Layered rafter system is a structure used in the construction of roofs of buildings with intermediate load-bearing walls, support pillars or columns. It rests not only on the walls outside, but also on the internal central support (in some cases, two).
If we talk about use, then layered rafters are the most common for residential private houses, which, as a rule, have interior walls- partitions.
Components of the layered system: two rafter legs, the lower edges of which rest and are fixed on the external walls (mauerlat), and the upper edges on the horizontal ridge girder. The purlin, in turn, is supported by vertical posts resting against the intermediate wall.
This classic scheme devices of a layered system, suitable for a gable roof. With a pitched roof, the same rules apply, but with a different implementation. The rafters included in the rafter system are laid with support on opposite load-bearing walls (it turns out that only on two supports). Internal partition not needed here. In fact, its function is performed by a higher wall.
To increase load-bearing capacity truss structure, struts are introduced into the system. Their presence allows you to increase the length of the overlapped spans.
For one pitched roofs it is possible to use layered rafters without the introduction of struts for spans of up to 4.5 m. The presence of a strut increases this possible length to 6 m. A similar trend can be seen with gable roofs. Gable design with one intermediate support is used for spans up to 9 m. Installation of struts increases maximum length span up to 10 m. And the combination of struts with a scramble (a horizontal beam connecting a pair of rafter legs) - up to 14 m.
There are several options for implementing layered systems, among which there are non-thrust and spacer structures with additional supporting struts, tie rods, and rafter beams.
Let's look at the basic designs of layered rafters.
Unbraced rafters without struts
This type of layered rafters does not provide expansion external walls. Leveling of thrust loads occurs thanks to a special combination of fastenings. One edge of the rafter is always fixed rigidly, and the second - on a sliding support. This gives the absence of thrust.
Rigid fastening may mean that the node is fixed, but the beam is allowed to rotate at the hinge (one degree of freedom). There is also a rigid pinching of the rafter beam, in which any displacement is impossible (zero degree of freedom).
More freedom is provided by a sliding fastening, which allows the rafter leg not only to rotate, but also to move horizontally (two degrees of freedom).
A non-thrust design is characterized by the fact that it always contains both a rigid and a sliding fastening. Due to this, under the influence of load, the rafters bend without transferring thrust to the walls.
Options for securing rafter legs
The bottom of the rafter is fixed rigidly, the top is fixed freely (sliding support)
The lower edge of the rafter is attached rigidly to the Mauerlat (one degree of freedom), by cutting with a tooth. In another case, a cut is used with fixation with a support bar.
At the upper end of the rafters, a horizontal cut with a bevel is made. If cutting is not possible, then the edge of the rafter leg is hemmed from below with a piece of beam and fastened on both sides mounting plates. The upper edge of the rafter is attached to the purlin using the type of sliding support. In this case, opposite rafters are laid on the ridge alternately, without fastening to each other. That's why gable roof, made according to this scheme can be perceived as two pitched roofs, adjacent to each other.
The complexity of the scheme is that any error in the implementation of the ridge unit turns the non-thrust structure into a spacer one. Therefore, this option is rarely used for gable roofs, more often for single-pitch roofs.
The bottom of the rafter leg is fastened loosely, the top – rigidly
The most common scheme for private houses.
The lower edge of the rafter is secured to the Mauerlat on a slider (metal bracket), due to which it can move and bend under load. To prevent the rafter from “moving away” in the lateral direction, it is fixed on both sides with metal corners or bars.
The top of the rafter legs is fixed on a hinge with rotation tolerance (one degree of freedom). At the same time, the ridge nodes of the layered rafters of this type They do it this way: the edges of the rafters are tied together and connected with a bolt or nails. Or they join the ends, previously cut at an angle, and then tie them with metal or wooden overlays.
The bottom of the rafter leg is fastened loosely, the top is firmly pinched
This scheme differs from the previous one in that the connection of the rafters in the ridge unit is performed with rigid pinching. The rafters rest against each other with their beveled ends, and then tie them together and the ridge girder with two bolts. The result is a pinched knot.
The bottom of the rafter legs is connected to the Mauerlat freely, on a slider.
This fastening option has an increased load-bearing capacity, allowing it to be used in regions with high levels of snow precipitation.
Ways to increase the stability of non-thrust systems
All three considered rafter systems show themselves to be stable under uneven loads only in the case of rigid fixation of the ridge girder. That is, when its ends are brought out to the gables or supported by additional slanted rafters.
If the ridge girder rests only on the posts, the roof may lose stability. In the second and third options considered (the bottom of the rafter leg is on a slider, the top is rigidly fixed), when the load on one of the slopes increases, the roof will shift towards the increased load. The first option will retain its shape, but only with perfectly vertical posts (under the purlin).
So that, despite the non-rigid fixation of the purlin and uneven loads, the layered rafter system remains stable, it is supplemented with a horizontal screed. A scrum is a beam, usually with the same cross-section as the rafters.
It is fastened to the rafters with nails or bolts. The intersection of contractions and racks is fixed with nails. The work of the scrum can be described as emergency. In the event of an uneven heavy load on the slopes, the contraction is activated and protects the system from distortion.
You can strengthen a system with a rigidly fixed top and a free bottom (second and third options) using a slight transformation of the lower unit. The rafter legs extend beyond the edge of the walls. At the same time, the mount itself remains sliding, like a slider.
Another option for increasing stability is to rigidly fasten the bottom of the posts that hold the horizontal ridge girder. To do this, they are cut into the frame and fixed to the ceilings, for example, using overlays made of boards or bars.
Spacer rafters without struts
In this case, the rafters rest on the load-bearing walls and transmit thrust to them. Therefore, such systems cannot be used for houses whose walls are built of aerated concrete. Aerated concrete blocks do not resist bending at all and collapse under thrust loads. And other materials, such as brick or concrete panels, easily withstand such loads and do not deform.
The rafter spacer system requires a rigidly fixed Mauerlat. Moreover, in order to withstand thrust, the strength of the walls must be high. Or there should be an unbreakable reinforced concrete belt along the top of the walls.
For spacer rafters, the same fastening options discussed above for non-spacer systems are used. But with one caveat: all existing sliding fasteners (slides) are replaced with hinged ones with the ability to rotate. To do this, a support beam is nailed to the bottom of the rafters or a tooth is cut into the mauerlat. The hinged fastening in the ridge unit is performed by placing the rafters on top of each other and fastening them with a nail or bolt.
The spacer structure is a cross between layered non-spacer and hanging systems. The ridge girder is still used in them, but it no longer plays a significant role. After all, the rafters are rested with their lower edges against the walls, and with their upper edges against each other. When the walls settle or the ridge girder bends under its own weight, the girder stops working altogether. By their nature, such rafters become hanging.
To increase the stability of the system, a contraction is included in it, which works for compression. It partially, albeit to a small extent, removes the thrust on the walls. In order for the contraction to remove the spread completely, it must connect the lower edges of the rafter legs. But then it will no longer be a fight, but a drag.
The expansion is also reduced by installing a rigidly fixed ridge girder.
Rafters with braces
Such systems can be arranged using both spacer and non-spacer schemes. Their difference from the already considered options is the presence of a third supporting part under the rafter leg - a strut (rafter leg).
The strut changes the system. The rafter turns from a single-span beam into a continuous two-span beam. This allows you to increase the overlapped span, up to 14 m. And also reduce the cross-section of the rafters.
The brace is connected to the rafter in such a way as to prevent it from moving. This is done as follows: the strut is placed under the rafter and secured with wooden plates on the sides and bottom.
Layered system with rafter beams
This design of layered rafters is suitable for buildings with two longitudinal load-bearing walls or intermediate transverse walls. The racks in this case are located not under the ridge, but under the rafters. There is no ridge run.
The rafter legs in the scheme rest on two rafter beams (through purlins), which, in turn, are laid along the roof slopes and rest on vertical racks. The racks are attached to the load-bearing intermediate walls through the beams.
Through runs may not be included in the diagram. Then the racks will have to be placed directly under each rafter and secured with tightening nails.
From above, the rafter legs are joined together and connected with metal or wood overlays on both sides.
The absence of a ridge purlin automatically means that the rafter system forms a thrust. To neutralize it in the non-thrust version of the system, a tie is secured below the through purlins. Under load, it will stretch and eliminate unwanted expansion. To maintain stability, the system uses a grip attached to the lower part of the rafter legs. Also, the structure will be protected from folding by special joints that are secured crosswise between the posts.
In the spacer system, the scrum is installed above the through girders. Then the contraction under load will compress and, in fact, turn into a crossbar.
Installation of racks under rafters or through purlins (and lack of B-pillars!) makes it possible to use layered rafters of this type to construct spacious attic spaces. Other schemes are only suitable for attic spaces and attics with partitions.
Key points for installing layered rafters
Having the calculated diagram of the device in hand, you can begin installation rafter system. The installation is carried out in several stages, the main ones being:
1. A mauerlat - a board or timber - is laid on the top of the external walls. To prevent the Mauerlat from rotting, a waterproofing material is laid between it and the wall - roofing felt, roofing felt, etc.
2. A bench is laid along the top of the intermediate wall, which is necessary for attaching the vertical posts.
3. Stands are attached to the bed in increments of 3-6m.
4. A ridge girder is installed on top of the racks.
5. The rafters are set in increments of 0.6-1.2 m. From below, the rafter leg is attached to the Mauerlat in accordance with the selected fastening pattern (on a hinge or on a slider). From above, the rafter legs are either laid out separately on the ridge girder, or the upper edges are connected to each other, resting on the ridge.
6. If the diagram provides, the rafter legs are connected by horizontal contractions.
7. Again, according to the requirements of the diagram, struts and supporting elements are installed.
When carrying out work on installing rafters, mistakes must not be made. It should be remembered that the rafter system is a roof frame that must withstand all possible loads. An incorrectly designed or installed system can easily lead to misalignment and even destruction of the entire roof.
At the stage of building a house Special attention need to be paid to the roof. The reliability of the entire structure will largely depend on how reliably, correctly and efficiently it is made. One of important details roof is a rafter system that supports finishing material, insulation, waterproofing and other elements. For gable roofs of houses, hanging rafters are often used, the design and components of which, as well as installation, are described in this material.
The rafters are the main load-bearing element of a pitched roof, which takes on the main load experienced by this structure. There are two types of rafter system - hanging or layered. You have to choose between them based on whether there are internal walls in the structure under construction that can serve as support for parts of the roof. The hanging rafter system is used if there are no such walls, and here only the external walls of the building will serve as support. That is, these rafters do not have a central support as such and rest on the wall on one side and on the opposite rafter on the other. The individual elements are fastened using slotted tenons, nails, anchors or metal plates.
Hanging rafters are usually used to cover spans 7-10 m wide. The scope of application can be different - such a system is used for constructing roofs in warehouses, industrial buildings, residential buildings.
On a note! The maximum distance between walls that allows the use of a hanging rafter system is 14 m.
Despite the fact that such rafters are installed at a slope, they do not act on the walls as struts - they transmit only vertical loads. This can be achieved through the use of braces made from timber located at the base of the roof.
On a note! The higher the load it experiences, the higher the tension between the rafter legs.
Rafter elements are made of timber, boards or logs, and metal. IN mandatory before the beginning installation work the material must be treated with special protective compounds that will provide the wood with a long service life. They will protect it from fungus and mold. If you treat the material with fire-fighting compounds, the house will become additional protection from fires.
What does a hanging rafter system consist of?
A system of this type has several constituent elements. And it is imperative to familiarize yourself with them in order to understand the purpose of each of them and not get confused in the concepts.
Table. Components of the system.
| Element | Description |
|---|---|
| This is the basis of the system. It is thanks to them that it is possible to form the shape of the roof and slopes. They are created from timber (or boards) with a cross-section of 50x150, 100x150 mm and placed in increments of about 60-120 cm. The selected dimensions and spacing will directly depend on the parameters of the roof, its design features, and the calculated load. |
| The place where two roof slopes are connected. There is usually an additional ridge beam. |
| With the help of these elements, which represent beams located horizontally between the rafters, the rafter legs are tightened and strengthened. Due to the tightening, the load experienced by the rafter legs is compensated. |
| A kind of puff, which is located near the ridge of the roof. It is made from very durable timber, as it experiences a colossal level of load. |
| A beam that is installed on the top of the load-bearing walls; the rafters rest on it. Helps distribute the weight of the entire roof along the perimeter of the building, which allows the foundation and walls to experience the load evenly. For production, timber with a cross section of 100x100 or 150x150 mm is used. Interestingly, it may not always be used - some types of roofs are built without it. |
| An element that is installed under the roof ridge and is designed to support a long tie. |
| These construction details are used for buildings that have too much large spans between load-bearing walls. They are necessary to support the rafters, providing them with minimal bending load, that is, they will not allow them to bend and sag. |
Prices for timber
Types of structures
There are and can be used in construction five main types of roofing structures of this type, which can be obtained on the basis of hanging rafters. They are distinguished by the presence of some structural elements, configuration, etc.
The simplest option is triangular articulated arch. Its shape is triangular, the tie here experiences a tensile load, and the rafters experience a bending load. This scheme is used only if the height of the ridge is at least 1/6 of the entire length of the span between the walls, otherwise it will not function correctly. Quite often used in the construction of attics. In this case, two rafters are butted and connected to each other at the ridge part. At the bottom, the triangle can be connected with a long tie. The system can only be used if the span between the walls is no more than 6 m.
A slightly different system from the above - articulated arch with headstock. In this case, the structure has a support in the form of a headstock installed under the ridge. Due to tightening, it works in tension. The headstock can be made of either metal or wood. The system is used if the spans between walls are 6 meters or more. The headstock will not allow the tightening to sag - it acts as a kind of suspension and allows you to adjust the degree of deflection of the horizontal part.
Articulated arch with drawstring that is raised, is usually used when planning the construction of attics and spacious attic spaces - that is, where ceiling height is important. In this case, the tie rods will be installed at the top of the rafters. And the higher the tightening is located, the greater the tensile force it absorbs.
If the support is a fixed hinged element, then the structure will be called arch with crossbar. Here the roof is attached to the mauerlat by forming a tooth on the rafters. The crossbar itself resembles a small puff, but in this case it works not in tension, but in compression. The rafter legs are fixed quite rigidly.
Arch with struts and headstock resembles an arch system with a suspension. Here, during the installation of rafters, struts are used, which ensure that the structure is relieved of excess stress. The option is suitable for covering spans 9-14 m wide.
On a note! Any design scheme for hanging rafters requires accurate calculations that will allow you to clarify all external and internal loads.
The rafter system is quite complex to install; sometimes its parts are assembled directly on the ground, and then raised to the roof. But rafters can also be installed directly on site. In general, a roof with hanging rafters is the most simple option, therefore it is often made by novice builders or independently.
Perforated plates used for fastening rafters
Prices for various types of fasteners for rafters
Rafter fasteners
Methods for attaching rafters
These structural parts can be secured in two ways.
- Bolt and clamp, where in this case the end part of the rafter is cut off, and it itself is placed on the beam in the place where the recess is made. The individual elements are fastened using bolts. They are connected so that the attachment point is perpendicular to the top of the rafter. Sometimes clamp systems are used for connection.
- Double tooth. The method is applicable if the slope of the roof slope relative to the horizontal is less than or equal to 35 degrees. In this case, two stops are made on the beam, a special hole is made in the rafter leg, and a tenon is cut out at the place of the extreme stop on the beam. These elements must match each other in size.
System design
To correctly create a system layout, it is important to consider:
- rafter installation step;
- load on the structure and structure;
- rafter section;
- material that will be used to create the roof;
- type of construction;
- roof slope angles
- dimensions of the building as a whole;
- climatic conditions of the region.
The more accurately the calculation is made and the more correctly the rafter system is designed, the stronger and more reliable the entire structure will be. For convenience, you can use existing online calculators, which is important for those who are not familiar with the exact sciences, or for novice craftsmen.
Video - Roof truss system, what you need to know for proper design
Installation rules
Installation of hanging rafters must be carried out in compliance with certain rules. Only in this case will it be possible to produce a correct and stable structure. The work rules are as follows:
Installation of hanging rafters
To build a rafter system, certain tools and materials may be required. The list below is quite approximate, but can be used as a basis when preparing to begin work.
Tools for creating a rafter system:
- hammer;
- axe;
- jigsaw or hacksaw;
- building level, twine, tape measure;
- thread with a weight (plumb line);
- pencil, ruler.
Prices for popular models of jigsaws
As necessary materials you can list directly the beams and boards for the rafters, screws and nails, anchor bolts, waterproofing material, antiseptic compositions for wood processing.
Sigma-Extra - external works, antiseptic for wood
Let's look at the process of installing the rafter system step by step.
Step 1. All wood material processed antiseptics, preventing the development of rotting processes, the occurrence of fungus and mold. Before installation begins, the material is thoroughly dried.
Step 2. Construction material delivered to the top of the building.
Step 3. Along the perimeter of the load-bearing wall, a mauerlat is nailed, consisting of two boards of such width that they can cover the entire upper part of the wall. It is recommended to lay a layer between the Mauerlat and the wall waterproofing material(for example, roofing felt). This will seal the connection, protect the material from moisture and reduce the number of drafts in the future.
Step 4. Next, beams are created from the timber, which will be attached to each pair of rafters in the lower part. They are attached to the Mauerlat and connect two load-bearing walls. You can temporarily move on them while work is being done, and boards that are used to create rafters are also stacked on them.
Step 5. A ridge beam is cut to form the top of the roof. It should have a length from one part of the future roof to another and be located parallel to the long load-bearing walls.
Step 6. The required roof height is determined - the level at which the ridge beam will be attached. It is recommended to make the slope angle within 25-45 degrees. You can find out the angle of the roof and the length of the rafters using the Pythagorean theorem. To do this, you need to measure the distance from the ridge, the height of the roof and the distance to the external wall.
Step 7 The ridge board is installed at the future junction of two rafter legs.
Step 8 Future rafters of the required length are measured and cut from the prepared lumber. They will be attached with the selected step to the ridge board and mauerlat.
Step 9 To make it possible to attach the rafters to other elements, special triangular cutouts are made. To do this, the board is installed so that one side lies on the ridge board, and the other on the Mauerlat. The cut marking is applied. Next, the marked areas are cut out.
Step 10 The second rafter is prepared in the same way. Now they should be docked with each other. To do this, the boards are moved as close to each other as possible, and markings are applied to indicate the location of the cut. The cut is made and the boards can be joined.
Step 11 The rafters are attached to the mauerlat using metal corners and self-tapping screws. With the help of them, the boards are also secured to the ridge of the roof.
Step 12 The two rafters are connected to each other using metal plates and self-tapping screws. Thus, the first two structures are installed, which will be located along the edges of the roof.
Step 13 To make it easier to install other rafters evenly, a rope is stretched strictly horizontally between the outer structures.
Rafter installation process
Step 15 If necessary, the rafters can be reinforced with ties that are attached to two joining rafters.
Video - Installation of the rafter system
Video - Installation of rafters
Video - Using a crossbar (screed) for rafters as an interior element
Hanging rafters are considered a fairly simple design compared to other types of rafter systems. But there should be no rush during their installation. It is important to remember that at the slightest distortion all the work will go down the drain, so you need to work carefully and carefully.
Rafters and sheathing - the roof frame that bears all the weight roofing pie, thermal insulation, snow load, so they must have high load-bearing capacity and strength. The idea of using metal rafters to make the roof structure more rigid is not new, but previously it was used mainly for covering industrial or utility structures. Nowadays, rafter frames and metal profile sheathing are considered a real alternative to wooden roof elements if the length of the slope exceeds 10 meters.
Design rafter frame The roof consists of many interconnected elements that form trusses. The pitch, the cross-sectional size between the rafters and other supports is determined by the calculation of the loads to which they are subjected during operation. The roof frame performs the following functions:
- Load distribution. Interconnected nodes, reinforced with corners, evenly distribute the weight of the roof, which can reach up to 500-600 kg, taking into account the snow load. The larger the cross-section of the rafters and the smaller the pitch between them, the greater the load-bearing capacity of the structure.
- Giving slope and shape. The rafters, located at an angle to the base of the roof, form inclined plane slopes, due to which snow and water do not accumulate on the roof surface.
- Forming a base for fixing the roofing material. Fastening finishing coating the roofing pie occurs to the roof frame. The sheathing acts as a basis for fixing the covering, distributing its weight evenly across the rafters.
Note! What all components, rafters and sheathing of the roof structure should be like is determined by engineering calculations. To determine the required load-bearing capacity of the frame, it is necessary to calculate the total load to which it will be subjected. To do this, add the weight of the roofing material, insulation, waterproofing, maximum snow load with the weight of the rafter system.
Types of rafter systems
The most common material from which the sheathing and rafters of the roof frame are made is wood. However, if the weight of the roofing material is large enough and the length of the slope is more than 6 meters, then the structure turns out to be too massive. Builders have to reduce the pitch between the rafter legs and increase their cross-section, which is why the roof assemblies acquire a lot of weight, increasing the load on the foundation. You can relieve load-bearing walls and the base of the building by using stronger, but lighter metal rafters. According to the type of material used, they are distinguished following types rafter systems:
- Wooden. Rafters and lathing made of wood are used for the construction of roofs, the length of the slope of which does not exceed 7-10 meters. The frame elements are fastened to each other using self-tapping screws, nails or movable metal elements. The step between the legs is usually within 50-80 cm.
- Metal. Metal roofing frames are made from steel metal profiles with a zinc coating, which is not afraid of moisture. Rafters and sheathing made of this material are light and durable, so the step between them can be increased to 1.5-2 meters. The metal profile is fastened using welding or fasteners. Metal roof components are used for slope lengths of 10 meters or more.
- Combined. Rafter frame combining metal and wooden knots, called combined. The combination of support elements made of wood and galvanized steel allows you to create a cheaper structure with high load-bearing capacity, increasing the pitch between the rafters.
Please note that metal and wooden elements frames cannot be connected to each other without a lining of waterproofing material or treatment with an antiseptic drug. Since metal has high thermal conductivity, its proximity to wood leads to the formation of condensation and rotting of the rafters.
Fastening methods
Metal rafters are assembled into triangular, trapezoidal or arched trusses. Internal stiffeners are attached to the frame beams, which form corners that significantly increase the load-bearing capacity of the frame. This system allows you to make the pitch between the rafters larger, taking into account the support capabilities of each truss. Fastening the metal elements of the roof frame is performed using one of the following methods:
- Fastening by welding. If you weld the parts of the rafter system using welding machine, you can get a rigid structure with high strength and load-bearing capacity. If you correctly calculate the roof, you can lighten the frame and reduce the load on the foundation of the structure. The disadvantage of this method is that only a professional can perform welding using special equipment.
- Fastening with bolts. Fixing the rafters with fasteners allows for a less rigid fastening. This method of assembling a rafter frame based on a metal profile is used in private housing construction, where the length of the slopes does not exceed 10 meters. Eliminating welding allows for faster roof installation.
Experienced craftsmen rely on the fact that rafters can withstand more weight than wooden ones, so you can increase the step between them and reduce the thickness of the cross-section of the elements. Moreover, construction stores sell ready-made trusses, fastened by welding, suitable for covering buildings of standard width.
Advantages
Metal rafter frames are used for the construction of roofs of any shape, any slope with a slope of 1-2 degrees. The materials from which rafters and sheathing are made are steel angles, round and rectangular pipes, and brands. In order to correctly select the thickness of the frame elements and choose the pitch between them, a calculation of the roof structure is performed, taking into account the permanent and temporary loads that are transferred to the rafter beams during operation. The advantages of a rafter system made from this material are:
- Fire safety. Unlike wooden ones, metal frame beams are not flammable, which increases the fire safety of the building.
- Easy to maintain. Zinc electroplating, which covers steel corners, protects the roof frame from corrosion throughout its entire service life. They, unlike wooden ones, do not require annual treatment with an antiseptic.
- Zero waste. Although metal constructions are quite expensive, it is considered cost-effective, since fastening is carried out by welding, and there is practically no waste left.
- Long service life. If you correctly calculate the loads, then roof frame made of metal will last more than 100 years, which exceeds the service life of even the most durable roofing coverings.
Professional roofers believe that it is advisable to use welded metal structures for the manufacture of roof rafter frames with a slope length of 10-12 meters. In this case, the main task is to correctly calculate the loads, and then determine the step between the legs in accordance with the climatic characteristics and properties of the roofing material.
Flaws
Despite the obvious advantages, metal rafter systems are not the most popular design solution in private housing construction. Even big step between the rafters and sparse lathing with high cost metal cannot make a structure cheaper than wood. The disadvantages of metal rafters are:
- High thermal conductivity. Metal has high coefficient thermal conductivity, which is why the rafters form cold bridges. From the point of view of energy efficiency, a frame made of metal profiles is not the best solution.
- Problematic transportation and installation. When choosing a metal rafter system, take into account what to transport, lift to a height and fasten long and heavy elements more complex than standard length lumber rafters.
- Difficult to install. To lift the trusses to a height and fix them, special equipment is used, the rental or purchase of which is expensive.
- Deformation under action high temperature. Although metal is considered a non-combustible material, during a fire it is greatly deformed, which usually leads to the collapse of the roof.
To determine whether it makes sense to use more expensive rafter elements made of metal profiles, you need to calculate the roof frame. If the length of the slope exceeds 10 meters and the load is more than 450-600 kg, then installation metal frame economically feasible.
Video instruction
Rafters are the most essential part of the roofing skeleton. The legs of the rafters transmit the thrust to the mauerlat and load-bearing walls of the house. What does the quality of the entire rafter system and the roof as a whole depend on? Experts remind that the reliability of any roofing structure will depend, first of all, on the quality of fastening the rafters to the Mauerlat. What principles and features of this process should every novice builder know and how to attach the rafters to the Mauerlat with your own hands, you will learn right now.
Attaching the rafters to the mauerlat: a little “materiel”
To begin with, we suggest looking at an illustration that shows two types of rafter systems:
The place where the rafter leg is attached to the base is called the support node. There are many such units in the system, but we will touch in detail on the lower fastenings of the frame with the rafters. The mauerlat and rafter legs are almost always made of wood, less often - of metal. It is more profitable and convenient to use wood, since this material is light in weight, easy to install and durable. correct installation and operation.
Types of support units by level of rigidity
Support nodes are the places where the rafters are attached to the roof elements:
In the case of iron connections, the nodes are fixed and rigid (welded or bolted). Wood is a softer and more dynamic material that can swell, dry out and deform. In this regard, experts recommend making support units with allowances for possible changes in the shape of the wood. Such nodes can have different degrees of mobility:
- Zero mobility node- rigid fastening with corners on both sides, in which the fastening of the rafters to the mauerlat remains motionless.
- Connection of the first degree of freedom– the beam can rotate in a circle.
- Connection of the second degree of freedom– circular rotation with displacement, provision is made for the installation of special sliders or slides.
- Third degree movable joint– possibility of horizontal, vertical and circular movement.
For any node, whether it is movable or not, at least two types of fixation should be used. For example, notched planks are additionally fixed from the inside using a support beam, and dynamic connections are strengthened with bolts and special steel angles.
About the types of fastening of rafters to the base
Let's talk about modern fasteners. For the strength and durability of support units, a variety of metal fastenings: beam holders, equilateral, fastening, anchor, reinforced corners, plates, supports, profiles, connectors, anchors, etc. All these spare parts are made of high-quality metal. For dynamic units (types 1, 2 and 3), sliders, angles and perforated plates are used. For rigid fastening, stationary connectors, anchors and corners are used.
Such fasteners for the rafter system are most often used:
For self-assembly, perforated fasteners are more suitable than others, since they have many holes for self-tapping screws and bolts.
Rigid or flexible connections: what to choose
So, we noted that the support nodes connecting the Mauerlat to the boards can have different degrees of mobility: from “0” to “3”. Zero degree - these are rigid fastenings that exclude any changes in the position of the beams.
Rigid connections: when they are needed
The Mauerlat is installed when it is necessary to transfer the thrust load from the rafters to the load-bearing walls. This is done mainly in houses made of bricks, panels and blocks. Deformation and shrinkage of the roof in in this case they try to exclude it in order to prevent changes in the load on the supporting walls. This is where the need arises for a fixed connection of the rafter system with the Mauerlat.
Fixed units secured with a notch
Many experts recommend making appropriate cuts at the point where the rafters are attached to the floor beams for greater strength and immobility of the connecting nodes. These cuts should fit tightly with the Mauerlat. Additionally, such units are strengthened with bolts, anchors and metal plates:
Or with long screws:
And one more important point: the size of the cut of the bar should not exceed 1/3 of its cross-section. Otherwise, the rafter system may lose its load-bearing capabilities:
Hard knots without notching rafters
The fastening method using a hemming block is used in layered rafter systems. The rafters are cut according to a template and beveled (to give the roof the desired slope) at the point of contact with the mauerlat. From the inside, such rafters are reinforced with support bars and reinforced with corners on both sides to the base frame:
Another option for a non-joint joint is a rigid fastening of the rafters, reinforced with overlay beams on both sides. Two boards at least a meter long border each rafter leg. One end of such supports is cut at an angle that corresponds to the slope of the roof slope (including rafters). The boards are fastened with a cut to the Mauerlat using long bolts and reinforced steel corners. The bars are attached to pre-marked places, first one at a time. Then the rafters themselves are mounted close to the overlays on one side, which are immediately reinforced with the same overlay on the other side. There is an option to install two beams at once, and then rafters, but this method is used less often, since it requires more accurate calculations.
When do you need to make moving connections?
Here we come to dynamic support nodes - connections that can change their position. What is it for? Let's remember physical properties materials - many of them shrink or swell. First of all, this applies to buildings made of pure wood - timber, logs, etc. Natural wood necessarily shrinks, due to which your roof can not only be deformed, but also completely collapse. To avoid such fatal consequences, craftsmen recommend sliding fastenings of rafter legs with a mauerlat (or the upper crown of a log house).
A prerequisite when installing sliding units is to support the rafter frame on a strong ridge beam. Since the supporting lower nodes are dynamic, maximum rigidity should be achieved at the roof ridge. The upper edges of the rafters are sawed down for a tight connection between themselves and the ridge beam, connected and reinforced with crossbars, metal strips, plates and corners. It is better to connect the rafter element already fixed in the ridge to the crown of the log house.
What is a sliding mount?
The movable connection is made by installing sliding fasteners called “slides” or “sliders”. Such a unit provides a certain freedom for the rafter legs, which helps prevent deformation of the roofing system after the natural shrinkage of wooden buildings:
Here are the types sliding supports there are:
Should I make a gash on the rafters if the house is made of timber: an alternative expert opinion
I still suggest cutting down, but not the rafters, but the top crown. Firstly, in this case, the risk of rafter bending is reduced, secondly, the “cold bridge” is reduced, thirdly, the tangential pressure on the mauerlat (top beam of the log house) is reduced, and fourthly, the insulation of the roof in the future is simplified. The disadvantage of this method is that when cutting down the upper crown of the beam, the height of the ridge decreases, therefore, if in the future it is planned to raise the height of the ceiling, one more crown should be provided. But! Such schemes are only suitable for houses made of wood, since in brick and concrete buildings the mauerlat must be intact in order to maintain its load-bearing qualities.
How to properly attach rafters to beams?
In simple houses, mostly frame ones, the Mauerlat can be abolished. In this case, the rafters are attached to the floor beams. The most reliable option for assembling such a system is the preparation of roof trusses. Each truss consists of two rafter legs, a connecting crossbar and a bottom tie. For strength, the trusses are reinforced with central beams and struts:
The floor beams are laid in such a direction that the trusses intersect them perpendicularly in different planes. The tie performs the main load-bearing functions, is attached to the floor beams with anchors, through bolts, studs and reinforced with plates and metal corners.
It is possible to attach the rafter legs directly to the beam, which will additionally serve as a tightening. To create high-quality support units in such a system, two fastening methods are recommended:
- Connecting the rafters to the beam with a double tooth - cuts are made on the beam and the bevel of the rafter leg (two butt notches on each).
- Fastening with bolt and clamp. Through fastening may be provided, but if the boards have a large cross-section, notches are made and the parts are connected with long bolts.
Attaching the rafters to the mauerlat: step-by-step description
As an example, we will describe the process of rigidly attaching rafter connections to the Mauerlat on a simple gable roof.
I. Preparing the mauerlat and rafters for work
At this stage, it is necessary to cut out the rafter legs of a given length and mark their step on the base. Optimal length the pitch of the rafter legs is 60-200 cm. You should also accurately determine the angle of inclination of the rafters.
Master class on installing the Mauerlat:
II. Creating a gash
We make a gash on each rafter leg for a tight connection with the base. To increase strength, you can provide an additional notch on the frame or rafter and install a thrust beam under each rafter leg.
III. Installing rafters on the mauerlat
The rafters must be laid carefully so as not to damage other elements of the building (windows, walls, etc.). We place the boards with a cut on the beams and rest them on the ridge beam. First, we install the outer rafter legs, between which you need to stretch a thread to align all the other rafters.
IV. Fastening each rafter to the mauerlat
Now that all the trusses are in place, you need to firmly secure the support units. For this we use several of the mounts suggested above:
- Nails+ steel corners on the left and right sides of the joints between the board and the base.
- Through bolts or studs+ support beam for rafter bevel.
- Anchors or bolts+ angles or steel plates, etc.
Two overhead boards on both sides of the rafter leg, installed on pre-marked places on the mauerlat, will help to strengthen the fastening. You can also use wire fastening as a strength enhancer for connections. To do this, you need to prepare a steel wire twist of 2-3 wires. Its length should be enough to wrap around the rafter leg at the junction with the Mauerlat and secure the ends of the twist to a metal crutch. As a crutch, you can take a long steel bolt, which is mounted into the wall 30-40 cm below the Mauerlat, strictly under the support unit.
We will also consider one of the old-fashioned methods - fastening with staples:
And finally, we suggest you watch the video:
What is the secret of the quality of the rafter system: three main rules of the master
- High-quality lumber is half the battle on the path to successful construction. The mauerlat and rafters should not have cracks, wormholes or knots.
- The accuracy of measurements, cuts and uniformity of the position of the fasteners is an equally important point. If all the rafter legs are the same length and cross-section, then it is better to prepare a template for making cuts and notches.
- Gaps on the Mauerlat – loss of the load-bearing functions of the base by 50% or more. The percentage of strength reduction depends on the depth of the notches.
When the rafter system is ready, it’s time to install the sheathing, insulation and roofing material. But this is something else interesting topic, which we will definitely talk about in the next article. In the meantime, we wish you good materials, easy work and good helpers!
When designing any residential building, architects pay special attention to the roof, since it performs not one, but several functions at once, depending on its design features. It must be said that not all future homeowners are satisfied with the usual gable roof, although it can be called the most reliable, since it has only two pitched planes and one joint between them. Many people are more attracted to complex designs, which add special attractiveness and originality to the building. Other, more practical homeowners prefer attic structures, which can simultaneously serve as a roof and a second floor.
The basis of any roof is an individual rafter system, which has its own design features. It will be much easier to choose the right roof frame if you figure out which ones in advance. types and diagrams of rafter systems used in construction practice. After receiving such information, it will become more clear how difficult such structures are to install. This is especially important to know if you plan to build the roof frame yourself.
Main functional tasks of rafter systems
When arranging pitched roof structures, the rafter system serves as a frame for covering and for holding the materials of the “roofing pie”. With proper installation of the frame structure, the necessary conditions for correct and non-insulated types of roofs that protect the walls and interior of the house from various atmospheric influences.
Roof structure is also always the final architectural element of the exterior design of a building, supporting it with its appearance stylistic direction. However, the design features of rafter systems must first of all meet the strength and reliability requirements that the roof must meet, and only then the aesthetic criteria.
The frame of the rafter system forms the configuration and angle of inclination of the roof. These parameters largely depend on natural factors, characteristic of a particular region, as well as the desires and capabilities of the homeowner:
- Precipitation in different periods of the year.
- The direction and average speed of the wind in the area where the building will be erected.
- Plans for the use of space under the roof - arranging residential or non-residential premises, or using it only as an air gap for thermal insulation of the rooms below.
- Type of planned material roofing.
- Financial capabilities of the homeowner.
Atmospheric precipitation and the strength of wind currents place a very sensitive load on the roof structure. For example, in regions with heavy snowfall, you should not choose a rafter system with a small slope angle, since snow masses will linger on their surface, which can lead to deformation of the frame or roofing or to leaks.
If the area where the construction will take place is famous for its winds, then it is better to choose a structure with a slight slope of the slope so that sudden gusts that occur do not tear off individual elements of the roof and roofing.
Main elements of roof structure
Parts and components of rafter systems
Depending on the chosen type of rafter system, the structural elements used can vary significantly, however, there are parts that are present in both simple and complex systems roofs
The main elements of a pitched roof rafter system include:
- Rafter legs that form the roof slopes.
- - a wooden beam fixed to the walls of the house and used to fix the lower part of the rafter legs on it.
- A ridge is the junction of the frames of two slopes. It is usually the highest horizontal line roof and serves as a support on which the rafters are fixed. The ridge can be formed by rafters fastened together at a certain angle or fixed on a ridge board (purlin).
- Sheathing is slats or beams mounted on rafters at a certain pitch and serving as the basis for laying the selected roofing material.
- Supporting elements, which include beams, purlins, racks, struts, ties and other parts, serve to increase the rigidity of the rafter legs, support the ridge, and connect individual parts into an overall structure.
In addition to the mentioned design details, it may also include other elements, the functions of which are aimed at strengthening the system and optimally distributing roof loads on the walls of the building.
The rafter system is divided into several categories depending on different features of its design.
Attic space
Before moving on to considering different types of roofs, it is worth understanding what an attic space can be, since many owners successfully use it as utility and full-fledged residential premises.
The design of pitched roofs can be divided into attics and attics. The first option is called this way because the space under the roof has a small height and is used only as an air layer insulating the building on top. Such systems usually include or have several slopes, but located at a very slight angle.
An attic structure that has a sufficiently high ridge height can be used in different ways, be insulated and not insulated. Such options include an attic or gable option. If you choose a roof with a high ridge, then it is imperative to take into account the wind loads in the region where the house is built.
Slope slope
To determine optimal inclination roof slopes of a future residential building, first of all you need to take a closer look at the already built low-rise neighboring houses. If they have been standing for more than one year and can withstand wind loads, then their design can safely be taken as a basis. In the same case, when the owners set a goal to create an exclusive original project, unlike the neighboring buildings, it is necessary to familiarize themselves with the design and operational features of various rafter systems and make the appropriate calculations.
It should be borne in mind that the change in tangents and normal values wind forces - the steeper the angle of inclination, the greater the importance of normal forces and the less tangential forces. If the roof is flat, then the structure is more affected by the tangential wind load, since the lifting force increases on the leeward side and decreases on the windward side.
Winter snow load should also be taken into account when designing the roof. Usually this factor is considered in conjunction with the wind load, since on the windward side the snow load will be much lower than on the leeward slope. In addition, there are places on the slopes where snow will certainly accumulate, putting a large load on this area, so it should be reinforced with additional rafters.
The slope of roof slopes can vary from 10 to 60 degrees, and it should be chosen not only taking into account the consolidated external load, but also depending on the roofing that is planned to be used. This factor is taken into account because roofing materials differ in their mass, their fastening requires a different number of elements of the rafter system, which means that the load on the walls of the house will vary, and how large it will be also depends on the angle of the roof. Of no small importance are the characteristics of each coating in terms of resistance to moisture penetration - many roofing materials in any case require one or another slope to ensure the free drainage of storm water or melting snow. In addition, when choosing a roof slope, you need to think in advance about how the process of cleaning and repair work on the roof will be carried out.
When planning a particular angle of the roof slopes, you need to know that the fewer joints between the sheets of roofing, and the more airtight they are, the less you can make the slope of the slope, of course, if it is not intended to be arranged in attic space residential or commercial premises.
If a material consisting of small elements is used to cover the roof, for example, ceramic tiles, then the slope of the slopes must be made steep enough so that water never lingers on the surface.
Considering the weight of the roofing material, you need to know that the heavier the covering, the larger the angle of the slopes should be, since in this case the load will be correctly distributed over the rafter system and load-bearing walls.
Can be used to cover the roof following materials: or profile sheet, galvanized steel, corrugated asbestos concrete and bitumen-fiber sheets, cement and ceramic tiles, roofing felt, soft roof and other roofing materials. The illustration below shows the permissible slope angles for various types roofing coverings.
Basic designs of rafter systems
First of all, it is worth considering basic types rafter systems relative to the location of the walls of the house, which are used in all roof structures. Basic options They are divided into layered, hanging, and also combined, that is, including elements of both the first and second types of systems in their design.
fastenings for rafters
Layered system
In buildings where internal load-bearing walls are provided, a layered rafter system is often installed. It is much easier to install than a hanging one, since the internal load-bearing walls provide reliable support for its elements, and in addition, this structure will require less materials.
For rafters in this system, the defining reference point is the ridge board, on which they are fixed. The non-thrust type of layered system can be arranged in three options:
- In the first option, the upper side of the rafters is fixed on a ridge support, called a sliding one, and their lower side is fixed by cutting to the mauerlat. Additionally, the rafters in the lower part are fixed to the wall using wire or staples.
- In the second case, the rafters in the upper part are cut at a certain angle and connected to each other using special metal plates.
The lower edge of the rafter legs is attached to the Mauerlat with movable fasteners.
- In the third option, the rafters are rigidly fastened in the upper part with bars or treated boards located horizontally, parallel to each other on both sides of rafters connected at an angle, and a ridge girder is clamped between them.
In the lower part, sliding fasteners are used to secure the rafters, just as in the previous case.
It is necessary to explain why sliding fasteners are often used to secure rafters to the mauerlat. The fact is that they are able to relieve load-bearing walls from excessive stress, since the rafters are not rigidly fixed, and when the structure shrinks, they are able to move without deforming the overall structure of the roofing system.
This type of fastening is used only in layered systems, which also distinguishes them from the hanging version.
However, in some cases, for layered rafters, a spacer system is used, in which the lower end of the rafters is rigidly fixed to the Mauerlat, and to relieve the load from the walls, tie-downs and struts are built into the structure. This option is called complex, as it includes elements of a layered and hanging system.