What is built up roofing
The roof of the house needs a solid support. For pitched built up roofs its role is performed by the rafter system, a complex engineering structure made of wood. It consists of many elements, has several varieties. Consider the features and specific features of the rafters, the order of installation and other nuances of construction. This information will help save expensive roofing and reduce the frequency of repairs. Read to the end to get the most complete information about the rafter system of a private home.
What tasks the rafter system solves
The main function of the roof – protection from precipitation, wind, external influences. For most areas are characterized by pitched roofs, providing a drainage of rain or melt water and self-runoff of excess snow. Flat roofs are found only in the southern regions, so there is no point in talking about them.
The composition of the roof includes two main elements:
The roof acts as an airtight covering, laid at a certain angle on a supporting structure – rafters. Their tasks include:
- Maintaining the roofing canvas;
- Creating a stable and reliable system, taking on snow and wind loads;
- Ventilation of the attic space;
- Protection of the upper floor against temperature fluctuations, sunlight, moisture and other effects.
In addition, the rafters provide an even distribution of load on the walls and foundation of the house. For regions with snowy winters the task of the system is to form a certain angle of slope, which provides an optimal balance between snow and wind loads. All these functions have to perform simultaneously, so the calculation and installation of the system are important and responsible stages of home construction.
The structure of the rafter system is quite complex.
Let’s consider the nodes of the system more carefully:
- Mauerlat. This is a horizontal bar, laid on the upper edge of the walls around the perimeter of the house. It takes the load from the rafters, distributes and transmits it to the walls and foundation.
- Ridge. A horizontal bar laid along the longitudinal axis of the roof. Sometimes two planks are laid if it is required by the width of the roof.
- Piers. The vertical bars placed on the ends of the scaffolding. The height of the struts determines the angle of slope of the slopes.
- Ridge Purlin. Horizontal board installed on the rib and attached to the upper ends of the struts. It is supported by the rafters.
- Rafter Legs. Sloping boards mounted on the rib. In the lower part they rest on the mauerlat, in the upper part they are attached to the ridge purlin, or simply connected in pairs.
- Purlin. Horizontal bar attached to the ends of the struts. Used on roofs with two purlins, dividing the whole structure into two tiers.
- Transom. Horizontal slats laid perpendicular to the purlins and connecting the rafters.
- Struts. These are inclined strips that fasten the elements of the structure and ensure their immobility.
- Lattice. Several rows of horizontal slats that connect the rafters together and serve as direct support for the roof. It can be a solid planking of boards. Sometimes an intermediate layer of sheet material (OSB, chipboard or plywood) is laid on top of the sparse battens to make the underlay tight.
In addition, an important element of the system is the eaves. This is an extension of the bottom of the rafters, beyond the maueralt and overhanging over the plane of the walls. Thanks to the overhangs, rain or melt water drains to the ground without reaching the walls or foundation. This allows you to save the supporting structures and increases the life of the building. The rafters themselves rest on the mauerlat and there end. They are continued by separate elements – fillets, which are fastened at the butt to the rafters and do not feel the loads imposed on the main part of the system.
The main elements of the rafter system are listed, but the order of their connection and placement may vary. Depending on how they are assembled, there are different types of rafter systems:
- Strung. The rafters are based on the bottom of the mauerlat, and from above are attached to the ridge purlin.
- Hanging. The rafters are set on the bottom of the mauerlat. And from above are connected to the opposite legs from the other slope. Typically, this is the design of the roof of pitched roofs of small one-story houses of frame type, or buildings of lightweight foam concrete.
- Sprengelnye systems. They consist of a chain of individual trusses assembled on the ground and mounted on the roof in finished form. This method allows you to reduce the danger of working at height, provides better quality and accuracy of assembly.
The choice of one or another design is not a whim of the homeowner. The type of roof laid in the construction project based on the results of calculations, taking into account possible loads and climatic conditions of the region.
Construction selection criteria
Roof framing system, or simply rafters – a load-bearing system that serves as a supporting structure for the roofing, insulation and other layers of the roofing pie. In addition, the rafters take on all the external loads caused by climatic and weather factors. When calculating the design takes into account two main factors:
- Wind loads. In some regions there are real hurricanes, when the wind speed is 20 m/s or more.
- Snow load. This is a significant value. Suffice it to say that for Moscow the weight of snow on 1 m2 is 180 kg (III snow region on SNIP maps). This means that even for a small building (eg garage) with a roof area of 20 m2, the snow load will be 3600 kg, or 3.6 tons.
To reduce the wind load is required to reduce the angle of inclination of slopes. At the same time, the conditions for the descent of the snow arise with a large slope of the roof. Therefore, designers have to choose a certain average value. Consider the relative rarity of strong winds and snow load stability, its seasonal nature. If the winds are rare, and there is a lot of snow, the gradient increases, and vice versa.
In addition, takes into account the type of roofing. For each material has its own limitations – if the metal roof to the snow is possible with a fairly flat slopes, then for a soft roof is allowed a higher slope. The ability to self-converging snow is considered very useful, but a dangerous property of the roof. On the one hand, are removed unwanted stress, on the other – there is a risk of damage to property, personal injury to pedestrians under the weight of the snow. The problem is that the descent is always unpredictable and occurs at an unknown moment. Therefore, slopes are trying to make either steep, so that the snow had no time to accumulate, or shallow, if the amount of precipitation in the region is not too great.
Types of roof structures
Nodes of the rafter system, linked in one way or another, form a certain shape of the roof. Depending on local climatic conditions, as well as the configuration of the house, there are different design options:
- Single-pitch. As a rule, these are auxiliary or outbuildings. The entire roof is a single plane. Sloped to the opposite side from the entrance.
- Gable roof. Roofing is divided into two slopes, representing two identical (most often) of the half with a slope in opposite directions.
- Waltz. This is a type of gable roof, which has a longitudinal (ridge) section is shorter, and the ends are additional sloping plane. There are gable roof, which end sections do not continue to the top, leaving a small vertical gables.
- Log (mansard). Each roof slope is divided into two longitudinal parts with a different slope angle. This design increases the volume of the attic, allowing it to be used as a living space (attic).
- Hipped roof. A roof without a ridge. All inclined planes converge to one upper point.
- Multi-pinching. This is a roof design for a house made up of several sections adjacent to each other at right angles.
Each type of roof requires a different rafter design. The least different are mauerlats, rafters or purlins – in any design they are supporting nodes and perform their tasks in the same way. The main changes come to the rafters and the elements that provide their rigidity – struts, transoms, etc. It is possible to change the roof structure on the used house, but it is a complicated and time-consuming procedure that requires almost complete removal of the old system and assembly of a new one.
Truss structures are made of wood. There are quite strict standards for the quality and size of the material. The main requirements for the wood:
- Edged boards of coniferous or deciduous species are used for assembly;
- The humidity of the material should not exceed 18%;
- On each linear meter of the board allowed to have three knots (no more), the maximum diameter of which does not exceed 3 cm;
- Through cracks are not allowed;
- The length of through cracks must not exceed half the length of the board.
Specialists advise to give preference to conifers. They are impregnated with resin, which increases the material’s resistance to rot, fungus or mildew. The thickness of the boards for the assembly of the rafter system of the house should not be less than 50 mm. However, for outbuildings is allowed to use 40 mm boards. The width is also regulated – for the rafters of the house the minimum value is 150 mm. If the length of the slope is more than 6 m, the minimum width will be 180 mm. Since the standard length of the lumber is 4 or 6 m, the boards will have to be lengthened. The joint area is at least 50 mm and is made closer to the roof ridge. Sometimes a double board is used, joined in such a way that the joint sections of both layers are in different places.
For the manufacture of mauerlats, scaffolding and poles use a bar. Requirements for the quality of wood (moisture, the presence of defects and rot) are the same as those for edged boards. The optimal size is considered 100 × 150 mm, which allows you to connect two boards 50 × 150 mm, if necessary.
The material should be treated with antiseptic and antiprene compositions. They exclude the risk of wood damage by insects or microorganisms, increase resistance to fire. Impregnation of wood should be in accordance with the requirements of the technology for the compositions used. As a rule, a double treatment with intermediate drying is carried out.
Briefly about the main points
Installation of the rafter system is the main stage of roof construction. This is a supporting structure that takes and distributes the snow and wind loads, keeps the roof covering in a given position, protects the walls and the base of the house from the effects of moisture.
The composition of the rafters includes many elements, each of which performs its own task. Some serve as strapping the perimeter of the walls, others support the rafters legs, and others connect them and hold them immobile. The result is a strong balanced structure, the weight of which is not great. And the load-bearing capacity is quite significant.
Elements of the system can be connected in several ways, which allows for different roof designs. The choice of one shape or another is made at the design stage, but if necessary, you can change the shape and configuration of individual sections or the entire roof.
It is recommended to entrust the installation to experienced professionals who are able to competently perform the assembly and avoid mistakes.