How to install a truss system

Recommendation points

• Types and forms of roofing
• Rafter system
• Mauerlat
• Installation of the rafter system
• Fire-biosecurity

The roof is the most important and complex element of any building. Difficulties begin already during design work, namely, when choosing the shape of the roof, the type of rafter system, coating, insulation, etc..

The process of arrangement and installation is very responsible and time-consuming, as it is associated with high-rise work, in which the issue of safety is especially acute.

When installing the roofing, it is imperative to comply with all the requirements and standards of the manufacturer, since even minor mistakes can lead to serious consequences in the future (roof leaks, deformations and other unpleasant moments).

Very big problems are caused by the work process itself, in view of the fact that it is associated with work on inclined planes, and from my own experience I want to note that it is impossible to work on roofs with angles of more than 30 degrees without taking additional measures.

In terms of cost, roofing usually costs within 30-40% of the total cost of building a box, therefore, it is worth treating this stage with appropriate care and delving into all the nuances and subtleties.

Types and forms of roofing

You should start with the development of a draft design of the house. Currently, a computer is often used for this, since there is a fairly large range of programs with which it is even possible to sketch a small sketch on your own and make changes at any time. I recommend trying to draw all the facades of the house and the shape of the roof, having analyzed several options, and then contacting the architect, having at least minimal layouts, including the location of windows, doors, external views of the facades and the shape of the roof. At the same time, it is advisable to plan the location of the interior partitions, especially on the attic floor.

Schematic representation of the main forms of the roof

There are several types of roofs in terms of shape, but today almost no one makes roofs with one or two slopes, but stop at the combinations shown below. But here it is already a matter of taste and choice of everyone, so it does not make much sense to give advice at this stage..

After you have decided on the shape of the roof, you need to decide what material will be used as a coating in the future roof, since the weight of the selected coating affects the calculations in the development and design of the rafter system. For example, the weight of flexible tiles or ondulin is 5-7 times less than the weight of ceramic or sand-cement tiles. Accordingly, the design of the system itself will differ when using certain materials. This is perhaps the most critical part of the calculation work in the design of the roof. I offer universal advice for all types of roofs.

Rafter system

When designing a rafter system, it is necessary to take into account the fact that most brands of roofing insulation are supplied to our market with an actual cutting width of 60 cm or 120 cm. Therefore, it is advisable to take the step of the rafter legs with this interval. In the future, you can save a lot by eliminating the cost of trimming and adjusting the insulation that fits between the rafters. The necessary safety margin of the rafters is calculated and selected optimally by the section of the rafter leg itself. When calculating the safety margin of wood, take into account the following nuances: be sure to ask when ordering which season the forest is (winter forest is stronger than summer), and it is of no small importance whether the resin came down from the wood, because this affects the durability of the future roof. The resin, with which the entire solid wood is impregnated, is a natural antiseptic.

The project of a rafter system that combines almost all forms of roofs, the basis of which is a half-hip form.

It is also worth considering that at the junction of the rafters, namely on the edges and valleys (joints of roof planes), it is necessary to use rafters of a larger section, since the load in these places is summed up from all joining rafters. Particular attention should be paid to the valley rafters, in view of the fact that these places are considered the most “problematic” in the roof, both in terms of load and possible leaks. I recommend in these places to use rafters in width 30-50% thicker and (or) wider than standard calculated.

Mauerlat

The beams that lie directly on the walls and on which the rafters rest are called Mauerlat. The sequence of their laying is as follows.

Before laying the Mauerlat, it is necessary to fill in the monolithic belt, having previously inserted the studs with a step of no more than 120 cm.

To begin with, a monolithic belt should be poured around the perimeter of all walls, into which the studs should be concreted (it is preferable to do this in order to avoid additional labor costs when installing anchors in hardened concrete). Choose studs with a minimum diameter of 10mm. In this case, it is necessary to use galvanized steel studs, since this material does not corrode. Install them in half steps less than the step of the rafters, but at least every 120 cm. When calculating, it is necessary to take into account that the pins get between the rafters, otherwise they will greatly complicate their docking with the Mauerlat. Next, an armored roofing material is placed on the belt in two layers (it is allowed to use building material such as “TechnoNIKOL”). This will provide waterproofing between the wood and concrete. Before laying, holes are drilled in the bars at the distance of the studs. To do this, the timber is preliminarily laid on the wall along the studs, and the places of future holes are marked on it. After that, the Mauerlat is pushed onto the studs, which should protrude at least 2-3 cm above the timber after it has been laid. Next, washers are sequentially put on and nuts are clamped. During installation, it is necessary to use reinforced washers (i.e. large diameter) in order to avoid the tree being pushed through when tightening the nuts. Both nuts and washers should be galvanized steel.

Installation of the rafter system

There are a lot of methods for connecting rafters with a Mauerlat, but there is no need to dwell on each of them, since I recommend using the following (due to its simplicity and reliability with minimal labor costs).

On the rafter leg, a notch is made to a depth of 20-25% of the rafter width (with a standard width of 150mm, the notch is made to a depth of 3-4cm). Further, with the help of a bracket made of galvanized steel and self-tapping screws, the rafter is fixed in the transverse direction (thus, possible movement along the Mauerlat is eliminated). First, the bracket is screwed on 3-4 screws to the rafter (after leaving a small gap of 3-4 mm between the other end of the bracket and the Mauerlat), and then the rafter is finally fixed by screwing the screws into the Mauerlat. At the final stage, the rafter is attracted to the Mauerlat by a bracket. The staple is initially driven into the rafter, in which the hole is pre-drilled with a drill that is 2-3mm smaller in the diameter of the staple (this is done to prevent wood splitting). And then, under a slight slope, the bracket is driven into the Mauerlat, pulling the rafter. The bracket and bracket should be on opposite sides of one rafter leg. Staples are usually used with a diameter of 8-10 mm. With a given thickness, the safety factor is sufficient even with a margin.

Studs are used in the places where the rafters are connected to each other. For ease of installation, it is better to use studs with a diameter of 8 mm, but this diameter is allowed in roofs with a light covering (flexible tiles, ondulin, metal tiles). In roofs with a high static load (ceramic or cement-sand tiles), the use of pins of this diameter is not allowed (in the case when the length of the rafters is more than 6 m).

The rafters are connected as follows. First, two through holes are made in pairs in all connected rafters. Then the studs are inserted, reinforced washers are put on them and the nuts are clamped in turn. The use of this method excludes subsequently, when the wood dries, “walking” of the rafters along its axis.

When connecting the rafters to each other, you must use the studs. The cross-section of the rib or valley rafter should be 30-40% larger.

There are situations when the length of the rafter is not enough, then you have to build up. In this case, the overlap of the rafters should be at least one meter. Drill 4 holes, in pairs as close as possible to the edges of the rafters to be connected, and clamp with pins, as in the previous description. It is better to assemble this structure on the ground to avoid sagging rafters..

For a reliable connection of the rafters with the ridge beam, it is necessary to use brackets that “sit” on self-tapping screws.

At the joints of the rafters on the ridge, it is also necessary to fasten them together with pins, using an additional rail for this. The resulting design will resemble the letter “A”. To simplify the installation of rafters on a ridge, a ridge girder is most often used (a bar – on which all rafters in the ridge area rest). In these places, brackets are used that fasten the rafters to the ridge beam.

When laying a ridge run, it is convenient to use supports, in which grooves are cut to fit the size of the bar itself.

If the length of the rafters is more than 6 meters, it is necessary to take additional measures to prevent the sagging of the rafters under the influence of gravity, both of the roof itself and of the snow cover (in winter). There are two main ways to avoid this problem. The first one is the creation of a “middle girder” structure, which provides for the location of the supporting beam, which is located across all the rafters and is a kind of support and support for them. And the second is the device of struts or cross beams, which will subsequently be the basis for the ceiling of the attic floor. It is worth choosing in each specific case the best option based on the technical features of your roof. But it is more practical, from the point of view of minimizing costs, the second method, since in the form of beams you can use the same timber that was used to make the rafters. As a result, you will have a ready-made base for the future ceiling. On top of that, mechanically speaking, the compressive strength of wood (in the second case) is much higher than the flexural strength (in the first). The cross-section of the timber, which should be used in the first version, should be an order of magnitude larger. This value increases quadratically with increasing distance between the support points of the middle run. On the other hand, there are situations when it is impossible to use the second method simply technically, therefore it is necessary to consider and analyze each specific moment separately. The ideal possibility to use the second method is to have a load-bearing wall in the center of the house..

All jumpers and braces in this case are also connected to the rafters in pairs with studs with a diameter of at least 10 mm.

Fire-biosecurity

It is important to consider the issue of bio-fire protection. According to modern SNiPs, fire protection is mandatory in any roof of a dwelling. You can do it in two ways: call a service that deals with this type of activity, or do everything yourself. The first option is the simplest, but also the most impractical. Since the processing of the roof occurs when it is completely assembled, thereby eliminating the possibility of processing problem areas at the joints of the elements. Therefore, it is better to carry out it yourself, since the process itself can be directly monitored and, as it should, missed everything.

Currently, there are many impregnations and coatings, which can be selected by experienced POS consultants. I would like to dwell directly on the processing itself. To do this, it is better to arm yourself with an ordinary paint brush and respiratory and vision protection (respirator and special glasses). Whoever tells you about the convenience of using spray guns, do not pay attention to the fact that, supposedly, the impregnation consumption is reduced, and the work is done faster. It is better to smear and soak everything with a brush, because the depth of penetration of the product is of great importance, and not its surface application. When using a spray, the composition will penetrate shallowly. It is advisable to brush through all surfaces twice, then the result will be durable protection. After application, the product must be allowed to dry completely before installation..

After finishing the installation of the rafter system, it is necessary to cut all the rafters at the level of the eaves.

Before nailing the lathing and counter-lathing, it is worth cutting the rafters along the line of the future cornice. It is better to make the length of the eaves overhang a little longer for several reasons: firstly, it improves the aesthetic appearance of the house, and secondly, it excludes possible undermining of the foundation by precipitation flowing from the roof. Will rationally stop at a length within 70-90 cm.

Having followed all the above recommendations, the roof will not bring unpleasant surprises and will last for many years, and will also save your money on installation work and material.