 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 length of the leg, you just need to divide half the span of the building by the cosine of the slope angle. Approximately 40 cm for overhangs should be added to the resulting figure.
Knowing the length of the rafter legs and determining the distance between them, you can easily find out what section they should have. To do this, you need to use special tables. Most often, a beam with a section of 150x150 cm is taken under the rafters. Under the Mauerlat, since it has a serious load during operation, they usually use thicker material - 150x200 or 200x200 cm.
How to attach at the bottom
Mauerlat to concrete and brick walls is usually fixed to anchors, previously embedded in masonry or mortar. There are several ways to fix the rafters themselves:
- by cutting;
- using galvanized angles and bolts;
- using nails, clamps and wire.
The rafters are mounted when assembling the roof frame either directly on the mauerlat, or on beams protruding beyond the plane of the wall. In the latter case, fastening can be done in two ways:
- On steep roofs (more than 35 degrees) cuttingrafters into a beam are often made with a single tooth. In this case, a spike is cut out in the leg, and a special stop in the counterpart. Actually the fastening itself is made using a bolt.
- On flat roofs, to increase the support area, the connection is made by cutting using a double tooth.
When mounted directly on the Mauerlat, only the landing nest is cut out in the rafter. In this case, rigid fixation is most often carried out using special galvanized perforated corners. They are installed on the sides of the leg and secured with bolts. To reduce the horizontal load on the walls from the roof, sometimes a beam is also stuffed onto the rafter from below (for emphasis on the Mauerlat). Such an attachment point 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 layered rafters are fixed on the Mauerlat and on a knot of three nails. It's also pretty reliable. In this case, one nail is driven in the middle of the rafter, and the other two at a slight angle on either side of it. With this method of fastening, it is also advisable to additionally use wire rod. Sometimes, instead of it, special clamps are used, clasping the rafter above the Mauerlat.
How to attach from above
At the top of the layered rafters can also be fixed in several ways. Sometimes they are attached directly to the ridge run by cutting, but more often they are connected in pairs above it. In the latter case, farms can be assembled using the “half-tree” method.or using thick steel plates or bars.
Since the layered spacer rafters also transmit horizontal loads to the walls, on large spans they, like hanging ones, are sometimes additionally connected in pairs with a crossbar under the ridge.
The layered rafter system of this type of house is assembled as follows:
- A template is cut out of a long board, according to which the rafters are subsequently cut. Its use allows you to make exactly the same elements, and therefore, to assemble a neat roof.
- Extreme racks are installed (along the gables).
- An inelastic orientation cord is stretched strictly horizontally between them.
- Intermediate racks are attached to the floor beams.
- The ridge run is fixed on them. You can fix it on racks using the half-tree method or corners.
- Extreme roof trusses are mounted.
- Intermediate steps are set with the selected step.
After the base of the roof frame is assembled, proceed to the installation of insulation, waterproofing, lathing and roofing material.
Additional elements of layered 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, consequently, unnecessary costs, the roof structure is usuallyinclude additional elements - struts or rafter beams. The first ones are fixed with one end at the junction of the rack and the bed, and with the second, at an angle of 45 degrees, to the rafter itself. The result is additional support that does not allow the leg to bend too much under serious loads. You need to cut the strut exactly at an angle of 45 degrees. The presence of gaps between it, the rack and the leg is not allowed.
The use of rafters allows you to assemble even more reliable roofs on houses with very wide spans. In this case, the ridge run is not installed. Instead, two beds are first mounted along the walls, on which racks are attached. Beams are laid on them, reliably supporting the layered rafters. The attachment points in this case are made according to the same scheme as on the Mauerlat. That is, a landing nest is first selected in the rafter, and then it is attached to the beam using corners.
What you need to know
The spreading rafters are used mainly only on houses with brick, block or monolithic walls. This is primarily due 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 capable of shrinking strongly. As a result, the roof rigidly fixed on the upper crown can simply lead. On such houses, the rafters are fixed to the Mauerlat on special elements - “sleds” that ensure their mobility.
As you can see, it is not so difficult to calculate and install layered spacer rafters on your own. 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.