Rafters perform a number of significant roofing functions. They set the configuration of the future roof, absorb atmospheric loads, and hold the material. Among the rafter's responsibilities is the formation of smooth planes for laying the covering and providing space for components roofing pie.
In order for such a valuable part of the roof to flawlessly cope with the listed tasks, information is needed about the rules and principles of its design. The information is also useful for those who are constructing the rafter system gable roof with their own hands, and for those who decide to resort to the services of a hired team of builders.
In device rafter frame For pitched roofs use wooden and metal beams. The starting material for the first option is a board, log, timber.
The second is constructed from rolled metal: channel, profile pipe, I-beam, corner. There are combined structures with the most heavily loaded steel parts and wood elements in less critical areas.
In addition to its “iron” strength, metal has many disadvantages. These include thermal qualities that are unsatisfactory to the owners of residential buildings. The need to use welded joints is disappointing. Most often, industrial buildings are equipped with steel rafters, and less often, private cabins assembled from metal modules.
In business self-construction Wood is the priority for rafter structures for private houses. It is not difficult to work with, it is lighter, “warmer”, and more attractive in terms of environmental criteria. In addition, to perform nodal connections you will not need welding machine and welding skills.
Rafters - a fundamental element
The main “player” of the frame for constructing a roof is the rafter, called among roofers rafter leg. Beams, braces, headstocks, purlins, ties, even a Mauerlat may or may not be used depending on the architectural complexity and dimensions of the roof.
Rafters used in the construction of gable roof frames are divided into:
- Layered rafter legs, both heels of which have reliable structural supports under them. The lower edge of the layered rafter rests against the mauerlat or the ceiling crown of the log house. The support for the upper edge can be a mirror analogue of the adjacent rafter or a purlin, which is a beam laid horizontally under the ridge. In the first case, the rafter system is called spacer, in the second, non-spacer.
- Hanging rafters, the top of which rests against each other, and the bottom is based on an additional beam - a tie. The latter connects the two lower heels of adjacent rafter legs, resulting in a triangular module called a rafter truss. Tightening dampens the tensile processes, so that only vertically directed load acts on the walls. Design with hanging rafters Although it is a spacer, the spacer itself does not transfer to the walls.
In accordance with the technological specifics of rafter legs, the structures constructed from them are divided into layered and hanging. For stability, the structures are equipped with struts and additional racks.
To support the top of the layered rafters, planks and purlins are installed. In reality, the rafter structure is much more complex than the elementary templates described.
Note that the formation of a gable roof frame can generally be done without truss structure. In such situations, the supposed planes of the slopes are formed by slabs - beams laid directly on the load-bearing gables.
However, we are now interested specifically in the structure of the rafter system gable roof, and it can involve either hanging or layered rafters, or a combination of both types.
Subtleties of fastening rafter legs
Fastening the rafter system to brick, foam concrete, aerated concrete walls is carried out through the Mauerlat, which in turn is fixed with anchors.
Between the Mauerlat, which is a wooden frame, and the walls made of the specified materials in mandatory a waterproofing layer of roofing material, waterproofing material, etc. is laid.
The top of brick walls is sometimes specially laid out so that along the outer perimeter there is something like a low parapet. This is so that the mauerlat placed inside the parapet and the walls do not push apart the rafter legs.
Roof frame rafters wooden houses rest on the upper crown or on ceiling beams. The connection in all cases is made by notches and is duplicated with nails, bolts, metal or wooden plates.
How to do without mind-boggling calculations?
It is highly desirable that the section and linear dimensions wooden beams were determined by the project. The designer will provide clear calculation justification for the geometric parameters of the board or beam, taking into account the entire range of loads and weather conditions. If available home handyman there is no design development, its path lies on the construction site of a house with a similar roofing structure.
You don’t have to pay attention to the number of floors of the building being constructed. It is easier and more correct to find out the required dimensions from the foreman than to find out them from the owners of a shaky self-built building. After all, in the hands of the foreman is documentation with a clear calculation of the loads per 1 m² of roof in a specific region.
The installation pitch of the rafters determines the type and weight of the roofing. The heavier it is, the smaller the distance between the rafter legs should be. For laying clay tiles, for example, the optimal distance between the rafters will be 0.6-0.7 m, and for corrugated sheets 1.5-2.0 m is acceptable.
However, even if the pitch required for proper installation of the roof is exceeded, there is a way out. This is a reinforcing counter-lattice device. True, it will increase both the weight of the roof and the construction budget. Therefore, it is better to understand the pitch of the rafters before constructing the rafter system.
Craftsmen calculate the pitch of the rafters according to the design features of the building, simply dividing the length of the slope into equal distances. For insulated roofs, the pitch between the rafters is selected based on the width of the insulation slabs.
You can find it on our website, which may also help you a lot during construction.
Rafter structures of layered type
Layered rafter structures are much simpler to construct than their hanging counterparts. A reasonable advantage of the layered scheme is to ensure adequate ventilation, which is directly related to long-term service.
Distinctive design features:
- It is mandatory to have support under the ridge heel of the rafter leg. The role of support can be played by the run - wooden beam, resting on racks or on the internal wall of the building, or the upper end of an adjacent rafter.
- Using a Mauerlat to erect a truss structure on walls made of brick or artificial stone.
- The use of additional purlins and racks where the rafter legs, due to the large size of the roof, require additional support points.
The disadvantage of the scheme is the presence of structural elements that affect the layout of the internal space of the attic being used.
If the attic is cold and it is not intended to organize useful rooms, then the layered structure of the rafter system for installing a gable roof should be given preference.
Typical sequence of work for the construction of a layered truss structure:
- First of all, we measure the heights of the building, the diagonals and horizontality of the upper cut of the frame. When identifying vertical deviations of brick and concrete walls, we remove them with a cement-sand screed. Exceeding the heights of the log house is cut off. By placing wood chips under the mauerlat, vertical flaws can be combated if their size is insignificant.
- The floor surface for laying the bed must also be leveled. It, the Mauerlat and the girder must be clearly horizontal, but the location of the listed elements in the same plane is not necessary.
- We treat all wooden parts of the structure with fire retardants and antiseptics before installation.
- On concrete and brick walls We lay waterproofing under the installation of the Mauerlat.
- We lay the mauerlat beam on the walls and measure its diagonals. If necessary, we slightly move the bars and turn the corners, trying to achieve the ideal geometry. Align the frame horizontally if necessary.
- We mount the Mauerlat frame. The beams are joined into a single frame using oblique notches; the joints are duplicated with bolts.
- We fix the position of the Mauerlat. Fastening is done either with staples to wooden plugs placed in the wall ahead of time, or anchor bolts.
- Mark the position of the prone position. Its axis should recede from the mauerlat bars at equal distances on each side. If the run will rest only on posts without supports, we carry out the marking procedure only for these posts.
- We install the bed on a two-layer waterproofing. We fasten it to the base with anchor bolts, with internal wall We connect with wire twists or staples.
- We mark the installation points of the rafter legs.
- We cut out the racks to uniform sizes, because... Our bed is exposed to the horizon. The height of the racks should take into account the cross-sectional dimensions of the purlin and beam.
- We install racks. If provided by the design, we secure them with spacers.
- We lay the purlin on the racks. We check the geometry again, then install brackets, metal plates, and wooden mounting plates.
- We install a test rafter board and mark the cutting areas on it. If the Mauerlat is set strictly to the horizon, there is no need to adjust the rafters on the roof after the fact. The first board can be used as a template for making the rest.
- We mark the installation points of the rafters. For marking, folk craftsmen usually prepare a pair of slats, the length of which is equal to the clearance between the rafters.
- According to the markings, we install the rafter legs and fasten them first at the bottom to the mauerlat, then at the top to the purlin to each other. Every second rafter is screwed to the Mauerlat with a wire bundle. IN wooden houses The rafters are screwed to the second crown from the top row.
If the rafter system is made flawlessly, the layer boards are installed in any order.
If there is no confidence in the ideal structure, then the outer pairs of rafters are installed first. A control string or fishing line is stretched between them, according to which the position of the newly installed rafters is adjusted.
The installation of the rafter structure is completed by installing fillets, if the length of the rafter legs does not allow forming an overhang of the required length. By the way, for wooden buildings the overhang should extend beyond the contour of the building by 50 cm. If you plan to organize a canopy, separate mini-rafters are installed under it.
Another useful video about building a gable rafter base with your own hands:
Hanging rafter systems
The hanging variety of rafter systems is a triangle. The two upper sides of the triangle are folded by a pair of rafters, and the base is the tie connecting the lower heels.
The use of tightening allows you to neutralize the effect of the thrust, therefore, only the weight of the sheathing, roof, plus, depending on the season, the weight of precipitation, acts on walls with hanging rafter structures.
Specifics of hanging rafter systems
Characteristic features of hanging type rafter structures:
- The obligatory presence of a tie, most often made of wood, less often of metal.
- Possibility to refuse to use the Mauerlat. A timber frame can be successfully replaced by a board laid on double-layer waterproofing.
- Installation of ready-made closed triangles on walls - roof trusses.
The advantages of the hanging scheme include the space under the roof free from racks, which allows you to organize an attic without pillars and partitions. There are disadvantages.
The first of them is restrictions on the steepness of the slopes: their slope angle can be at least 1/6 of the span of a triangular truss; steeper roofs are strongly recommended. The second disadvantage is the need for detailed calculations for the proper installation of cornice units.
Among other things, the angle of the truss will have to be installed with pinpoint precision, because the axes of the connected components of the hanging rafter system must intersect at a point, the projection of which must fall on the central axis of the Mauerlat or the backing board replacing it.
Subtleties of long-span hanging systems
The tie is the longest element of a hanging rafter structure. Over time, as is typical for all lumber, it becomes deformed and sags under the influence of its own weight.
Owners of houses with spans of 3-5 m are not too concerned about this circumstance, but owners of buildings with spans of 6 meters or more should think about installing additional details, excluding geometric changes in tightening.
To prevent sagging, there is a very significant component in the installation diagram of the rafter system for a long-span gable roof. This is a pendant called a grandmother.
Most often it is a block attached with wooden pegs to the top of the truss. The headstock should not be confused with the racks, because its lower part should not come into contact with the puff at all. And installation of racks as supports in hanging systems does not apply.
The bottom line is that the headstock hangs, as it were, on the ridge assembly, and a tightening is attached to it using bolts or nailed wooden plates. To correct sagging tightening, threaded or collet-type clamps are used.
The tightening position can be adjusted in the area of the ridge assembly, and the headstock can be rigidly connected to it by a notch. Instead of a bar in non-residential attics, reinforcement can be used to make the described tension element. It is also recommended to install a headstock or hanger where the tie is assembled from two beams to support the connection area.
In an improved hanging system of this type, the headstock is complemented by strut beams. The stress forces in the resulting rhombus are extinguished spontaneously due to the proper placement of vector loads acting on the system.
As a result, the rafter system is stable with minor and not too expensive modernization.
Hanging type for attics
In order to increase the usable space, the tightening of the rafter triangles for the attic is moved closer to the ridge. A completely reasonable move has additional advantages: it allows you to use the puffs as a basis for lining the ceiling.
It is connected to the rafters by cutting with a half-pan and duplicating with a bolt. It is protected from sagging by installing a short headstock.
A noticeable disadvantage of the hanging attic structure is the need for accurate calculations. It is too difficult to calculate it yourself; it is better to use a ready-made project.
Which design is more cost effective?
Cost is an important argument for an independent builder. Naturally, the price of construction for both types of rafter systems cannot be the same, because:
- In the construction of a layered structure, a board or beam of small cross-section is used to make rafter legs. Because layered rafters have two reliable supports underneath them; the requirements for their power are lower than in the hanging version.
- In the construction of a hanging structure, the rafters are made of thick timber. To make a tightening, a material with a similar cross-section is required. Even taking into account the abandonment of the Mauerlat, the consumption will be significantly higher.
It will not be possible to save on the grade of material. For the load-bearing elements of both systems: rafters, purlins, beams, mauerlat, headstocks, racks, 2nd grade lumber is needed.
For crossbars and tensile ties, grade 1 will be required. In the manufacture of less critical wooden overlays, grade 3 can be used. Without counting, we can say that in the construction of hanging systems, expensive material is used in greater quantities.
Hanging trusses are assembled in an open area next to the facility, then transported, assembled, upstairs. To lift weighty triangular arches from timber, you will need equipment, the rental of which will have to be paid. And the project for complex nodes of the hanging version is also worth something.
Video instruction on the installation of a hanging category truss structure:
There are actually many more methods for constructing rafter systems for roofs with two slopes.
We have described only the basic varieties, which in reality are applicable for small country houses and buildings without architectural tricks. However, the information presented is enough to cope with the construction of a simple truss structure.
Perhaps the main decoration of any house is the roof, without which it is difficult to imagine any structure. The roof gives a private home a holistic appearance and defines his style. However, first of all, it acts as an engineering structure designed to protect people from the destructive effects of the external environment.
Today, the use of various architectural elements in the construction of houses, undeservedly forgotten earlier, is returning to fashion. For example, mezzanines, bay windows, and attics are becoming increasingly common, giving the building a stylish and interesting exterior. At the same time, you can achieve a noticeable increase in internal space at a relatively low cost of money.
The roof truss system is considered the most strong, durable structure. It is characterized by ease of installation, ease of adjustment and the ability to withstand heavy loads, with a relatively light weight. The rafter system is a special structure, with a triangle at the base. Its rigidity is achieved by fastening the rafters and the roof frame together. The most common options are made from wooden logs or beams, however, there are also systems made from galvanized steel.
Types of rafter systems
Experts distinguish several types of rafter systems. They are accepted classified by roof shape and type:
- hipped;
- tri-slope;
- gable;
- hip;
- half-hip;
- broken
Gable rafter system roofing is the cheapest and simplest option. The main advantages of this design include simplicity, reliability and the ability to withstand significant loads. At the same time, it is worth describing some disadvantages. Namely, the impossibility of implementing any special design solutions due to the simplicity of the shape of such a roof. However, this minus can easily be compensated by decorating the house with various decorative elements. The second drawback can be called more significant - inner space small in size, significantly different from the parameters sloping roof.
The most common and recognized among lovers of interesting architectural solutions, is sloping roof truss system. In this case, there is an opportunity for the manifestation of imagination, of course, subject to established construction standards and strength characteristics. This design can be regular or asymmetrical in shape and is determined based on the structure of the house itself, as well as the design and layout of the interior of the room.
Sometimes you can get an extremely interesting room under the roof by correctly installing the rafter system and organically using a second light or mezzanine. In order for relatively small financial costs To maximize the usable area of the house and make it cozy, it is very important to be able to effectively and fully use the available space.
Features and requirements for the rafter system
The size of the rafters, which are elements that determine the strength and slope of the roof, must be calculated individually for each private house, taking into account total area and the geometric shape of the roof, as well as the distance between the walls. Depending on the results obtained, it will be possible to determine the appropriate cross-section of the beams. As a rule, this figure is about 1 meter.
In practice they use two types of rafters:
- An inclined structure that rests on the walls of the house along its entire perimeter, and is supported in the center by an additional or intermediate support. It is advisable to use if the distance between several supports is no more than 6.5 m.
- Hanging rafters, fixed at the top at the ridge and resting on each other. This system is particularly reliable when connecting walls to each other.
In general, the design of the rafter system directly depends on the type of material from which the house is made. For example, for a building built of brick, a rafter system is recommended that rests on a separately constructed support made of small-section beams or a mauerlat. As a support for rafters wooden structure the top crown of a log house is best suited, and for houses frame type use the top harness.
Main parts of the roof truss system
The basis of the roof on which it is made roofing pie installation And internal lining premises, is precisely the rafter system. It is often also used as a basis for communications and engineering systems. The set of various components and elements that make up the roof truss system performs the function of transferring power loads directly from the roof to the load-bearing walls of the structure. The main parts include:
- Hanging and inclined rafters.
- Mauerlat.
- Ridge and side girders.
- All kinds of connecting elements, such as struts, diagonal braces, spacers.
As a rule, the load on the roof truss structure is very high. It is determined individually, taking into account the design features of the roof and the wind and snow loads characteristic of the area. Installed rafters must guarantee the retention of such loads, and also have the necessary margin of safety to withstand possible gusts of wind or snowfall.
In order to determine the main parameters of the components of the roof truss system, it is necessary perform special calculations. They will help to accurately determine the profile, length and cross-section of each of the structural elements, as well as the features of the interaction of the parts with each other. The strength of the entire structure largely depends on how tightly the units of the roof truss system are connected. Recommended use the following types connections:
What type of connection is applicable for specific system, can be determined based on the design load, design features and materials used.
Installation principles
Installation work carried out in three main stages. In particular, markings that take into account the location of window openings, hoods, chimneys, installation of the Mauerlat and reliable fastening it to the supporting support. Next, the trusses are erected and, finally, the ridge is installed.
Installation should begin by laying the Mauerlat on top of the walls and attaching the rafters directly to it. They must be installed in the desired position and secured at one end to the support, and at the other to the ridge beam. For reliability, the connections can be additionally secured to each other with studs with a diameter of 8 -12 mm.
When constructing a truss structure, the most common an error is an incorrect calculation rafter sizes, which can lead to sagging of the roof.
It is necessary to provide for each element and node and understand the technology at the preliminary stage. At the same time, it is also recommended to plan the eaves overhang with a length of 60 cm, which will reduce the effect of weather phenomena on the walls of the house.
For wooden structures that may dry out over time, the most appropriate fastening option would be bolted connections, capable of adding rigidity to the entire structure.
If you follow all the basic rules, installing a roof truss system can be done relatively easily. However, it is important to strictly adhere to the chosen technology to avoid the risk of various problems.
For the manufacture of rafters in individual construction Wooden materials are usually used: boards, timber, logs. Despite their relative cheapness, wooden roof rafters are strong enough to withstand all roofing loads and serve faithfully for many years.
Other possible materials for rafter systems - metal and reinforced concrete - it is not practical to use for private construction, due to their heavy weight, complex installation and high cost.
The rafter system must be strong, but not heavy. Of course, for the load-bearing base of the roof of large industrial buildings and high-rise buildings, you have to use metal or reinforced concrete. But for ordinary private houses this is an unnecessary excess. In this case, the rafters are made of wood - from boards, timber (regular or glued), logs.
Logs are rarely used, exclusively for log houses. This material is too heavy, requiring high professionalism from the carpenter and the ability to make complex cuts in the fastening areas.
Timber is the best option from which you can mount strong and durable rafters. The only drawback of the timber is the high price.
Cheaper boards with a minimum thickness of 40-60 mm are often used as a replacement for timber. The list of their advantages also includes low weight, ease of installation and a high margin of safety.
The following requirements apply to the selected lumber:
- The minimum acceptable types of wood are 1-3. The presence of knots is allowed in small quantities (it is better to do without them at all!), no more than three knots, up to 3 cm high, per 3 m.p. Cracks are also acceptable, but they should not penetrate right through the wood, and their length cannot exceed half the length of the material.
- It is allowed to use dried wood with a moisture content of up to 18-22%. If these indicators are higher, the rafters, as they dry, may crack or bend and lose their shape.
- The load-bearing parts of the rafter system are made of material with a thickness of 5 cm and a width of 10-15 cm.
- Length of elements from coniferous species- up to 6.5 m, and from hard deciduous trees - up to 4.5 m.
- All wooden parts of the rafters must be treated before use. protective compounds, preventing their rotting, fire and damage by wood-boring insects.
Main parts of a wooden rafter system
The main component unit of a wooden rafter system is a truss - a flat triangular structure. The sides of the “triangle” form rafter legs connected at the top at an angle. To connect the rafters horizontally, tie-rods, crossbars, and clamps are used.
The rafter system is made up of several trusses placed on the mauerlat and fastened together by purlins.
To better understand the specifics of the farm, let’s define its elements. Their composition and quantity in one structure depends on the type of roof, its dimensions and the type of rafters used.
So, the components could be like this:
- Rafter leg- These are the rafters themselves, onto which the sheathing is stuffed and the roofing material is laid. The truss consists of two rafters (beams) connected at the top at the ridge in the form of a triangle. Their angle of inclination equal to angle slope of roof slopes.
- Puff– a crossbar that fastens the rafter legs horizontally and prevents them from moving in different directions under load. Used in a hanging rafter system.
- Rigel- a beam similar to a puff, but working on a different principle. In the system it is compressed, not stretched. Fastens the rafter beams in their upper part.
- Fight– also a horizontal crossbar that connects the rafter beams and increases the stability of the truss. Used in a layered rafter system.
- Rack– a horizontal beam that serves as an additional support for fixing the rafter legs.
- Strut- an element mounted at an angle to the horizontal, giving the rafters additional stability.
- fillies– used to lengthen rafter legs when it is necessary to create overhangs.
Also, the rafter system can include parts that are not directly related to the trusses, but are used for their installation and assembly. They are:
- Run- a beam running along the slopes, connecting the rafter legs of the trusses. A special case is a ridge purlin, which is installed along the roof slopes at its highest point (ridge).
- Lathing- consists of beams or boards placed on the rafters from above along the roof slopes. Roofing material is mounted on the sheathing.
- Mauerlat– timber or boards laid along the perimeter of the external (main) walls of the building. The presence of a Mauerlat is provided for securing the lower ends of the rafters to it.
- Sill- an element similar to a Mauerlat, but laid along the inner wall of the building. Vertical posts are fixed to the bed.
Types of rafter systems
From wood you can assemble many options for trusses and, accordingly, rafter systems. But all of them can be divided into two types: hanging and layered.
Hanging rafter systems
Used for rooms without internal walls. Trusses made up of rafters rest solely on the outer walls; there is no need for additional support. That is, hanging rafters cover one span, 6-14 m wide.
An obligatory part of hanging trusses, in addition to the rafter legs connected at the top at an angle, is a tie - a horizontal beam connecting the rafters. The tie becomes the basis of the “triangle” of the truss. In most cases, it is located at the bottom of the structure, connecting the lower ends of paired rafters. But raised-pull designs are also used. And also with its modified version - a crossbar, which looks like a raised tightening, but works on compression, and not on tension, like a true tightening.
The need to use a Mauerlat depends on the presence of the tie and its location in the farm. If the tightening is located at the base of the rafter legs, then the Mauerlat is not needed. During installation, the truss is supported on the external walls through the existing tie, which at the same time becomes a floor beam. If the tie is raised upward or a crossbar is used instead, then the Mauerlat must be included in the diagram as the basis for attaching the rafters to the upper edges of the walls.
As additional elements in a hanging system, headstocks and struts are used. They serve to strengthen the truss when covering wide spans.
The headstock in appearance resembles a vertical post running from the center of the tightening to the top point of the truss (ridge part). In fact, the headstock is a suspension, the function of which is to support a pull that is too long (more than 6 m) and prevent it from sagging.
In tandem with the headstock, with an even greater increase in the tightening length, struts are used - diagonal beams. They are rested with one end against the rafter leg, and the other against the headstock. In one farm, two struts are used, on both sides of the headstock.
In country houses and small private houses, hanging rafters made of wood are good because they allow you to create spacious attic rooms no partitions inside. Of course, we are talking about schemes where there are no struts and headstocks. Their presence imposes on the developer the need to divide the attic into at least two rooms.
Layered rafter systems
The layered structure of wooden rafters is used for rooms with internal capital walls, which serve as additional support for the system. In this case, the distance between the external walls (total overlapped span) can be within 6-15 m.
Rafter trusses necessarily consist of rafter legs supported on the outer walls and a vertical drain supported on the inner wall. If there are two internal walls, two studs can be used in the scheme.
Unlike a hanging system, a layered system must have a Mauerlat to which the rafter legs are attached. The racks crash into a kind of mauerlat - a bench. This is a timber laid on top of an internal supporting wall.
For a span of 6 m or less, the simplest layered truss is used, consisting of two paired rafter legs and a stand. The construction of wooden rafters with an increase in span requires the addition of additional details to the diagram, such as contractions and struts (rafter legs).
The contractions are similar to the ties in hanging systems, but they are always located above the base of the rafters. The main purpose of scrum is to increase the stability of the system.
For stability, struts, also called rafter legs, are also provided. The strut supports the rafter leg, that is, in fact, it becomes an additional (third in a row, after the Mauerlat and ridge girder) support for it.
Layered wooden rafters are the most common for private residential buildings, cottages. As a rule, such buildings have one or more internal main partition walls, which can become a support and additional support for a strong rafter system.
Methods of fastening rafters to the Mauerlat
The attachment point of the rafters to the mauerlat is one of the most important; the functionality of the rafter system and its ability to bear loads depend on its correct execution.
There are two types of such fastenings: rigid and sliding. The choice of one of them depends on the design of the truss structure. Replacing a rigid fastening with a sliding one or vice versa, as well as insufficient provision of the required degree of shift of the rafter leg, will lead to the fact that the circuit will “break” and will not work.
The rigid fastening ensures a strong, motionless connection between the rafter and the mauerlat. Shift is not allowed, but the rafter can be rotated at the hinge. Such fastening is organized in two main ways:
- by cutting the rafter beam into the mauerlat and further fixing the assembly with corners, staples, nails;
- using metal corners and a support bar.
A sliding fastening (or, as roofers call it, a “sliding fastener”) is of a slightly different type and functionality. It allows the rafter leg to move relative to the support. Of course, this shift will not be noticeable to the eye, but will allow the rafter system not to deform during the natural shrinkage of the walls of the house. Sliding fastening is especially necessary in the construction of wooden log houses. It is also implemented in the construction of houses of any other type, if required by the design and calculations of wooden rafters.
To provide the rafter leg with a small range of movement relative to the Mauerlat, special fastening elements are used - sliders. Structurally, they consist of two metal elements, the first of which is static, and the second is capable of moving relative to the first. There are two varieties sliding fastenings: open and closed types.
The open type slider is a prefabricated structure consisting of two individual parts: a static guide bar and a corner with a bend at the top. The guide is threaded into the bend of the corner and fixed on the rafter leg, the corner is attached to the mauerlat. When changing the geometric dimensions of the building, the guide can move relative to the rigidly fixed corner by 60-160 mm.
Sliding mount closed type its properties are exactly the same. The design changes a little; it is no longer collapsible, but solid. The corner, which is attached to the Mauerlat, has a loop in the central part. A guide is inserted into it, which, in turn, is attached to the rafter leg.
What both fastening options are (sliding and rigid) is shown in the video:
Another important component of the rafter truss is the rafter attachment point in the ridge part. In private housing construction, the following fastening options are most often used for these purposes:
- overlap;
- end-to-end;
- using a half-tree cut.
Overlapping fastening is considered the simplest option. The upper edges of paired rafters are simply placed on top of each other. Then a hole is made at both ends and the connection is secured with a pin or bolt and nut.
To make a butt connection, the upper ends of the rafter legs are cut off at an angle to make it possible to align the sawn surfaces. Fastening is carried out using nails, which are driven into the end of the ridge part through both rafter legs. To further secure the nail joint, use wooden horizontal plates or metal plates that are placed over the joint on both sides of the truss.
A half-tree connection involves preliminary cutting out notches at the upper ends of the rafters to half the thickness of the beam. This allows you to combine the rafters in the ridge, like parts of a designer, without increasing the thickness of the ridge assembly (as happens with an overlap connection). After combining the parts, they are fixed with nails, bolts or dowels.
In addition to the methods described, there are other, less common ones. For example, a tongue-and-groove connection. It is not popular, as it requires great professionalism from the carpenter. The essence of the fastening is that a groove is made in one rafter, and a tenon is cut out on the other. The tenon and groove are combined and fastened with a nail or dowel.
One of the possible connections of the rafters in the ridge (end-to-end, through the ridge purlin) is discussed in the video:
Advantages and disadvantages of wooden rafters
However, working with wood and installing wooden rafters does not present any particular difficulties. In addition, there are other advantages of using wood as a material for rafters:
- low cost of wood;
- universal availability;
- relatively light weight, simplifying installation;
- there is no need to use heavy construction equipment;
- versatility, the ability to be used on buildings made of any material, regardless of the bearing capacity of the foundation.
The disadvantages of choosing wooden rafters are minor, but they also need to be known in person before starting construction:
- the need for treatment with protective agents that prevent fire and rotting of wood, as well as reducing its “attractiveness” to various insect pests;
- the use of wooden rafters is possible only on spans up to 14-17 m; for wider spans it is recommended to use metal or reinforced concrete;
- slightly reduced service life compared to metal or reinforced concrete trusses.
Thus, all the shortcomings are features rather than real negative aspects. This explains such a widespread use of wooden rafters in private housing construction.
At the heart of every roof is a large number of beams, rafters, posts and purlins, which are collectively called the rafter system. Behind centuries-old history Many types and methods of its organization have accumulated, and each has its own characteristics in the construction of nodes and cuts. Let's talk in more detail about what the rafter system of a gable roof can be and how the rafters and other elements of the system should be attached.
Design of a gable roof truss system
In cross-section, a gable roof is a triangle. It consists of two rectangular inclined planes. These two planes are connected at the highest point into a single system by a ridge beam (purlin).
Now about the components of the system and their purpose:
- Mauerlat is a beam that connects the roof and walls of a building, serves as a support for rafter legs and other elements of the system.
- Rafter legs - they form inclined planes roofs and are a support for the sheathing under the roofing material.
- Ridge run(bead or ridge) - combines two roof planes.
- A tie is a transverse part that connects opposite rafter legs. Serves to increase structural rigidity and compensate for thrust loads.
- Lezhny - bars located along the mauerlat. Redistribute the load from the roof.
- Side purlins - support the rafter legs.
- Racks - transfer the load from the purlins to the beams.
There may still be fillies in the system. These are boards that extend the rafter legs to form an overhang. The fact is that to protect the walls and foundation of the house from precipitation, it is desirable that the roof ends as far from the walls as possible. To do this, you can take long rafter legs. But the standard length of lumber of 6 meters is often not enough for this. Ordering non-standard is very expensive. Therefore, the rafters are simply extended, and the boards with which this is done are called “fillies”.
There are quite a few designs of rafter systems. First of all, they are divided into two groups - with layered and hanging rafters.
With hanging rafters
These are systems in which the rafter legs rest only on the external walls without intermediate supports (load-bearing walls). For gable roofs, the maximum span is 9 meters. When installing a vertical support and a strut system, it can be increased to 14 meters.
Hanging type The good thing about the rafter system of a gable roof is that in most cases there is no need to install a mauerlat, and this makes the installation of rafter legs easier: there is no need to make cuts, just bevel the boards. A lining is used to connect the walls and rafters - wide board, which is attached to studs, nails, bolts, crossbars. With this structure, most of the thrust loads are compensated, the impact on the walls is directed vertically downwards.
Types of rafter systems with hanging rafters for different spans between load-bearing walls
Gable roof rafter system for small houses
Exists cheap option rafter system when it is a triangle (photo below). Such a structure is possible if the distance between the external walls is no more than 6 meters. For such a rafter system, you can not make calculations based on the angle of inclination: the ridge must be raised above the tie to a height of at least 1/6 of the span length.
But with this construction, the rafters experience significant bending loads. To compensate for them, either rafters of a larger cross-section are taken or the ridge part is cut in such a way as to partially neutralize them. To give greater rigidity, wooden or metal plates are nailed on both sides at the top, which securely fasten the top of the triangle (also see the picture).
The photo also shows how to extend rafter legs to create a roof overhang. A notch is made, which should extend beyond the line drawn from the inner wall upward. This is necessary to shift the location of the cut and reduce the likelihood of the rafter breaking.
Ridge knot and fastening of rafter legs to the backing board when simple version systems
For mansard roofs
Option with installing a crossbar - used when. In this case, it serves as the basis for lining the ceiling of the room below. For reliable operation of a system of this type, the crossbar cut must be hingeless (rigid). The best option- half frying pan (see picture below). Otherwise, the roof will become unstable to loads.
Please note that in this scheme there is a Mauerlat, and the rafter legs must extend beyond the walls to increase the stability of the structure. To secure them and dock them with the Mauerlat, a notch is made in the form of a triangle. In this case, with an uneven load on the slopes, the roof will be more stable.
With this scheme, almost the entire load falls on the rafters, so they need to be taken with a larger cross-section. Sometimes the raised puff is reinforced with a pendant. This is necessary to prevent it from sagging if it serves as a support for ceiling cladding materials. If the tie is short, it can be secured in the center on both sides with boards nailed to the nails. With a significant load and length, there may be several such belays. In this case, too, boards and nails are enough.
For large houses
If there is a significant distance between the two outer walls, a headstock and struts are installed. This design has high rigidity, since the loads are compensated.
With such a long span (up to 14 meters), it is difficult and expensive to make the tie in one piece, so it is made from two beams. It is connected by a straight or oblique cut (picture below).
For reliable joining, the connection point is reinforced with a steel plate mounted on bolts. Its dimensions must be larger than the dimensions of the notch - the outermost bolts are screwed into solid wood at a distance of at least 5 cm from the edge of the notch.
In order for the circuit to work properly, it is necessary to make the struts correctly. They transfer and distribute part of the load from the rafter legs to the tie and provide structural rigidity. Metal pads are used to strengthen connections
When assembling a gable roof with hanging rafters, the cross-section of lumber is always larger than in systems with layered rafters: there are fewer load transfer points, therefore each element bears a greater load.
With layered rafters
In gable roofs with layered rafters, the ends rest on the walls, and the middle part rests on load-bearing walls or columns. Some schemes push through the walls, some don't. In any case, the presence of a Mauerlat is mandatory.
Non-thrust schemes and notch units
Houses made of logs or timber do not respond well to thrust loads. For them they are critical: the wall may fall apart. For wooden houses, the rafter system of a gable roof must be non-thrust. Let's talk about the types of such systems in more detail.
The simplest non-thrust rafter system diagram is shown in the photo below. In it, the rafter leg rests on the mauerlat. In this version, it bends without pushing the wall.
Pay attention to the options for attaching the rafter legs to the Mauerlat. In the first, the support area is usually beveled, its length being no more than the section of the beam. The depth of the cut is no more than 0.25 of its height.
The top of the rafter legs is laid on the ridge beam, without fastening it to the opposite rafter. The structure turns out to be two pitched roofs, which in the upper part are adjacent (but not connected) to one another.
The option with rafter legs fastened at the ridge part is much easier to assemble. They almost never push against the walls.
To operate this scheme, the rafter legs at the bottom are attached using a movable connection. To secure the rafter leg to the mauerlat, one nail is driven from above or a flexible steel plate is placed from below. See the photo for options for attaching rafter legs to the ridge girder.
If you plan to use heavy roofing material, you need to increase bearing capacity. This is achieved by increasing the cross-section of the rafter system elements and strengthening the ridge assembly. It is shown in the photo below.
Reinforcing the ridge assembly for heavy roofing material or for significant snow loads
All of the above gable roof schemes are stable in the presence of uniform loads. But in practice this practically never happens. There are two ways to prevent the roof from sliding towards a higher load: by installing a screed at a height of about 2 meters or by struts.
Options for rafter systems with contractions
Installing contractions increases the reliability of the structure. In order for it to work properly, it needs to be secured to them with nails at the places where it intersects with the drains. The cross-section of the timber for the scrum is the same as for the rafters.
They are attached to the rafter legs with bots or nails. Can be installed on one or both sides. See the figure below for attaching the screed to the rafters and ridge girder.
In order for the system to be rigid and not “crawl” even under emergency loads, it is enough to ensure a rigid fastening in this option ridge beam. In the absence of the possibility of its horizontal displacement, the roof will withstand even significant loads.
Layered rafter systems with struts
In these options, for greater rigidity, rafter legs, also called struts, are added. They are installed at an angle of 45° relative to the horizon. Their installation allows you to increase the span length (up to 14 meters) or reduce the cross-section of beams (rafters).
The brace is simply placed at the required angle to the beams and nailed on the sides and bottom. Important Requirement: the strut must be cut accurately and fit tightly to the posts and rafter leg, eliminating the possibility of it bending.
Systems with rafter legs. The top is a spacer system, the bottom is a non-spacer system. The correct cutting nodes for each are located nearby. Below are possible strut mounting schemes
But not in all houses the average load-bearing wall is located in the middle. In this case, it is possible to install struts with an angle of inclination relative to the horizon of 45-53°.
Systems with struts are necessary if significant uneven shrinkage of the foundation or walls is possible. Walls can settle differently on wooden houses, and foundations can settle on layered or heaving soils. In all these cases, consider installing rafter systems of this type.
System for houses with two internal load-bearing walls
If the house has two load-bearing walls, install two rafter beams, which are located above each of the walls. The beams are laid on the intermediate load-bearing walls, the load from the rafter beams is transferred to the beams through the racks.
In these systems, a ridge run is not installed: it provides expansion forces. The rafters in the upper part are connected to one another (cut and joined without gaps), the joints are reinforced with steel or wooden plates, which are nailed.
In the upper non-thrust system, the pushing force is neutralized by the tightening. Please note that the tightening is placed under the purlin. Then it works effectively ( top diagram on the image). Stability can be provided by racks, or joints - beams installed diagonally. In the spacer system (in the picture it is below) the crossbar is a crossbar. It is installed above the purlin.
There is a version of the system with racks, but without rafter beams. Then a stand is nailed to each rafter leg, the other end of which rests on the intermediate load-bearing wall.
Fastening the rack and tightening in the rafter system without a rafter purlin
To fasten the racks, 150 mm long nails and 12 mm bolts are used. Dimensions and distances in the figure are indicated in millimeters.