In any construction, the most important step is to create a roof. It accounts for the mechanical pressure of snow in winter, significant wind loads and the influence of precipitation. That is why the process of manufacturing a roof requires close attention and the execution of all work with extreme precision and accuracy.
The construction of the roof, like most other work on building a house, is divided into several stages. The device of hanging rafters is one of them. It is worth noting that truss systems can differ markedly in their technological characteristics.
The main difference between the hanging version is the support of structural elements on the Mauerlat on each other and from below or on the upper ridge detail. That is, they are located without a vertical support, which is indispensable when forming a layered system.
It is advisable to use this option in the absence of carriersinterior walls in a building. It is also possible to use, if necessary, the use of attic space. An example is the construction of a mansard roof.
The design of hanging rafters transfers a bursting mechanical load to the walls of the building - this is the inevitable cost of the absence of vertical supports in the composition. Therefore, the arrangement of such a rafter system is not complete without the so-called screed. It is a horizontally placed element that pulls the rafter legs, due to which the walls are subject to less bursting loads. It should be noted that with a decrease in the level of installation of the screed, the efficiency of the application increases.
The rafter system, made with a hanging arrangement, has many operational positive characteristics. High-quality and competent execution will give sufficient strength to the roof and reduce the impact of loads exerted on it. Compared to the layered system, it is lighter in weight, which reduces the overall load on the wall structures and, accordingly, on the foundation of the house.
Using fewer parts, less material is needed to create a hanging rafter.
Layer and hanging rafters
If the house has a major internal wall, when building a roof, a layered system is most often used as a supporting structure. It has the main difference, which is the presence of an internal additional element, which has an emphasis on the column or internalbearing wall. As a result, only bending load transfer is transferred. Among the advantages of this design, it is worth noting its relatively low weight and the need to use a small amount of material for construction.
For certain cases, for example, when arranging a single roof over several spans, the simultaneous use of both layered and hanging systems is required. Rafters, nodes of a layered structure in this embodiment are used in that part of the house where there are internal supports, respectively, where they are not, another system is set up.
Stages of construction
After determining the type of rafters, you can take on the construction itself. To begin with, a schematic drawing is made. It should be noted right away that it is advisable to entrust it to professionals, since it requires special knowledge and experience in this area. Making calculations at the layman's level is a rather dangerous undertaking, since design errors can be made, which can later lead to a change in the integrity of the roofing system or to its destruction.
After the drawing is created, further tasks begin, that is, the installation of the Mauerlat. A wooden beam with a section of 20x20 cm is used for this, it is placed along the perimeter of the walls and strengthened. It is necessary to pay attention to the fact that the Mauerlat requires protection against dampness, so a special material is mounted under it, for example, roofing felt.
When using blocks or bricks to buildwalls, a concrete leveling screed is required around their perimeter, only after that a Mauerlat can be installed.
As a material for the rafter legs, it is best to use a wooden beam with a section of 20x5 cm.
Hanging rafters can be installed using a ridge beam, which gives the structure additional rigidity. When organizing a roof without it, the connection of the rafters in the upper part should be made with an emphasis, and not with an overlap.
There are two mounting options: first install the two extreme pairs of legs using a ridge beam, after which the rest are placed, or in order. The first option is more preferable, as it can save time, as well as a more reliable design that is resistant to many types of loads.
The same material is used for the manufacture of puffs as for the rafter legs. The puff is mounted in a horizontal position. As noted earlier, the lower its location, the better its characteristics become. It is necessary to take into account this circumstance, which is of particular importance if the operation of the attic space is planned. In the absence of a timber of a suitable size, overlapped trimmings can be used to create a puff.
Hanging rafters: construction and knots
When installing a hanging system, the method is importantnode connections. In addition to fixing with screws or nails, there must be a connection with nuts and bolts, for this, holes are drilled in certain places.
Technological cuts are mandatory. For each connection point of the elements of the truss system, at least two fasteners must be made. An example is the fastening of a puff with two self-tapping screws or nails, followed by a threaded rod.
When a hanging rafter system is installed, it is worth remembering the formation of cornice overhangs. It is desirable that initially their length be included in the dimensions of the rafter legs, but the cost of the structure increases because of this. For a more budgetary production of overhangs, an edged board is used, the parameters of which are 10x5 cm. “Males” are formed from it, their length should be selected in such a way that the overlap on the rafter legs in the upper part is at least 50 cm. Parts are fixed using threaded parts.
In the Mauerlat, cuts of the required size are made for a tighter connection. To give increased mechanical strength in the middle, the "filly" is also fixed with a small support bar, which is fixed on the side of the overhang and on the top of the Mauerlat.
The connection of the rafters is carried out with an overlap and butt. In the first version, the hanging rafters are fixed to each other using a bolted connection. In the second, force is applied to the nodes with overlays, inwhich are metal plates or boards.
The cornice connection of rafters and puffs is performed by a frontal orthogonal cutting having a single or double tooth, as well as plates or boards.
Hinged triangular arch
This version of the truss truss is the simplest. The arch is based on three elements: a horizontal beam (puff), which secures the hanging rafters in the center of the span or at the base, as well as two inclined rafter legs, combined at the top point. The puff, formed from steel rods or a wooden beam, takes over the support force, so only a vertical load is transmitted to the external walls of the house. Due to this, it becomes much easier to support the truss on the wall structure: instead of the Mauerlat, a regular board is mounted on it, which is used to redistribute the existing load.
At the base of the rafters, a puff is placed if it is necessary to create an attic floor. It is used to support intermediate floors. In other situations, the tightening is mounted at a higher level. This increases the tensile stress that falls on it, but at the same time, bending forces decrease in the span of the rafters.
Articulated arch with crossbar
This truss visually looks like a gable roof with hanging rafters, described above, in the form of a triangle. The main difference lies in the wall support used. In the crossbar arch, the base of the rafter legs is fixed in the grooves,which are available in the Mauerlat. On the walls, in addition to vertical forces, there are also spacers. In the middle of the span, a puff is mounted (in this method it is called a crossbar) and used for compression. As a result, the attachment of the rafter legs and the crossbar is simplified.
Hinged arch with suspension
If the roof with hanging rafters has a width of more than 6 m, it is quite difficult to equip a three-element simple truss. Firstly, to produce a puff of this size, it takes a lot of time to find a bar. Secondly, due to the large weight, a long puff sags in the center. Therefore, as an addition to the arch, a suspension is used - this is a part that combines the center of the tightening and the upper part of the arch. The tightening element is made up of two parts, using a bolted connection. A wooden pendant is often called a headstock, and a metal one is called a strand.
Articulated arch with struts
To reduce the deflection in the rafters, struts are added to the arch with suspension, which are inclined elements to support the rafter legs in the center of the span. At the same time, a suspension is used to stop the struts themselves.
In conclusion of the above, it is worth noting that due to the competent execution of calculations and the total amount of work, hanging rafters in terms of operational properties will not be inferior to layered ones. In addition to stability and high strength, the use of this design will form an attic space suitable for use inbusiness or other purposes. In the case of arranging a mansard roof, such a truss system becomes an ideal option.