Most low-rise buildings are equipped with a sloping roof, the functions of which include protection from precipitation, wind and heat preservation in the premises. A slope is required to remove precipitation, making the structure less heavy and reducing pressure on the walls.
There are many varieties of roofs that differ in design and performance. Most often in residential buildings, gable and four-gable roofs, as well as their combinations, are used. A gable roof is formed by two intersecting surfaces called ramps.
The choice of the required design is based on the required functionality and appearance of the house under construction. Gable roofs are usually chosen to obtain the most durable buildings, the roof of which is additionally covered by side walls.
The photo shows the roof with the attic - simple and broken.
What is included in the rafter system?
At the ends of the upper part of such a house, walls having a triangular shape above are left. These walls are called gables, less often - forceps. From here came the name of a similar roof structure - gable.
There are two principal types of gable roofs - pitched and suspended. The roof rafter system rests on the interior walls of the room, giving them greater strength. The design benefits include the use of shorter lumber, which simplifies installation and reduces the cost of transportation.
The most commonly used hanging structure is a gable roof, the layout and arrangement of which are shown in Figure 1.
The common system for all roofs includes:
- rafters - thick wooden boards laid along the roof slope;
- a puff connecting the ramps and located at the bottom of the roof (represents a long beam);
- struts - serve to eliminate sagging slopes and perform the function of struts. At one end they are attached to a puff, and at the other to a ramp, thereby creating bursting stress;
- ridge beam - is fixed in the upper part of the roof at the intersection of the slopes. Its position must be horizontal, otherwise the design will not only be skewed and lose aesthetics, but also will not be strong enough;
- headstock (rack) - is a relatively short beam, propping up the ridge connection. With its lower end, the headstock is abutted, creating a monolithic structure;
- crossbar - is used as an alternative to the headstock and is located parallel to the tightening in the upper part of the floors. Its use allows you to avoid the use of struts or move them to the lowest part of the rafter system. All of these elements, when combined, form a farm;
- Mauerlat - is a thick and durable beam laid at the base of the roof (on the top of the wall). It is on this bar that the pressure of the slopes and the precipitation located on them will be exerted. The use of Mauerlat makes it possible to increase the tightness of the roof and to avoid wall deformations, evenly distributing the pressure received on itself;
- lathing - sheathed on top of the rafters and is used for subsequent roofing.
In different designs, the consumption of materials is different. Before collecting the roof, devote time to planning and calculate the required amount of timber and boards.
Calculation of the rafter system
To calculate the main parameters of the rafter system, use the following approach (in the figure):
- the height of the roof is equal to the ratio of half the width of the roof to the tangent of half the angle between the slopes. Example: the angle between the slopes is 90º, and the roof width (AC value) is 4. The height is 0.5 · 4 / tg (90/2) = 2;
- the length of the rafters is calculated by the Pythagorean theorem and is equal to the square root of the sum of the squares of the height of the roof and half of its width. Example: according to the values from the previous example, the length of the rafters will be the square root of expression 22+22 and it will be equal to 2.8 m. 0.3-0.5 m should be added to the obtained length - this distance is necessary for the visor;
- the roof area is calculated as the product of the length of the rafters and the length (width) of the slope, multiplied by 2. Example: For the previous case, with a slope width of 5 m, the roof area is 5 · (2.8 + 0.5) · 2 = 33 m2;
- the total length of lumber for the lathing is calculated as the product of the width of the ramp by the number of strips of lumber. The last value is equal to the ratio of the length of the rafters, multiplied by 2, to the step of the crate. The standard step of the crate is 0.3 m. Example: for the previously calculated roof, the number of running meters of the crate is (2.8 + 0.5) · 2 / 0.3 = 22 m (rounded to a larger integer value).
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In calculating the consumption of boards and timber for the remaining elements of the rafter structure, the Pythagorean theorem is also used. The step between the rafters to calculate the number of farms should be taken 0.6-1.0 m.
If the house is being built in a windy area, you need to reduce the step between the rafters. This will strengthen the structure and press it harder to the base of the walls.
Installation steps
Self-installation of the rafter system is quite complicated, but can be performed without special skills. It’s best to get the three of us to work - it will not only be easier, but also safer on high-altitude work.
Stages of self-installation of a gable roof:
- Mauerlat installation. The better the fit of the beam, the less it will blow into the building;
- fixing additional (temporary) supports if the width of the walls is large and auxiliary ceilings need to be used;
- installation of rafters, which are nailed to the Mauerlat;
- installation of the remaining structural elements giving it strength and rigidity;
- lathing (also on nails);
- waterproofing performance. For a metal tile, a diffusion membrane is required, and for a slate, an ordinary roofing material is enough;
- covering with roofing material.
In the case of a steep roof, provide a stationary staircase in one part of the house. This will simplify access to the roof for repairs, pipe cleaning, etc.
Rafters - installation technology and requirements for boards
Features of the choice of boards for rafters are in the selection of materials identical in their parameters. This is due to the requirement to ensure strength over the entire surface of the roof and to minimize distortions caused by uneven distribution of mass.
The selection of boards should be carried out by visual inspection, revealing cracks, large knots and other obvious defects. It will be much easier to work with homogeneous wood, as it will withstand any loads applied during installation.
The procedure for installing rafters for classic gable roofs:
- We connect the trusses on the ground, which then enter the frame of the truss structure (Fig. 1). During operation, we control the correct observance of the corners, since the correction of errors during the operation will result in significant time costs;
- We prepare the Mauerlat for laying the rafter “legs” - we make recesses under the rafters on the timber (use a hacksaw on a tree and a chisel);
- We install rafters assembled in farms using a plumb line.
When nailing rafters, long nails (200 mm) are used. In order not to split the wood, drill a hole along the path of the nail (diameter 6-7 mm). This will not reduce the strength of the rafters, but will prevent cracking.
We recommend watching a video that will help you understand the intricacies of calculating the rafter system and installing the roof:
From the strength and tightness of the roof depends on the comfort of people living in the house. When assembling a gable rafter system, you should find a balance between the amount of materials purchased and the cost. Saving for the sake of lower roof strength is not worth it - this is especially true in regions with severe weather conditions.