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 duties are the formation of smooth planes for laying the covering and providing space for the components of the 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 useful both for those who are constructing a gable roof truss system with their own hands, and for those who decide to resort to the services of a hired team of builders.
In the construction of the 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 the matter of independent construction of rafter structures for private houses, wood is a priority. It is not difficult to work with, it is lighter, “warmer”, and more attractive in terms of environmental criteria. In addition, to make nodal connections you will not need a welding machine or welder skills.
Rafters - a fundamental element
The main “player” of the frame for constructing a roof is the rafter, which among roofers is called a 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, the result is a triangular module called a truss. Tightening dampens the tensile processes, so that only vertically directed load acts on the walls. Although a structure with hanging rafters is braced, the bracing itself does not transmit 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 the frame of a gable roof can generally be done without a rafter structure. In such situations, the supposed planes of the slopes are formed by slabs - beams laid directly on the load-bearing gables.
However, what interests us now is specifically the structure of the rafter system of a gable roof, and it can involve either hanging or layered rafters, or a combination of both types.
Subtleties of fastening rafter legs
The rafter system is fastened to brick, foam concrete, aerated concrete walls through a Mauerlat, which in turn is fixed with anchors.
Between the Mauerlat, which is wooden frame, and walls made of the specified materials must be laid with a waterproofing layer of roofing material, waterproofing material, etc.
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 the 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 cross-section and linear dimensions of wooden beams be 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 styling clay tiles, For example, optimal distance between the rafters there 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 rafter pitch according to design features buildings, 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 a purlin - a wooden beam resting on posts 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 structural elements, affecting the layout of the internal space of the used attic.
If the attic is cold and organization is not expected in it useful premises, 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 eliminate them 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 process everything wooden parts structures before installation with fire retardants and antiseptics.
- 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 installed in advance in the wall, or with 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 attach it to the base with anchor bolts and connect it to the inner wall twisted wire 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 – trusses – on the walls.
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 the installation of racks as supports in hanging systems is not used.
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. For lifting heavy objects triangular arches made 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 that are actually 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.
Rafter system- the basis of any roof. The complexity or availability of the rafter structure depends on the type of roof chosen. Today we will talk about the simplest option - a rafter system for a gable roof. Experienced specialists talk about the structure of the roof frame, the features and functions of its elements, and how to build a gable roof rafter system with your own hands.
Gable roof: types and advantages
Let us recall that a gable roof is a type of roof consisting of two planes (slopes) connected at an angle of a certain degree. It can be simple (symmetrical or asymmetrical) and complex - broken.
The rationality of choosing a roof from two slopes is determined by its following advantages:
- Cost-effectiveness and ease of construction, in comparison with other roofing structures.
- Ease and availability of service at any time of the year.
- Reliability and durability in case of wind, snowfall, hail and other natural influences.
- Possibility of arranging an attic.
- Better hydro- and thermal insulation.
The theoretical minimum required for the construction of a rafter system
The pitched roof structure is made of metal or wooden beams. Metal is a more “problematic” material. It weighs everything down roofing system, cools and heats up quickly, is more difficult to install and requires the use of professional welding equipment. Based on this, wood is mainly used in the construction of private houses (especially with your own hands).
There are two main options for making a rafter system for a gable roof - a hanging type device (each rafter leg has two support points) and a layered method (the rafters are connected at the bottom by a tie, forming a triangular truss, with a load-bearing beam installed in the middle). A layered structure is necessary if there is a distance of more than 10 meters between the load-bearing walls. Look at the image:
What does the system of rafter elements consist of? Let's imagine a 3D projection. The roof skeleton consists of a mauerlat (rafter base), rafter legs, ridge, racks, purlins, beams, tie rods, struts and sheathing. The Mauerlat, the support and the tie rods are the lower parts of the system on which the entire future roof is erected. First, check out the illustration below, and then look at each element separately:
Mauerlat - the basis of all foundations
Mauerlat is a solid wood beam (mostly coniferous species) with a cross section of 10-15 cm. This optimal sizes for the required strength and durability of the entire roofing structure. The timber is laid on the load-bearing walls of the house to redistribute the thrust load on them.
There are two ways to install rafter beams - with transfer of load to the walls and without transfer of gravity. The choice of mounting option for the Mauerlat should depend on the weight of the roofing system, coating, thickness load-bearing walls and roof perimeter.
With the latter option, the Mauerlat is placed in a pocket, closer to the inner edge of the wall, and attached to wooden plugs with staples (each plug corresponds to the size of the brick and is part of the top row of brickwork).
The bars that take the load are mounted on the load-bearing walls from above using anchors. Craftsmen advise placing a rigid frame on a concrete base in the form of a belt in the wall. High-quality waterproofing must be installed under the Mauerlat.
You can find out more from the video:
Leather - basic load distributor
The bench performs functions similar to the Mauerlat and has the same dimensions. Beams are laid on internal load-bearing walls to evenly distribute the loads from vertical posts and struts.
For a master class on installing bed frames, see the illustrations:
Rafter legs - ribs of the roof skeleton
Rafters can be called the main component of the roof frame. This element cannot be omitted or replaced with another part. The legs of the rafters are wooden beams, the cross-sectional size of which can vary from 5 to 15 cm. The rafters rest on the mauerlat and are connected to each other by a ridge.
The rafter installation process can be seen in this video:
Horse - a small nuance with a big meaning
The final element at the junction of two slopes is called the ridge of the roof. This is an edge located vertically at the highest point of the roof. A ridge girder is installed at the junction of the rafters. Afterwards the roof ridge is installed on it. This element holds the rafters together, performs a ventilation function and gives the roof aesthetics.
Racks – receivers of the main loads
Racks are powerful beams that take part of the load of the rafter structure. They are installed vertically, usually in the center of the truss. If the project includes an attic, then the racks are placed on both sides, closer to the roof slopes. When the attic is divided into two rooms, the racks are placed both in the center and on the sides.
Purlins – rafter support
The ridge and side purlins serve as an amplifier of the rigidity of the trusses. The greater the load on the system (snowy winters, heavy roof covering, large roof area, etc.), the more purlins should be installed on the roof slopes.
Tightening – truss element connector
This structural detail performs the function of fixing the rafters at the base. Thus, a rafter triangle is formed - a truss. Tightenings may not be installed in layered systems.
Struts - structural strength
The struts serve as support for the racks and strengthen all structural elements. Experts recommend installing struts at an angle of 450. This increases the strength of the system and protects it from deformation under the influence of snow and wind.
Lathing - the basis for the roofing pie
Lathing - horizontal wooden slats with a cross section of 40-50 mm, located on the slopes perpendicular to the rafters. The main purpose of the lathing is to fix the roofing material. The frequency and thickness of the lathing slats depends on its type. In addition, the sheathing helps move materials during roofing and serves as an additional element of structural strength.
Overhang elements - final moments
The edge of a roofing system is called an overhang. This is a protrusion of the rafter system above the wall by about 40 cm. The eaves box consists of the following elements: fillets (slats connecting to the rafters), frontal and cornice boards. The purpose of the overhang is to protect the walls from getting wet during rains and melting snow.
Step-by-step guide to installing a gable roof truss system
To begin with, we suggest you familiarize yourself with the illustration that we have prepared for you:
Now let’s look at the three main stages of the process of installing a rafter system for a simple gable roof:
Stage 1: Calculations and design
Work should begin by preparing a roof project. It will contain all sizes, shapes and types of fastenings of structural elements. To create a high-quality project, you need to make the following calculations:
- Calculation of constant and variable loads on the rafter system. Constant loads include the weight of the roof and finishing materials(the attic is also taken into account additionally). Variable loads are the force of wind, rain, snow, etc. The maximum can be considered the main load of up to 50 kg per square meter roofs, and variable - up to 300 kg (taking into account possible snow debris).
- Taking into account seismic activity, storm winds and features of the location of the house. For example, if a house is surrounded by other buildings, then the load on the roof is significantly reduced.
- Choosing the angle of inclination of a gable roof. When calculating the angle of inclination, the following factors are taken into account: the higher the angle, the more materials will be spent on the roof (and money, accordingly); the slope depends on the roofing material - what softer roof, the smaller the angle of inclination (for example, for soft tiles, an angle of 5-200 is selected, and if you use slate or ondulin, you need to select a slope of 20-450).
- Calculation of pitch and length of rafters. The pitch length between the trusses varies from 60 to 100 cm. The heavier the roof covering, the more often the rafters need to be distributed. To calculate the length of the rafter, we use the Pythagorean theorem, taking the rafter leg as the hypotenuse of the triangle. The first side will be considered half the width of the house, and the second side will be the selected roof height. Then we will add another 60-70 cm of reserve to the hypotenuse we found.
When all the calculations have been made, you need to make a drawing of the parts, connections and the entire project as a whole.
Stage 2: Acquisition and preparation of necessary materials and tools
For the work, it is necessary to purchase lumber, according to calculations, bolts, angles, anchors and other connecting parts, and prepare the appropriate tools (drills, levels, meters, jigsaw, etc.). The wood for load-bearing beams and rafters must be solid and of high quality - knots and wormholes are unacceptable.
Antiseptic, anti-corrosion and fire protection treatment of wood is a must at this stage. You can start working with the material a day after processing.
Stage 3: Installation of the roof truss structure
It is better to install the roof frame in dry, non-windy weather, so as not to encounter additional difficulties during work. At this stage we will go into more detail and consider step by step instructions installation of the rafter system.
Installing a rafter system: step-by-step guide
Step 1. Laying the mauerlat and bed. There may not be a bed if there are no more load-bearing walls inside the house. Before laying the Mauerlat on the walls, it is necessary to lay out waterproofing material, for example, roofing felt. We measure the tape of the required width, cut it and lay the waterproofing on the edge of the wall (where the frame will be installed).
We mark the beams of the required section and length, make trims and begin to form the base frame. The Mauerlat should be located on the outer edge of the wall (if a hanging structure is planned) or in a special niche on the wall in front of the threshold (if the roofing system is layered). The beds under the racks are placed on the internal load-bearing partitions. The Mauerlat is attached to the wall and wooden plugs with staples, studs and anchors.
And we invite you to familiarize yourself with the illustration that we have prepared for you:
When laying the frame along the entire length of the wall, we may be faced with the need to connect the base beam. They should be performed by cutting the bars at an angle of strictly 90 degrees. Fastening is carried out with high-quality bolts.
How to avoid damaging bricks or blocks when lifting roof boards?
Select the edge of the wall through which it is most convenient to feed the beams for the roof frame. This edge must be protected with a wooden square. Two pieces of rough board about a meter long are suitable, which need to be knocked together at right angles. Place the square on the outer edge of the work wall. Now you can lift the boards without fear of damaging the walls or window sills.
Step 2. Installation of rafters. The first step is to install the outer rafters. In order to keep the rafters level, we install the racks in the center. We attach the racks to the Mauerlat using an iron angle and self-tapping screws. This temporary part is removed after all rafters have been installed. We fasten the outer rafters with crossbars and install a ridge girder. Type of fastening - metal corner, screws and studs.
The illustration shows the process of installing rafter legs and attaching the rafters to the purlin:
Here's how to attach the rafters to the mauerlat:
Between the outer trusses it is necessary to stretch a construction thread, along which we will level all the rafters of the slope.
Now let's install everything rafter elements according to a pre-designated scheme. We join the rafters above the ridge girder.
For more information about the rafter installation process, watch this video:
A system of special racks will help strengthen the rafter legs. They are attached to the Mauerlat wooden blocks from boards of the same thickness as the rafters. The bars must be secured in increments equal to the selected distance between the rafters according to the markings. The length of each board is approximately 40 cm. These racks will transfer the load to the Mauerlat and load-bearing floors. The bars must be secured to the base with steel corners. Now the rafter legs need to be installed so that one side of each is adjacent to the rack. Then, on the other side of each rafter, we attach the same stand and grab all three parts with 12 mm studs.
After installing all the legs, the posts are trimmed flush with the rafter slope on the street side. From the inside between the racks a empty corner, which needs to be covered with a wooden triangle (you can use trimmings from the bevels).
All rafter legs should be additionally strengthened with crossbars, racks, struts and reinforced joints with metal plates. Complete process strengthening the rafters can be seen in the video:
Step 3. Waterproofing and sheathing. On the finished rafter ribs you need to lay a high-quality waterproofing, vapor-permeable material under the sheathing. The overlap of the insulating sheet (sheet on sheet) is 15 cm. A counter lath of wooden slats is placed on the waterproofing along the edges of the rafters. A sheathing of the same slats is installed on top, perpendicular to the rafter legs.
When installing the frame, you need to take into account the presence of a chimney and mandatory ventilation of the ridge. The average distance between the sheathing boards is 300 mm. This scheme is suitable for all types of solid roofing. When choosing a soft roofing material, the sheathing is made of solid sheets of moisture-resistant plywood.
The rafter system is ready. Now it’s time to install the roofing material, internal insulation roof and attic arrangement (if provided for by the project).
So, it’s time to answer the main question of our topic: is it worth doing it all yourself? Don't believe anyone who tells you that it is easy and simple. But if you have golden hands and a great desire to make a high-quality roof “for yourself,” then go ahead! We wish you good luck!
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It's hard to imagine a house without a roof. Without it, the house cannot be called complete. The roof, as you know, protects the house from precipitation, inclement weather, etc. In addition, it is used to complete the architectural image of the constructed object. In this case, it is necessary to correctly calculate and install the roofing system, only then will the roof last for many years and will not need to be repaired every year.
The structure of the rafters can be very different. We propose to consider their most common types and types, methods of their fastening.
As you know, the roof can be flat or pitched. As for suburban construction, in this case the installation of a gable roof rafter system is especially popular. If the house is very large, then the roof will be of the appropriate size, for example, hipped (more details: ""). Thus, the developer must imagine the design of the rafter system hipped roof. The most simple option the device of the rafter system is considered pitched roof, since it is very labor-intensive and requires sufficient knowledge.
Still, it is worth noting that the installation of a gable roof rafter system is considered the most common option today. The design consists of two planes that are on the same level. They are also the load-bearing part that rests on the walls of the house. Usually there is an attic under the roof slopes. If it is heated, then it can be used as an attic floor where you can arrange a functional room.
It is worth noting that the angle of inclination of the future roof will depend on the roofing material. It is usually measured in degrees.
Determining the reliability of the rafter system
When installing a roof, developers Special attention given to the rafter system. It must be made reliable, because the service life of the roofing structure depends only on this system.
The reliability of the rafter system is usually affected by following points:
Construction of a rafter roof: structural components
We can conclude that if the rafter system is calculated incorrectly and unprofessionally, it may simply not withstand the load in the future. In this case, terrible consequences cannot be avoided.
When calculating the design of the rafter system, the following points must be taken into account:
- roof shape;
- location of internal supports;
- floor span size;
- expected operating loads.
In the design of the rafter system, the triangle is the main figure. Rafter legs are also important in such a design; maximum attention must be paid to their design. They are usually laid along the roof slope. The sheathing is supported precisely by the rafter legs.
What types of rafters are there?
Today rafters can be:
- hanging;
- layered.
The optimal sections themselves will be the following options:
- for boards – 16-18x4-5 cm;
- for bars – 16-18x12-14 cm;
- for round timber – 12-16 cm.
The axis distance between the bars and logs should be 150-200 centimeters. The distance of the rafters between the beams of the boards is usually 100-150 centimeters.
At the end of each rafter leg, so-called fillies must be nailed. A regular board is suitable for their implementation. Along the entire cornice along the slope of the filly, you need to nail the formwork, which will serve as the base for the boardwalk. It is on this decking that the roof material is attached.
The installation of a rafter system for a gable roof or any other may encounter such a difficulty as expansion (read: ""). To avoid expansion of the rafter system, the rafters must be attached to the ridge very firmly.
If the device is used incorrectly, displacement of the ramp may also occur. If you cut the rafter legs into the wood floor, this will not happen. The strength of the joint can also be achieved with an overlay, or with reliable fastenings, which are made using bolts and dowels.
In this case, it is necessary to take especially seriously the calculation of the maximum roof load, and we must not forget about the snow cap. Price roofing works in this case, it will increase significantly, since more lumber will be required, and the tiles themselves are not cheap.
Due to such high costs, many people choose soft roofing. In this case, you need to remember about the sheathing, which is made entirely of boards or plywood. This type of roof also requires underlay carpets. Therefore, when purchasing, the developer building materials You will also have to pay quite a bit of money.
Almost any modern roof of a low-rise building is built on a rafter frame system. In theory, the roof structure can be made in the form of a flat ceiling. But the simple manufacture of such a roof structure is offset by a lot of disadvantages; a significant increase in the thermal insulation layer will be required and provision of forced removal of snow and rain and melt water. Even when constructing garages or outbuildings, such a roof arrangement is resorted to in extreme cases, preferring a more complex rafter option.
Why rafter systems are so popular
The rafter system appeared as a result of natural selection among many various options building a frame. The modern design of the roof truss system is based on several basic elements designs:
- A rafter frame, which is a set of beams of equal length that form the plane of the roof slope. The rafters are laid symmetrically in a “hut”, with the upper edge on the highest horizontal part of the frame - the ridge girder, and rest on the mauerlat - a thick board sewn onto the upper horizontal plane of the brick frame of the building;
- The base or system of fastenings on which the rafter frame rests consists of a mauerlat, beams and ceiling beams on the top of the walls of the building. Thanks to this device, the load from the weight of the roof and rafters is redistributed, leveled and transferred to the internal and external walls of the house;
- The roof sheathing, together with additional strength elements - struts, spacers, crossbars, serves to impart additional rigidity to the rafter beams.
For your information! In addition, the sheathing boards form the basis for laying the roof covering.
Pine logs and beams are traditionally used to construct the roof truss system of a low-rise building. This allows for a lightweight and at the same time rigid roof structure. Attempts to replace wooden beam steel profile lead to an increase in the weight and cost of the rafter system by at least two to three times, and due to numerous cold bridges it will be necessary to lay an additional layer of thermal insulation.
One of the most popular rafter systems is a device in the form of a two- or hipped roof with a pair of rafters. In this case, the frame made of symmetrical load-bearing elements perfectly absorbs the load in the vertical and transverse direction in relation to the ridge girder.
If the predominant wind direction in a given area is approximately the same, then the longitudinal force on the roof structure resulting from the air flow is most often compensated by folding brick gables. In strong and changeable winds, it is more rational to use a hipped hip structure.
Design and features of the rafter system
It is clear that the use of rafter technology is aimed at forming roof slopes with the most rational slope angle for a given area. The steeper the angle of inclination, the faster and easier it is to remove rainwater and snow.
Meteorological service information on power and maximum pressure layer of snow on a square meter of flat roof for different regions of the country.
For a rafter system, the load on the elements of the rafter system is reduced based on the angle of inclination of the roof slope:
- For options with an inclination angle of up to 10-20 o, the reduction in snow mass pressure is extremely insignificant; on average, a low roof accounts for a force of 80-90% of the value for a flat surface;
- For roof slopes installed at an angle of 25°, the load will be 70% of the “flat” value, while for an angle of 65° the snow pressure will decrease by 70-80%;
- On steeper slopes, pressure is not taken into account at all; in this case, the strength of the rafter system is calculated based on the wind load.
Important! Even small cottage, with a roof slope of 45o, located in central Russia, with high level precipitation receives an additional load from snow, reaching 5 tons.
Therefore, even in small cottages and houses, a log or beam with a cross-section of at least 100-150 mm is used as a material for constructing a rafter system.
Types of rafter systems
The design of the roof frame rafter system is most often carried out according to the scheme with hanging or layered rafters. The use of a specific scheme is determined by many factors, among which the dimensions of the house and ceiling, the presence of interior walls or partitions, the nature of the use of the attic space.
The main differences between layered and hanging rafters are as follows:
In the case of a free fit of the end of the rafters on the ridge girder, each pair of beams of the gable roof is not fastened together, but is cut in according to a sliding pattern. In the lower part, the rafter legs are attached to the mauerlat in the form of a rigidly fixed hinge, using bolted connection or nails. Under load, such a device works like a non-thrust rafter system, due to the fact that any vertical or lateral force on the rafter system does not lead to the appearance of horizontal thrust forces at the support points on the Mauerlat.
Important! Important feature Such a structure of the frame ensures a minimal bursting effect on the walls of the house, which is extremely important for wooden houses made of timber or logs. But the practical assembly of such a structure requires the most accurate and careful adherence to the dimensions and accuracy of installation of the elements.
In the second case, the layered beams on the ridge girder are rigidly attached with reinforcing linings made of metal or boards, much like in the case of hanging rafters. The lower edge is installed on a mauerlat with a cutout in the rafters of the supporting surface and side guides that prevent the board or beam from twisting.
Knots of the rafter system
To ensure the necessary strength of the rafter structure, especially for buildings with spans of more than 8-9 m in length, it is necessary to use logs and beams of considerable thickness, which makes assembling the roof frame a very difficult and expensive task. It is easier and more efficient to install additional power elements, compensating for deflection or transferring the main part of the force to less loaded parts of the frame.
For example, to compensate for the deflection of a rafter leg, two main elements are used - struts and vertical posts. Depending on the design of the rafter system, power racks can be installed in the central part and support the ridge girder, taking on part of the load from the weight of the frame. The elements can be combined with struts in the middle part of the rafters, thereby transferring the load from the side purlins to the tie rods or beams - longitudinal beams resting on the ceiling or internal main walls. The struts do not cut into the body of the rafters, but are fastened with nails, bolts, screws through steel plates or wooden plates.
The second most popular element for strengthening hanging rafters is a raised tie. This element makes it possible to reduce the horizontal pushing action of the rafter legs and the entire system, unlike the previous ones, works in tension, so the device is attached to the side surface of the rafters using a cunning self-tightening unit called a half-frying pan.
For layered rafter beams, a similar element called a scrum is used. If the structure of the frame, the length and thickness of the rafter beams do not provide adequate stability of the triangle, in this case it is necessary to install an additional horizontal strut - a screed. This method of strengthening the system is effective for counteracting uneven asymmetrical loads, for example, heavy oblique rain or sudden gusts of wind.
To obtain a long ceiling beam or tie, more than 8 m long, it is often necessary to splice two six-meter pieces according to the diagram shown in the figure.
One of the problems characteristic of hanging rafters when long length span, there may be a deflection in the center of the tightening of the base of the ceiling. In this case, they resort to a suspension or headstock device. Despite the external similarity with the rack, this element works in tension, so its cross-section can be significantly smaller. When installing the headstock, it is necessary to provide a tension device that allows you to select gaps and equalize the deflection of the tightening.
Fastening the elements of the rafter system in nodes and connections is usually done using 150-200 mm nails, driven at different angles and distances from the edge of the beam. On the reverse side, the nail is bent with a twist. This fastening device allows you to avoid the effect of “self-pulling” of the nail from being planted in a log or beam. If timber is used in the rafter system, it will be most convenient to make the connection using overhead profiled steel plates, corners and holders.
In some cases, the use of such devices makes it possible to perform temporary or preliminary assembly of rafter beams on self-tapping screws, accurately measure the dimensions and locations of cuts, and only after that make permanent fasteners.
In previous articles that described the structure of the roof, we already said that hanging rafters the lower end rests on the Mauerlat, and the upper ends of adjacent rafters rest (either directly or through the ridge board) against each other. In the very simplified version this is shown in Fig. 1:
Picture 1
I think it is obvious to everyone that with such an arrangement, bursting loads appear on the walls. To reduce them, tie downs are added to the truss. But let's talk about everything in order.
As an example, let’s take a house with a semi-attic in the Volgograd region. The sum of snow and wind loads is 155 kg/m2. The dimensions of the house box are 8x10 meters. The thickness of the walls is 50 cm. The angle of inclination of the slopes is 40° (see Fig. 2):
Figure 2
STEP 1: We install. In this design, in addition to the usual loads, pushing forces will act on it, tending to move it from the wall. For more reliable fastening, you can add metal fastening plates to the anchor bolts (or studs) (see Fig. 3). The plates can be secured to the wall, for example, with frame anchors, and to the Mauerlat using nails, self-tapping screws, and wood grouse.
Figure 3
STEP 2: We determine the required section of the rafters. We carry out the calculation in the “Arch” tab (see Fig. 4):
Figure 4
The cross-section of the rafters is taken to be 50x200 mm in increments of 60 cm.
A question may immediately arise here. Where do we get the distance from the ridge to the tie rod? For us it is 2 meters. Earlier on the site, we already said that before we start building a roof, we need to make a drawing of it on paper, always to scale (with all proportions respected). If you know how, you can draw on the computer. Further, using this drawing, we determine all the dimensions and angles that interest us.
Tie-downs are installed between hanging rafters to reduce bursting loads on the walls. The lower the tension, the more benefit it will provide. Those. the less bursting load there is on the walls. But since in our example the puffs still play a role ceiling beams attic floor, then we determine the height of their location based on the ceiling height we need. I took this height of 2.5 meters (see Fig. 5):
Figure 5
STEP 3: We make a template for the lower cut of the rafters. To do this, we take a piece of board of the desired section about a meter long, apply it to the mauerlat at our angle of inclination of the slopes of 40° (focus on the pediment) and make markings, as shown in Figure 6:
Figure 6
We draw the vertical and horizontal lines we need (shown in blue) using a level. The depth of the cut is 5 cm.
So, let's make a template.
STEP 4: We install a ridge board, through which all the rafters will be connected to each other. First you need to outline the location of its installation.
We take the template we made earlier and apply it to the Mauerlat. We are interested in the size shown in Figure 7 (here it is 18 cm):
Figure 7
Let's call the lowest point on the Mauerlat point “A”.
We transfer the resulting size to the top of the pediment, make markings in accordance with Figure 8:
Figure 8
We will designate the lower right corner as point “B”. Now we can measure the distance from the attic floor to point B (the length of the temporary racks).
We install strictly vertically temporary racks from 50x200 boards and place a ridge board of the same section on them. To fix them, you can put a board under the posts, secured with simple dowel nails to the floor slabs (see Fig. 9). There is no need to fasten it too much, then we will remove it. The distance between the posts is no more than 3 meters.
Figure 9
We attach the ridge board to the gables with metal brackets. The stability of the racks is ensured by the jibs.
Perhaps you could see somewhere how hanging rafters are installed without a ridge board (see figure on the left). I am very familiar with this method; we did this before too.
But when we tried the option with a ridge board, we settled on it. Despite the fact that it takes some time to install the racks and ridge boards, subsequent installation of the rafters is much more convenient and faster. As a result, in time you win. In addition, the design is more stable and more geometrically smooth.
STEP 5: We manufacture and install rafters.
We make a rafter like this: we take a board of the required length, apply a template to one end, mark it and make the bottom cut. Then use a tape measure to measure the distance between points “A” and “B” (see Fig. 7-8). We transfer this size to our workpiece and make the top cut. The angle we need for the top cut is on our template (see Fig. 10). For us it is 90°+40° = 130°
Figure 10
This is how we install all the rafters (see Fig. 11)
Figure 11
The connection of the rafters with the Mauerlat here does not look the same as it was, for example. I think you already understand that this is due to the presence of bursting loads, which were not present in that version. However, in future articles you will see that this option is just one of the possible, and not the only correct one. We will also use cuts that are more familiar to us. The main thing is to securely fasten the rafters to the Mauerlat.
At the top point, the rafters will protrude beyond the ridge board. You can drive small bars between them, or you can leave them as is. This, in principle, does not play any role (see Fig. 12):
Figure 12
We fasten the rafters to the ridge with nails or self-tapping screws. Any additional fastening elements It is not necessary to put it here. In general, in this design, thanks to the lower gash, the rafters seem to be sandwiched between the Mauerlat and the ridge board.
STEP 6: We install the tightening.
We make them from boards of the same section as the rafters. It is not necessary to make any cuts or cuts here. We make the puffs overlapping the rafters. We fasten them with several nails and tighten them with a threaded rod with a diameter of 12-14 mm (see Fig. 13):
Figure 13
Thus, we install all the tightening and remove our temporary stands on which we placed the ridge board:
Figure 14
Now you can guess the purpose of the small windows in the upper part of the pediment. Through them, ventilation of the insulation will be carried out, which will lie on the ceiling of the semi-attic floor (between the puffs).
STEP 7: We attach the eaves overhang fillets to the lower ends of the rafters (see Fig. 15). We make them from boards with a section of 50x100 mm. We make the length of the filly such that we get the eaves overhang of the width we need (40-50 cm), and so that it overlaps the rafter by at least 50 cm. We fasten the filly with several nails and tighten it with 2 threaded rods. In the middle part, for additional support on the wall, you can fasten a small block to the fillet with nails or self-tapping screws.
Figure 15
Please note that at the junction of the overhang fillet with the mauerlat, we do not make a cut on it, because this will reduce its already small cross-section. Here we first make a small cut in the Mauerlat itself (see Fig. 16):
Figure 16
To make the cornice even, use lace. Place the outer fillies first, then pull the string between them and place the rest. In Figure 17, the lace is shown in blue.
Figure 17
Step 8: We already know the following steps from previous articles. We place the fillies on the pediment and attach the wind boards (see Fig. 18):
Figure 18
STEP 9: Now we can leave the cornices as they are.
Let's look at another version of the eaves overhangs (see Fig. 19):
Figure 19
These “earrings” are made from inch boards 10-15 cm wide. We fasten them with self-tapping screws.
Thus, now all we have to do is hem the siding belts to the bottom of the eaves; secured to the rafters protective film, make counter-lattice and sheathing; cover the roof with roofing material. We discussed these steps in previous articles. I think there is no point in repeating myself here and when considering other roof designs in the future.
