Greetings, comrades! We have to figure out how the rafter system of the attic roof works. I will introduce you to its main elements, their functions and share my own experience in building an attic floor. But first, a few definitions to help us avoid confusion.
Attic roofs are traditionally called a very specific type of roof - broken, that is, with two slopes with a variable slope. However, the traditional definition is incomplete. In fact, this can be called any roof that allows you to place an attic underneath it - a living space limited by the roof slopes.
A semi-mansard roof differs from a mansard roof in that it rests on solid side walls at least one and a half meters high. The semi-attic uses space more profitably: it does not have areas with low ceilings that are unsuitable for use as living space.
Here are the main types of attic roofs:
| Image | Type and Brief Description |
| Single-pitch: a single roof slope rests on main walls of different heights. To effectively use the entire attic area, the smaller wall must have a height of at least 1.5 meters, which implies an increase in construction costs. | |
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Gable: in cross-section, it is an isosceles or (less commonly) asymmetrical triangle. Uses usable area less efficiently than a broken line. |
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hip: a four-slope option with two pairs of slopes of different sizes. |
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Half-hip The roof differs from the hip roof by the presence of shortened vertical gables. |
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Broken has two slopes with variable slope. It benefits from the most rational use of attic space: areas with low ceilings near the side walls have minimal dimensions. |
Elements
In order not to make the reader confused in terms, I will give a few more definitions. Here are the main elements of the rafter system:
| Image | Rafter structure element |
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Mauerlat: a timber laid on a main wall or monolithic ceiling that serves as a support for the rafters. |
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Rafter legs: inclined beams that serve as support for the roof. Hanging rafters (that is, resting only on the walls of the building) allow you to build a roof up to 6-6.5 meters wide. Layered rafters (with intermediate supports) allow you to increase the span up to 12 meters with one support and up to 15 meters with two supports. |
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Crossbar or tightening: a beam that holds together the rafters of a gable roof. Its task is to prevent deformation of the rafter system in the event of heavy snow loads. |
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Rack: a vertical support under the rafter leg, ensuring its stability in strong side winds. In addition, studs usually serve as the basis for the frame of the side walls of the attic. |
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Sill: horizontal beam on which the posts rest. |
Scheme
Now it's time for drawings and diagrams.
Gable roof
The large span of the roof forces the use of a central post on which the layered rafters rest. The side posts give the slopes additional rigidity and serve as a frame for the attic walls.
Resistance to snow loads is provided by a pair of crossbars: the first serves as the basis for a horizontal insulated ceiling, the second is hidden in a cold attic.
Another, simpler rafter system for a gable roof with an attic. There is no central pillar. A shortened crossbar makes the ceiling broken: the horizontal central part is adjacent to sloping areas.
broken roof
For a broken mansard roof, the racks are always installed exactly under the break. The crossbar, which tightens the kinks together, ensures maximum rigidity of the structure. Alas, this scheme has a serious drawback: the ceiling remains relatively low even in the center of the attic, although the height of the ridge allows it to be raised another few tens of centimeters.
A shortened crossbar connecting the upper rafters approximately in the middle of their length allows you to raise the ceiling with virtually no damage to the strength of the rafter system.
Hip roof
Here, rigidity is provided by slanted (corner) rafters with posts in the middle of their length. The racks are connected to each other by horizontal connections. External rafters rest on the rafters and form a solid foundation for the roof.
A feature of the hip roof is the absence of vertical gables, so natural lighting is provided by skylights embedded in the roof.
Shed roof
With a single slope, the primary problem is ensuring resistance to snow loads, so when spanning over 4.5 meters, the rafters need additional supports.
The diagram shows their installation options:
- When spanning up to 6 meters, sufficient rigidity will be ensured by installing an oblique rafter leg;
- The central post with a pair of rafter legs allows you to increase the span to 12 meters;
- Two intermediate posts with oblique legs and a tie between them make it possible to make a 16-meter span.
Half hip roof
The height of the gables allows the main load to be transferred to them. A prefabricated truss rests on the gables, which serves as a support for the side rafters. For greater rigidity, the rafter legs are connected to each other by crossbars and longitudinal girders.
Nodes
How to install rafter system connections with your own hands? At your service is a description of how to install the main components.
Attaching the Mauerlat to the walls
Mauelllat is made from timber with a section of 100x100 - 150x150 mm. The timber must be treated with an antiseptic. The walls underneath are waterproofed to prevent capillary suction of water from the wood; Usually the role of waterproofing is performed by a couple of layers of roofing felt.
To fasten the Mauerlat, anchor pins are usually used, installed when pouring the armored belt around the perimeter of the wall. Holes are drilled in the beam for them, and after laying, the beam is attracted to the reinforced belt with nuts and wide washers.
Attaching the rafters to the mauerlat
To maximize the rigidity of the connection between the rafter legs and the Mauerlat, a cutout is usually made in them by a third of the rafter width. For fastening the following can be used:
- Steel staples. They are driven into both beams on both sides;
- Galvanized corners. They are attached to both beams with several self-tapping screws with a length of at least 2/3 of the rafter thickness.
Galvanized corners and overlays are used to connect rafter legs to each other, to racks, to horizontal purlins and floor beams. The overlays can be replaced with thick (at least 15 mm) plywood, oiled to protect against moisture.
Attaching the crossbar to the rafters
The connection of the crossbar with the rafters of a gable or sloping roof experiences the heaviest loads in winter, when there is snow on the roof. A simple instruction will help you make it as strong as possible: the crossbar is connected to the rafter with an overlay and attached to it with a pair of bolts with nuts and wide caps through pre-drilled holes.
Materials
The best material for a rafter system is cedar, which is lightweight, durable and rot-resistant. However, in practice, cheaper ones are used much more often: spruce, fir and pine. All loaded elements of the rafter system (rafter legs, crossbars and racks) must be free of wood defects that affect strength:
- Large falling knots;
- Cross-layer (deviation of the direction of the fibers from the longitudinal axis of the beam);
- Oblique cracks;
- Rot.
The typical cross-section of the beds and uprights is 100x50 mm. The cross-section of the rafters is determined by their length and the pitch between the rafter legs: the larger it is, the greater the load falls on a separate beam. You can select the optimal rafter section using the table below.
My experience
When building the attic, I chose a sloping roof. For the installation of the rafter system, pine beams with a section of 50x100 mm were purchased. The pitch between the rafter legs is 90 cm, the longest span is 3 meters. The roof slope angle is 30 degrees for the upper slopes and 60 for the lower ones.
The sheathing for the roofing material (corrugated sheet) is assembled from unedged boards 25 mm thick. Precisely from unedged - simply because its price is lower, and the appearance when laid under the roof does not matter at all. The sheathing pitch is 25 cm.
The crossbar tightens the upper rafters approximately in the middle of their length. The suspended ceiling made of gypsum plasterboard is assembled on ceiling profiles attached to the rafters and crossbars with direct hangers.
The design of the rafter system has proven its strength: for four seasons it successfully withstands the strongest winds typical of Sevastopol winters.
Conclusion
I hope that I was able to answer all the reader’s questions. As always, the attached video will provide you with additional materials. I look forward to your comments and additions. Good luck, comrades!
Recently, there has been a return to the use of various architectural elements that were undeservedly forgotten during the era of industrial construction. Again you can find bay windows, mezzanines, and attics, which make the exterior of the house more interesting and, at a relatively low cost, allow you to increase the interior space. For example, an 8x10 attic roof rafter system will increase costs by approximately $4,500, while adding 60-65 m2 of usable space to the area of the house.
Types of mansard roof truss systems
In practice, several types of rafter systems for the attic are used. They are classified according to the type and shape of the roof: two-, three- and four-slope, hip and half-hip, broken. The simplest option is gable rafter system attics. Its advantage is a simple and reliable design that can withstand heavy loads. However, there are also disadvantages - the simple form does not allow you to create an interesting design, which, however, is compensated by the addition of decorative elements. Another more significant drawback is the limited internal space, which is significantly less than in the case of a sloping roof.
Idea You can get rid of the lack of space by building a semi-attic - that is, a room with side walls 1.5-1.8 meters in height. This will increase the internal volume while maintaining the advantages of the gable roof design.The drawing shows an example of a half-attic type mansard roof with walls 1.8 m high
The most interesting from an architectural point of view is the rafter system of the sloping mansard roof. It allows you to show your imagination, of course, subject to compliance with building codes and, above all, strength characteristics. This design can have a regular and asymmetrical shape. The configuration is selected based on the characteristics of the house and the layout of the interior space of the attic. In some cases, the rafter system of the attic roof allows you to create a rather interesting interior space, especially if it is possible to plan a second light or mezzanine. It is important to use the volume as fully and efficiently as possible - this is not an easy, but very interesting task, which allows you to increase the usable area and make the house more comfortable at a relatively low cost.
Advice Since space is limited where the attic floor meets the roof, it is advisable to organize storage areas, install furniture, and lay utility lines.
Spaces near the walls of the attic are best used for installing convenient cabinets, shelves and built-in furniture
Elements and components of mansard roof rafter systems
The rafter system is the basis of the roof, its skeleton, on which the roofing pie and the inner lining of the attic are mounted. It is often used as a basis for laying engineering systems and communications. In turn, the rafter system of the attic roof consists of component units and elements, the combination and relative arrangement of which ensures the transfer of wind and snow loads from the roof to the load-bearing walls of the building:
- rafters (hanging and layered);
- Mauerlat;
- purlins (ridge and side);
- connecting elements (struts, spacers, diagonal connections).
The loads acting on the rafter structures of mansard roofs are quite large and reach an average of 200 kg/m2. However, in each specific case this is determined based on the characteristics of the roof structure, as well as the wind and snow loads present in the area. In any case, the rafters must be guaranteed to withstand these loads and have the necessary strength margin that can withstand very strong winds or heavy snowfall.
When calculating the load on the rafter system, it is very important to take into account the likely amount of precipitation and wind strength
The parameters and main components of the mansard roof rafter system are determined by calculations. Calculations make it possible to determine the length, profile and cross-section of each structural element and the features of how the parts are connected to each other. The strength of the entire structure largely depends on the strength of the connection of the nodes of the rafter system of the attic roof. The most common types of connections are tongue-and-groove, screw, bolted and welded. The choice of connection type depends on the design loads, the material used and design features.
Calculation and drawing of the mansard roof rafter system
Having chosen the type and determined, it is necessary to calculate the rafter system of the attic roof. It should be taken into account that the rafters not only provide the strength and reliability of the roof, but also shape the internal space of the attic. It is allowed that part of the area will have a low ceiling height. Furniture is usually placed there, or used to organize storage areas. Despite some restrictions, the interior space should be spacious enough and not cause discomfort.
The first stage of design is the selection of the truss system for the mansard roof. The diagram determines the type and location of the rafter system elements. In particular, the type of rafters is determined: layered or hanging. The difference between these elements is as follows: layered rafters on the attic floor rest on side walls or other supports. Hanging rafters form a single, rigidly interconnected structure. Depending on the width of the span, the truss system of the attic roof can be reinforced with additional connections.
The diagram shows the main elements and parts of the attic roof rafter system and their relative positions
Next, it is necessary to perform calculations that take into account wind and snow loads that will act on the roof. The material, cross-section and distance between the rafters of the attic roof depend on these parameters. It is recommended to use the so-called “safety margin”. This means that the obtained result must be multiplied by a multiplying factor, thereby guaranteeing increased reliability of the attic rafter system. Taking into account the characteristics of operation, the value of this coefficient is taken from 1.5 to 3.
Important When making calculations, it is necessary to take into account the weight of the roofing system. In particular, when using ceramic tiles, the cross-section of the beam for the rafters must be at least 70x150 mm, with a pitch of 0.5 m.Quite often, the rafter system of an attic roof is created from wooden structures. It is recommended to choose wood that is highly durable and at the same time minimally susceptible to rotting. Larch can be considered the best option, however, it is also possible to use cheaper wood with sufficient strength. In this case, more thorough treatment with antiseptic compounds is necessary. It is desirable that these compositions also have fire retardant components.
The rafters of the attic roof must be treated with a special antiseptic to prevent wood rotting
However, it is not always possible to use wood to make rafters. For heavy loads, the use of timber with a large cross-sectional profile is required, or a significant reduction in the distance between the elements. This leads to a significant weighting of the entire attic roof structure and, as a consequence, to the need to increase the load-bearing capacity of the attic walls. In this case, metal rafters of attic roofs are often used.
The results of the calculations are reflected in the drawings, which indicate all the design decisions made for this attic truss system. If the work is done independently, then instead of a detailed drawing, it is permissible to develop a simplified sketch of the rafter system of the attic roof. In this case, the sketch must have all the information and parameters necessary for assembling and installing the rafters.
An example of a schematic drawing of an attic rafter system indicating the main parameters and distances
Construction of an attic rafter system using the example of a gable roof
The design of the attic roof rafter system depends on the type of roof chosen for the house project. The simplest, and at the same time reliable and effective, can be considered the classic gable roof. Of course, it looks simple compared to complex, broken roofs, but at the same time it contains many basic elements and parts characteristic of all types of mansard roof truss systems.
The rafter system of a gable mansard roof consists of several main parts and elements. In general this is:
- Linear parts and elements - beams, columns, rod systems;
- Planar parts and elements - slabs, panels, flooring;
- Spatial parts and elements - shells, vaults, volumetric elements.
In practice, not all parts and elements of the mansard roof rafter system are used. In particular, for a gable roof, rafters, crossbars, ties, struts and struts are used. Their dimensions and relative position are determined during design. When assembled, all these elements form the truss of the mansard roof truss system.
Six examples of roof truss design, indicating the height of the ridge and the location of the rafters
The truss is the most important part of the rafter system of the attic roof. The number of trusses depends on the length of the facade and the selected installation step. The type of truss and the installation pitch are interrelated quantities; the more complex and durable the spatial structure of the element, the larger the step can be used during installation. In turn, an important element of the farm are the rafters for the attic, or, as they are also called, rafter legs.
Rafter legs are the most important element of the truss, the distance between which determines the stability of the roof
The video attached to the article talks in sufficient detail about the rafter system of the mansard roof. From it you will learn about all the intricacies of constructing a rafter system and performing installation work. Options for roofing systems from a simple gable to a more complex - broken one - are considered. By following the instructions and recommendations in the video instructions, even a novice master can understand the features of this work and perform it independently.
Installation of the attic roof rafter system
The work of installing mansard roof rafters is also called raftering. This is a set of works on marking and installing rafters. At the same time, work is also carried out to remove the horizontal ridge and trim the pediment. You can do the raftering yourself. A good guide for this can be the video instructions below.
The installation of a mansard roof truss system has many features, many of which are unknown to novice craftsmen. The frame must be installed on hydro- and heat-insulating material. This increases service life and reduces heat losses. Another common mistake is coating elements with protective compounds after installation. Treatment with an antiseptic must be carried out before installation, then the entire surface will be treated and the rafters will last longer.
Note to the masterWood for rafter work must be pre-dried; its optimal moisture content is approximately 18%.The quality of fastening of the attic roof rafter system must be given special attention. You can do it correctly, purchase high-quality materials and professional tools, but if the connection of the rafter system parts is done poorly, then after some time a major repair will be required. The choice of fasteners is a separate section of the project, in which the length and diameter of the hardware is determined.
To assemble the attic roof rafters, you must use galvanized fasteners of the required size. An attempt to save on fasteners does not give a significant result, but it contributes to the emergence of serious problems during operation. The distance between fasteners is selected in proportion to their sizes. The fastener pitch should not be very small so as not to reduce the strength characteristics of the part itself.
Installation of a rafter structure for a gable mansard roof of a wooden country house
The first truss is mounted from one of the ends, while checking the correct installation vertically and horizontally. Then, from the other end, a second truss is mounted, and its verticality and horizontality are also verified. Two construction cords are pulled parallel between the trusses, which will serve as a guide for installing the intermediate rafters of the attic roof.
Advice To temporarily secure trusses, you can use struts made from substandard lumber or scraps.After all the trusses are installed in their places, the installation of side and ridge girders begins. Having assembled the rafter system of the attic roof, you can move on to other types of work: filling and covering the gables, installing a roofing pie, heat and sound insulation, interior cladding and finishing.
More and more architectural, original houses are appearing, decorated with bay windows, superstructures, and mezzanines. Attics are becoming fashionable, expanding the usable area of the house. But attic roofs, unlike a simple gable roof, require complex rafter systems. Correctly calculating them and carrying out installation is not at all easy. Below we will try to briefly highlight this issue and provide useful recommendations from experts.
At the design stage of a house, the roof structure must be calculated. Calculations using complex formulas should give the value of the load per meter of a square rafter system. For residential buildings, the standard load is 50 kg/m2.
In accordance with the calculations, the type of attic truss structure is selected. The main varieties include:
- hanging rafter system;
- sloping roof rafter system;
- layered gable roof system;
- combined.
The most common are combined rafter systems for pitched roofs. It is recommended to entrust the calculation of complex rafter systems to professional designers.
Advice!
To increase the usable area of the building, you should choose a multi-slope mansard roof design.
Rafter roof elements
Experienced specialists will perfectly understand what is being discussed in the article, since they know all the terms and names of structures. To make it clear to everyone, here are the common names of elements:
- Mauerlat is a beam attached to the upper crown or wall of the house on which the rafter system rests;
- Floor beams are a wooden structure that plays the role of an attic floor and, at the same time, the ceiling of the room below;
- Posts are vertically mounted posts on which rafters and beams are attached.
- Purlins - beams (boards) located horizontally serve as support for the rafters;
- Crossbars are horizontal beams in a U-shaped truss. They serve as a support and tighten the pitched rafters; they are also called “tightenings”;
- Rafters - timber or boards that form the basis of the roof structure;
- Suspension - a rack whose task is to support the crossbar, facilitating its operation, is also installed horizontally;
- Sheathing - a board or plywood base on which the roofing material is laid;
- The filly is a board that serves as the basis for installing the overhang and is installed at the bottom of the rafter system.
Rafter system calculations
The choice of truss structure largely depends on the size of the building, according to which the span length is determined, i.e. distance between main posts. For small houses, preference can be given to a gable structure.
Structural calculations are carried out to determine the load, in accordance with standards and building regulations. The basic rule for an attic roof is to limit its height, which should not be less than 2.5 meters indoors.
Thus, the roof must have a minimum height of 2.80 m, since it requires laying an insulating layer and finishing inside the attic itself.
Advice!
You should definitely make a design drawing, on which you should put all the dimensions and display the frame elements as much as possible. The drawing should show the dimensions of the house, the angle of the rafters, and the height of the roof.
To make accurate calculations for all loads on the roof and its elements, it is recommended to use a calculator located on many sites dedicated to the construction of roofs.
It is enough to enter the type of roofing, the material of the attic roof frame and dimensions. The program will calculate the section angles and pitch of the rafters and give recommendations on the design of the sheathing.
Calculation of the rafter system
Attic roof installation technology You should start with the installation of the Mauerlat, which is laid along the perimeter of the upper edge of the walls, if the house is brick or block. In houses made of timber or chopped with mauerlat, the upper crown can serve. The timber from which the Mauerlat is made must have cross-sectional dimensions of 100x100 mm or 150x150.
Next, you should begin installing the ceiling. Beams 150x200 mm placed on the Mauerlat should protrude beyond the walls of the house by about 0.3-0.5 m. The beams are fastened with corners and screws (wood screws), starting from the outer ones, then intermediate ones.
Important!
When laying, use a cord to control the horizontal level so that the beams lie in the same plane.
The distance between them is, as a rule, 0.5-1.0 meters; if you plan to lay insulation, the standard sheet width of which is 0.6 m, then it will be more convenient to withstand the same spacing of the beams.
Installation of roof truss system
Installation of support posts and purlins
The next stage is the installation of racks. For them, a 100x150 mm beam is used, which is fastened to the front floor beams. It is necessary, using a plumb line, to check the verticality of each support and fix it with jibs. Intermediate supports are also installed strictly vertically on the beams, forming two parallel rows.
Next comes the installation of purlins, which can be made from boards 100-150 mm wide and 40-50 mm thick. The purlins are fixed using nails and corners with screws. Then, on top of the purlins, plank crossbars are laid, for rigidity, installed at the end.
Advice!
As a result, the outline of the future attic space is formed. To provide additional strength, the supports should be strengthened with struts and braces.
Installation of rafter elements
Installation of rafters starts from the bottom. A board 40-50 mm thick and 150 mm wide is suitable for them. We rest one end on the mauerlat close to the floor beams, and the other is attached to the purlins using angles, screws and nails. To install rafters at the top of the roof, mark the center line of the roof. All rafters must be the same length. To do this, you should make a template board, making cuts on it at both ends. Then you can saw off the rest of the rafters using the template.
The rafters are mounted on purlins, fixed at the top with metal plates. On the purlins, the beams are secured with edge-to-edge cuts and secured with corners on self-tapping screws.
For strength, the lower rafters should be strengthened with struts (board 50x150 mm). After securing the struts, the temporary stops can be removed.
Installation of rafters
Lathing
In cases where floor beams are laid in wall pockets, fillers should be secured to the lower rafters to provide a roof overhang. If the floors lie on the mauerlat, then fillers are not needed, since the beams should protrude beyond the walls to form an overhang.
The sheathing is installed depending on the intended covering of the attic roof. Either the sheathing will be solid or with gaps. A waterproofing layer should be laid on top of the sheathing, after which you can begin sewing up the gables and laying the roofing material - metal tiles, corrugated sheets or slate.
Roof sheathing under metal tiles
Attic insulation
If the attic roof has a broken structure, then, as a rule, it is not insulated, since the air layer under the rafters protects the premises, while simultaneously ventilating the space under the roof. To do this, when sewing up the gables, you should leave ventilation windows (holes) above the attic floor. Thermal insulation should be done only inside the attic room.
Thermal insulation of the attic has its own characteristics, since the rooms have triangular or trapezoidal ceilings. Builders consider insulation to be one of the complex technological tasks in the construction of attic spaces. The main thing is to lay the thermal insulation so that on the sloping ceiling and on the fences it does not shrink during use.
Attic insulation
The rafter system is one of the final stages of building a house. The durability of the building itself and the comfort of living in it depend on how correctly it is executed. Simplicity of execution allows you to do the work yourself with the assistance of an assistant.
Rafter systems for the attic
Today there is a real boom in the use of various architectural elements in suburban construction, which, in the context of industrial construction, would seem to be a thing of the past forever. We are talking about devices such as bay windows, attics, mezzanines and the like. They can significantly increase the usable space and give the building an original, beautiful look.
Attics are especially popular, allowing you to create a full-fledged living space in the under-roof space. For this purpose, special rafter systems are used to ensure the possibility of creating such a room, provided that the structure is of sufficient strength.
The installation of a broken attic roof allows you to get a full-fledged living space on the second floor
Types of rafter systems
The main types of rafter systems for attic construction are presented in two options:
- gable;
- broken rafter system.
Photo gallery: what is an attic
A sloping roof allows you to obtain the optimal combination of the roof slope and the usable area underneath. It is possible to increase the volume of the attic in a structure under a gable roof only due to the high height of the ridge. The attic can be “equipped” with an external structure with a balcony under a separate roof. The balcony can be an integral part of the attic roof structure.
However, in practice, the roofs of a country house are so diverse that it is almost impossible to classify them. The design uses a variety of elements:
- hip bevels;
- birdhouses;
- awnings;
- semi-built-in translucent structures (greenhouses);
- lanterns and other architectural solutions in the most original and sometimes unexpected combinations.
Photo gallery: projects of houses with a gable roof and an attic
A gable roof can be decorated with an elegant “birdhouse”, which will also increase the area of the attic. In large houses, a gable structure can be used as one of the elements of the roof composition. The main idea of such a house is to combine simplicity of execution with perfectly selected finishing elements
Elements and components of the attic roof rafter system
Obviously, the most durable structure is a gable roof. But to obtain a sufficiently spacious room with such a rafter arrangement, it is necessary to reduce the angle between the slopes, and this leads to an increase in wind loads on the roof. A reasonable solution is to create a semi-attic, when racks are installed from floor to ceiling to a height of 1.3–1.8 meters, and rafters are already attached to them. Such a device requires the installation of low crossbars to compensate for the thrusting loads from the rafter legs on the racks.
To increase the useful volume of the attic, the crossbars that form its ceiling are installed as close as possible to the ridge unit The use of a sloping roof allows you to simplify the shape of the attic room and obtain a larger volume of living space.
The construction of a sloping roof allows you to get a much larger attic than the installation of a conventional gable roof The main elements of the rafter system are:
When installing a rafter system, additional parts are widely used to strengthen the fastening of structural elements.
The use of modern fasteners allows us to effectively strengthen the structure and significantly reduce installation time Calculation of the rafter system
The initial data for the calculation are taken from a previously developed project. For example, let’s look at a roofing diagram with an attic.
The design drawing indicates the dimensions, installation locations and material of all elements of the roofing system The main indicator, calculated first of all, is the pitch of the rafters, depending on the planned finishing coating. So, to install ceramic tiles, the pitch of the rafters should be no more than 60 centimeters, and if you plan to install a plastic covering or soft roof, this figure can be increased to 120–150 centimeters. Again, you need to take into account the nature of the loads - a combination of the effects of wind and snow - and set the optimal angle between the roof slopes.
The procedure for calculating the number of rafters
For example, consider the installation of roofs for a house 10 meters long. As a first approximation, we take the distance between the rafter legs to be 80 centimeters. Then you will need them: 1000: 80 + 1 = 13.5. Since the number of rafters must be an integer, we round the result to 13. In this case, the exact distance between them will be 1000: 13 = 769 (millimeters). This is the exact value of the gap between the axes of the rafter legs.
When calculating the need for materials, you need to take into account their cutting ability. In the case of wood materials, their length can be 4 or 6 meters. During the design process, it is necessary to select the dimensions of parts taking into account the formation of the smallest amount of trim . Coniferous wood trimmings are unsuitable even for firewood for stoves.
Materials for truss structure
The traditional material for rafters in Russia is wood. Larch is considered the best option, however, due to price and availability, it cannot always be used. Therefore, beams made of coniferous wood are used. The size of the section depends on the design of the building.
Composite profiled wood materials are becoming increasingly common. These include:
Construction of a rafter system for a gable roof with an attic
The gable roof rafter system is based on a triangle - the most rigid figure.
Only the main elements of the rafter system are listed. Additional parts can be used to increase the strength of the structure.
Installation of the roof truss system taking into account the formation of the attic
The rafter system of a gable roof can be formed in two ways:
- Assembly of the main structure below, followed by lifting to the ceiling and installation on the Mauerlat.
- Installation of rafter legs in detail directly at the installation site.
The first option is a more productive and convenient way.
Assembling the roof trusses below
This work is done in the following order:
Video: assembling roof trusses “on the ground”
Installation of rafter elements
The rafters, as the main load-bearing element of the roof skeleton, must be securely connected to each other. Many different methods have been invented for this, including specific ones designed for certain types of buildings. The rafter system of a wooden house requires special attention. The beam is attached to the Mauerlat by a sliding device, and the ridge connection is made on a hinge. This is due to constant seasonal movements of the log house, which must be compensated.
Photo gallery: methods of connecting rafter legs
The rafter legs are attached to the mauerlat using metal corners. The rafters are connected at the ridge end-to-end or through a cut. The critical nodes of the rafters are connected using metal connecting plates. The rafter legs are connected using special hinge joints, ensuring freedom of movement of the structure during seasonal deformations of the building. The joint in the ridge can be reinforced with bolts.
Installation of support posts and purlins
This is a responsible operation, because at this stage the surface of the front finishing of the walls and ceiling of the attic is formed. Therefore, the execution procedure is the same as when installing rafters:
For the manufacture of all parts of the rafter system, timber of the same dimensions is used, usually measuring 50x150 or 40x150 millimeters.
Video: quick installation of the rafter system
Lathing
This is a mandatory element of the rafter system. In the case where a warm attic space is formed in the under-roof space, the sheathing is done twice:
- External sheathing is used to secure the finishing roof covering. In addition, if each board is attached to the rafters with two nails, it serves as a fastening element of the frame. In addition, an insulating and moisture-proof roof pie is formed under the sheathing.
- The internal sheathing serves as a frame for insulating the attic and installing the final finishing of walls and ceilings.
In addition, counter-battens are also installed, with the help of which the ventilation system of the under-roof space is organized.
The counter-lattice is placed parallel to the rafters and provides clearance for ventilation of the under-roof space For sheathing, a board measuring 25x100 millimeters, edged or unedged, is used. Unedged boards must first be sanded. It is not recommended to use a board wider than the specified size. When warping, it can deform the finishing coating or damage the roofing pie.
The sheathing boards are fastened with nails at least 70 millimeters long and at least two nails for each intersection. This method increases the structure’s resistance to wind loads.
The spacing of the sheathing depends on the material of the finishing coating - for ceramic tiles and soft roofing it should be minimal (about five centimeters); for metal tiles or corrugated sheets, a distance between the boards of up to 70 centimeters is allowed.
A properly laid roofing pie must contain a ventilation gap between the insulation and the roofing material, which helps quickly remove moisture Video: lathing the rafter system
Rafter finishing
After installation of all roof elements, the rafter system becomes practically inaccessible for inspection and maintenance. Therefore, before installing each part in place, it must be carefully treated with protective compounds. There are enough special products on the building materials market to protect wooden structures from damage by bacteria and rot, as well as various fire-fighting impregnations.
Before installing parts of the rafter system, they must be treated with special protective compounds. It should be taken into account that carrying out protective operations on a finished structure is not always effective, since hidden surfaces remain untreated.
Any wood protection products are certified by government agencies, so when purchasing, you need to ask the seller for a certificate of conformity. It is issued by sanitary authorities and fire inspectors. First of all, the safety of the composition for humans and its compliance with the declared qualities are guaranteed.
Which of them to choose is decided by the consumer himself, depending on his financial capabilities. In principle, you can always run into a fake, but this will become clear only after many years of use.
Impregnations can be applied with a wide folded brush or roller, but a spray gun is often used.
Video: installation of a gable roof rafter system with an attic
A reliable roof largely determines the life cycle of a building. A properly installed roof ensures the comfort and cost of living in a home. In Russian conditions, when heating is needed most of the year, a properly installed insulated roof retains up to 30% of the heat. And high-quality insulation is possible only with a high-quality rafter system.
The difficulty lies in the fact that it is necessary to take into account several important components: snow load, gusts of wind, slope angle, weight of the rafters themselves, insulation, waterproofing, roofing materials. Rafters can be combined, metal or wood.
The attic roof should be light so as not to load the foundation. Developers are increasingly using thin-walled metal rafters for mansard roofs; their cost is much higher than wooden ones, but the speed and ease of installation of such systems more than makes up for this drawback.
In independent construction, the rafter system of an attic roof is traditionally made of wood, so we will take this material as a basis.
Good to know
In order to correctly calculate the truss structure of the attic, it is necessary to take into account 3 types of loads acting on the roofing system. First, you should calculate the weight of the attic roof itself: roofing material, sheathing, counter-lattice, waterproofing, rafters, attic insulation layer, vapor barrier, finishing. To do this, you will need to know the weight of each material per 1 m2; such parameters are indicated in the instructions and certificates of the manufacturers.
For example, polymer-bitumen waterproofing will require 5 kg/m2, one square meter of mineral wool weighs 10 kg, standard sheathing made of 25 mm boards - 15 kg, ondulin approximately 3 kg, metal tiles - the heaviest roofing material. When calculating the load, add up all the indicators and multiply by a correction factor of 1.1.
Secondly, the design of the attic roof rafter system must withstand snow loads; in some areas this figure exceeds more than 500 kg per m2. Formula for calculating snow load:
S=Sg x µ, where
Sg is the mass of snow on 1 m 2 of horizontal surface, the indicator is established by regulatory documents for each climatic region separately;
µ - coefficient depending on the slope of the roof: an angle of 25 o - coefficient 1.0, 25-60 o - 0.7, if the slope is above 60 o the coefficient is not taken into account.
And finally, wind loads. To calculate them, the formula is used:
W = Wo x k, where
Wo – normative data, which are determined individually for each district;
k – correction factor for different types of areas and building heights.
Based on the total load on the rafter system of the attic roof, the length, pitch and cross-section of the rafter leg are determined. Below we have provided a table of recommended parameters for installing a rafter system in a temperate climate zone (Moscow region):
| Distance between attic roof rafters, m | Leg length, m | ||||||
| 6 | 5.5 | 5.0 | 4.5 | 4.0 | 3.5 | 3 | |
| Cross section of beams, mm | |||||||
| 0,6 | 50*200 | 50*200 | 50*175 | 50*150 | 50*150 | 40*175 | 40x150 |
| 0,9 | 75*250 | 75*200 | 75*175 | 75*175 | 50*200 | 50*175 | 50x150 |
| 1,1 | 100*200 | 75*200 | 75*200 | 75*175 | 75*175 | 75*150 | 75x150 |
| 1,4 | 100*200 | 100*200 | 75*200 | 75*200 | 75*200 | 75*175 | 75x150 |
| 1,75 | 100*250 | 100*200 | 100*200 | 100*200 | 75*200 | 75*200 | 75x150 |
| 2,15 | — | 100*250 | 100*250 | 100*200 | 100*200 | 100*175 | 100x150 |
Other elements of the attic roof rafter system have their own parameters:
- the Mauerlat is made of timber 150*150, 100*150, 100*100 mm;
- crossbar – 100*200-150 mm;
- purlins – 100*200-150-100 mm;
- valleys, diagonal legs – 100*200 mm;
- tightening – 50*150 mm;
- strut – 150*150, 100*100 mm.
- hemming board – 25*100 mm.
Rafter system of a gable mansard roof, drawing of the main elements of the truss
Main nodes
The truss structures of attic roofs, regardless of whether they include many interfaces and butt joints, are connected to each other, cut into a tie and a mauerlat, have auxiliary elements of the system: crossbars, headstocks, supports or struts, and are additionally fastened with staples, bolts, clamps. The design of the rafter system of the attic roof must provide for the uniform distribution of all loads on the base and load-bearing structures of the building.
Scheme of the attic roof rafter system, main components and elements
Depending on the architectural solution, the type of attic roof is determined, in turn, not only its usable area depends on the configuration of the roof, but also what type of rafter system is applicable in each specific case: suspended or layered.
Layered system, resting the foot on the ridge beam and Mauerlat
Layered rafter legs rest on the load-bearing walls of the attic, and are supported in the middle by intermediate supports; in such systems, loads are transferred exclusively to bending. Hanging ones rest only on attic walls, they are usually installed where there is no internal load-bearing wall, they do not create a horizontal load, only compression and bending.
A hanging system is installed if the distance between the wall supports is no more than 6.5 m; installing an additional support makes it possible to cover an attic up to 12 meters wide; if you need to cover a wider area, then each 3 meters has its own support.
Hanging and layered rafters of the attic roof
If the attic design provides for not one, but several slopes, as in a classic attic roof, then hanging (upper) and layered (lower) rafters usually alternate.
Watch how the rafter system of mansard roofs is installed; the video lesson will help you understand all the intricacies of the construction of a broken structure.
The load-bearing elements of the system ensure the strength and stability of the entire attic roof structure. The load-bearing elements of an attic roof are: sheathing, rafters, and mauerlat. The design features of the system are determined by the type of attic roof: single-pitched, hip, multi-pitched, gable, etc.
Mansard roof truss system, layout diagram of single-pitched floor units
The shape of the attic roof depends on the area to be covered and the architectural design; it is better to choose simple types of attic roofs so that the beams and racks do not clutter up the space inside.
Attic, rafter system of hip hip roof
Stages of installation of the attic rafter system
The technology for installing a mansard roof requires a certain sequence:
- Mauerlat installation;
- laying cross beams;
- installation of vertical racks;
- tightening the side posts or installing a ridge beam;
- step marking and installation of rafter legs;
- installation of auxiliary units: crossbars, struts, rafter legs;
- lathing, waterproofing, roofing material.
Attaching the Mauerlat is an important step for the design of the entire system; it is the basis of the design
Docking technology
Despite the fact that the algorithm for constructing a rafter system is quite simple, installing attic roof rafters requires a competent approach to the installation of docking units. The main load in the system falls on the legs, so the safety and strength of the roof, and the attic as a whole, depend on the quality of their installation on the Mauerlat, as well as on the reliability of their connection to each other.
Rafter system of the attic roof, photo of attaching hanging rafters to the Mauerlat
There are 2 options for connecting the rafters to the Mauerlat: sliding and rigid. In wooden attic structures, it is impossible to make all connections rigid, since wood, under certain conditions, compresses and expands, and with a rigid connection, thrust loads arise on the load-bearing units, which together leads to deformation of the elements of the attic rafter system.
How to properly attach layered rafters for an attic
Important: A rigid connection is used if the rafters are layered, then the leg is rigidly attached to the mauerlat, but at the same time, on the ridge, between each other or on the run, the system nodes must be attached with a sliding connection.If a rigid fastening system is used on the attic roof, then it is necessary to exclude any forces: sliding, torques, turns, shifts. For rigid fastening there are two connection methods:
- notch in the rafter leg;
- support beam patch.
The saddle (notch) is made on the leg, the cuts should ensure a tight fit of the parts, and should not exceed 1/3 of the height of the beam. For rigid fastening, the rafter is notched against the mauerlat, 2 nails are driven from the sides, at an angle to each other, crosswise, the third nail is driven from above, vertically. It is recommended to further strengthen the fastenings with metal plates, corners, and bolts.
The sliding method of connection is that a recess for the leg is cut out in the Mauerlat, then the parts are connected to each other with a special metal corner - a slide. This is how the layered system is attached.
Sliding fastening of the attic rafter system
Regardless of the method of connecting the attic rafter system, the rafters and Mauerlat are screwed together with wire or additionally connected with anchors to prevent the roof from shifting due to strong gusts of wind. The struts, headstocks, spacers and headstocks are secured with staples and clamps.
How to build up and strengthen attic rafters
Often the rafter leg is not long enough for the timber, so they are built up to avoid ordering special boards and to avoid increasing the cost of attic construction.
Important: Since the rigidity of the rafter system is lost at the joining points, connections should be made where the bending moment tends to zero.Rafters made of timber are joined with an oblique cut. Inclined cuts are made in the parts, their length should be equal to twice the section, and the ends should be of a height equal to the cross-section of the beam, multiplied by a factor of 0.15. The joint is secured with bolts. This connection of beams harmoniously fits into the design of the attic, and allows you to leave the load-bearing elements of the roof open.
Sometimes attic rafters are attached with an overlap, the boards should overlap each other by at least a meter, then they are fastened with nails or bolts in a checkerboard pattern. If attic rafters are built up end-to-end, then the cut must be exactly 90°, the elements are applied at the ends, boards are laid on both sides, then the structure is secured with bolts or nails.
Options for extending the rafter leg
If the cross-section of the material is not enough, then the boards are reinforced. Paired rafters consist of two or more boards that fit tightly together; this structure is stitched with nails in a checkerboard pattern. Composite rafters are made from two boards, between which liners are inserted with a length of 2 board heights. The distance between the liners should not exceed 7 foot heights. The top of composite rafters may consist of a single plank.
Ways to strengthen the rafter leg
How to join rafters at the top
There are several options for connecting the rafters of the attic roof to each other at the top. If a ridge is not provided, the boards are cut at an angle, fastened together with nails, and additionally with metal and wooden overlays. Hanging systems for complex raftering of a multi-slope mansard roof are fastened into a tenon with a single or double tooth.
Upper rafter connection on a ridgeless mansard roof
If the rafter system provides a ridge, then the rafters can be attached to the ridge beam with an overlap, or by sawing the ends at the desired angle, the joints must be reinforced with metal or wooden overlays.
Joining rafters on a ridge beam
It is better to assemble trusses on the ground, and then lift and install the structure on the attic floor. First of all, you need to make a template. To do this, the board is placed in place, the corner of the cut is drawn for the upper joint, and a groove is marked for fastening to the Mauerlat, then the second board is tried on in the same way.
On the ground, according to the marks obtained, the boards are sawed down, joined together, the template is lifted onto the roof and tried on. If the geometry of the attic roof is met, then the entire rafter system can be made according to one template, but in case of small deviations, it is recommended to do only the top cut and joining, and make the tie-in into the Mauerlat locally.