Fastening rafters to beams as an optimal variant of the
design. For your house you can use two types of rafter systems - hanging and inclined, each of which has certain advantages. Thus, hanging farms are used more often for the construction of a light roof of a large area, while the sloping ones are considered to be more thorough and easier to stack, because each rafter "leg" in this case has two points of support, whereas only one has a wall.
We are more interested in the hanging version, as more suitable for the attic. The simplest assembly of a roof frame of this type is based on horizontal beams that are laid on the edges of walls, perpendicular to them. In the beams, closer to the ends, special grooves-grooves are cut, into which the rafters are inserted.
The beam laid on the crests of the walls should extend beyond the limits of the building by 40 centimeters, for accuracy only the projections of the extreme beams are measured, between which the string is stretched and the rest of the crossbars are already laid along it. Of course, the grooves can not hold the heavy rafters in any way, so they are additionally fixed with special metal fasteners or, which is simpler and more reliable, with the lining of the boards between the beam and the rafter. In fact, the bar acts as a connecting puff, an element that reduces the compression load. Such spacers are installed also when the cross bar is not used in the construction, and the rafters are supported by the Mauerlat.
Sometimes these tightenings are made of metal, especially if it is obvious that the load on the roof will be large, or use pre-stressed beams for rafters, that is reinforced with spring-loaded flexible elements. If you plan to build an attic, the tightening can become part of the overlap, that is, lags, for which they are placed in the lowest part of the rafters, just where the bar should be laid.
How do the joists and rafters fit together?
As already mentioned, the suspended rafters very often rely on cross-members from the bar, which, in turn, are laid directly on the wall. However, this design has a point pressure on a certain section of the wall and therefore this way of building trusses for roofing is acceptable only for wooden log houses.
If the house is built of brick, with undistributed pressure on the walls, their edge begins to crumble at the locations of the beams. Just to spread the pressure on the whole wall, a Mauerlat is used - a thick bar that runs along the crest of the wall.
In general, it turns out that the beams of the ceiling and the rafters lie on the Mauerlat, the first ones being laid close to the beam of the truss support, thus creating an uninterrupted overlapping surface. Experienced masters recommend using a bar with a cross-section of 150x150 mm for the Mauerlat.
Such dimensions are enough to ensure that the unloading element for the edge of the walls does not deform under the weight of the roof. For additional wall insurance, you can install a reinforced belt, to which and attach the Mauerlat corner.
Approximate calculation of rafters, beams and spans
Before you build a roof frame, you need to find out how you can more accurately what the snow load can please you the weather in the habitable area. On the basis of the data obtained, the rafters, the roof beams and the elements of the trusses, which are to retain the roof, should be calculated. In particular, it is necessary to make structural calculations for different sections of wood or metal frame elements, to determine the size of the spans between the rafters and the thickness of the beams.
So, at a snow cover pressure within 1.5 kPa and a section of rafters from coniferous wood 38x140, at a step between "legs" of 300 millimeters the roof span should not exceed 4.28 meters. Such data is easy to find on the Internet in the form of tables.
By the way, the step between the rafters can be different, but it is on it that the span of the roofing structure depends. The span is nothing more than the distance between the supports that support the ridge beams .If there are none, the span is considered to be the entire distance between the outer rafters.
From this distance, by the way, the entire structure of the roof can also depend, since if the span is 4.5 meters, roofing beams are used for the design, and for larger values it is more practical to use the trusses. The inclined rafters are more convenient when flying up to 6.5 meters, and the roof of 7 meters is already executed on a suspended structure.