There are several roof options under which you can place a comfortable living space. In order to ensure the maximum volume of the attic space under the roof, it is necessary to select the optimal angle of inclination of the slopes and not to forget about the snow and wind load on the roof. We will look at the most popular designs of attic rafter systems in the middle zone.
Design of the attic roof truss system
A mansard roof makes it possible to obtain additional usable space with a relatively small financial investment, which is why this architectural solution has gained great popularity. So what is called an attic?
Attic (from the French mansarde) is an usable attic space (both residential and non-residential premises), formed on the top floor of a house, or the top floor of a part of a house, with an attic roof.
Wikipedia
https://ru.wikipedia.org/wiki/Attic
The attic is located within the load-bearing walls of the building and rests on them through the mauerlat, horizontal beams (strings) and rafters. The larger the attic space, the greater its useful volume, which is achieved by increasing the angle of inclination of the slopes and is formed by a more complex rafter system. The location of the rafters of the attic roof depends on the structure, which can be of different types, namely:
- Tent or pyramidal structure with a minimum volume of under-roof space.
The slopes of the tent structure rest on the side rafters and the central post, so the volume of the under-roof space here is minimal
- A hip or half-hip roof, in which the main living space is located under trapezoidal slopes.
The rafters of the hip roof form two triangular and two trapezoidal slopes
- A gable structure, which is a symmetrical gable roof with gables cut in at right angles, which provides a significant volume of attic space.
A multi-gable roof allows you to equip a full attic floor
- A gable symmetrical roof with an attic is a classic option, characterized by ease of installation and resistance to wind influences due to its rigid structure.
A gable roof requires the least amount of working time and low consumption of building materials
- The rafter system of an attic sloping roof provides the maximum volume of living space at a relatively low cost.
A sloping roof is the optimal solution in terms of the ratio of the cost of construction to the volume of usable space in the attic
The rafter system must withstand constant loads, which consist of the weight of structural elements, insulation and roofing. In addition, there are variable loads depending on the strength of the wind and the weight of the snow on the roof. The choice of the cross-section of load-bearing elements and the method of connecting them should be aimed at creating the most durable and rigid structure that evenly distributes the load on the walls of the building.
Depending on the width of the building, different types of attic rafter systems are used, which are divided into hanging, layered and combined.
- Hanging rafters are those that rest on the walls of the building through the mauerlat and tie, and form a ridge in the upper part. With this connection method, there is no intermediate support, and the bursting pressure on the walls of the house is reduced with the help of crossbars, racks and struts. The hanging rafter system is usually used when the building width is no more than 6 m.
To compensate for bursting forces in suspended rafter structures with a span of up to 6 m, tie rods and crossbars are used
- Layered rafters are called rafters with intermediate support on the internal wall of the house. They are used when the width of the building is from 6 to 16 m. The larger it is, the more elements are used to evenly distribute the load.
Layered rafters have one or more supports inside the house
- The combined type of rafter system is used in mansard roofs with variable slope angles. The most typical example is a broken mansard roof, where the lower rafter legs are layered and supported by a post and a mauerlat, and the upper ones are mounted as hanging rafters supported by a tie rod and a headstock. When constructing mansard roofs, all types of rafter systems are used, and their choice depends on the structure in which they are used.
In the design of a sloping roof, the upper rafters are hanging, and the lower ones are layered
Scheme of the rafter system of the attic roof
To erect a roof, you must have a project that specifies the list and size of structural elements, as well as the method of connecting them. To understand the principle and sequence of installation, you need to know the purpose of the elements of the rafter group and the way the roof adheres to the walls of the building. The attic roof consists of the following elements:
- the connecting element between the wall of the building and the rafter group is the Mauerlat, which is attached to the walls of the house with studs, brackets or anchors;
- strings are attached to the Mauerlat parallel to the short wall of the building, and beds are installed along the long side;
- vertical posts are mounted on the central floor;
- the ridge girder rests on the posts;
- the upper part of the rafters rests on the ridge girder, and the lower part is connected to the tie, forming a cornice overhang;
- the rafter legs in the upper part are connected by crossbars;
- on hip roofs, diagonal rafters and shortened eaves are used;
- trusses serve as additional support for the diagonal rafters;
- for intermediate fastening of rafters, racks and struts are used;
- if necessary, the rafters are extended with fillets.
The main load-bearing elements of the attic roof are rafters, beams and tie rods, as well as vertical posts and a ridge girder.
The diagram indicates the dimensions of the elements of the rafter system, their location, angles of inclination and methods of insertion at the connection nodes. Information about the location of double rafters, the presence of additional supports, and the dimensions of eaves and gable overhangs is also required.
The diagram is the main document for assembling the rafter system; it reflects all the necessary parameters. However, before cutting the material, it is necessary to check the calculations again and create templates for the main parts. If there is no project, you need to carry out calculations and create a diagram yourself.
Mansard roof rafter spacing
Before starting work, you need to select the pitch of the attic roof rafters. The distance between the rafters and the eaves (in the case of a hip roof) depends on several factors:
- building size;
- type of rafter system;
- constant and variable load on the roof;
- sections of rafters, racks and slopes;
- type of roofing;
- type and pitch of sheathing;
- insulation sizes.
For rafters, sheathing and counter-lattens, softwood material is selected in accordance with SNiP II-25, and the load on the rafters is calculated according to SNiP 2.01.07 and ST SEV 4868. Based on what is stated in the building codes and regulations, we can say that for rafters less than 9 m applicable beam cross-section from 50X150 to 100X250 mm with a pitch of 60 to 100 cm. The size of the building affects the design of the truss and the presence of racks, struts and crossbars, the use of which increases the strength of the rafter legs and allows you to increase the pitch between the rafters to 120 cm or more . Usually, to select a step, reference tables are used, which contain recommendations taking into account the length of the rafters and the cross-section of the beam.
Table: dependence of the pitch between the rafters on the section of the beam and the length of the rafters
The type of roofing used also affects the choice of rafter spacing, because different materials have different weights:
- tiles, depending on the type, weigh from 16 to 65 kg/m2, slate - 13 kg/m2. Such heavy coverings imply a reduction in the pitch of the rafter legs to 60–80 cm;
- the weight of metal coatings and ondulin does not exceed 5 kg/m2, so the pitch of the rafters can be increased to 80–120 cm.
On hip roofs, in any case, the step of the roofers is chosen to be 50–80 cm to give the slope greater rigidity.
In addition, the installation step of the rafters depends on:
The length of the rafters and sheathing of the attic roof
When doing independent calculations, the dimensions of some structural elements of the roof have to be calculated based on the existing dimensions of the building and the angle of inclination of the slopes. The length of the rafters sometimes has to be adjusted for different types of attic roofs, selecting the optimal dimensions of the entire structure as a whole.
Let's assume that the main dimensions of the building are known and it is necessary to calculate the length of the rafter joists for several proposed options for the angle of inclination and type of roof. Let half the width of the building L be 3 m, and the size of the eaves slope be 50 cm.
Additional calculations show that increasing the angle of inclination of the lower slope from 60 to 70 o will increase the width of the attic by 10%.
The length of the sheathing connecting the rafter legs is determined taking into account the gable overhangs that protect the facade walls from precipitation. The length of the gable overhang depends on the height of the building and is selected in the range from 40 to 60 cm. Therefore, the total length of the slope will be equal to the length of the house, increased by twice the length of the overhang.
Let's assume that the length of the house is 10 m, and the gable overhang is 0.6 m. Then the dimensions of the sheathing must be calculated taking into account the length of the slope, equal to 10 + 0.6 ∙ 2 = 11.2 m.
The parameters of the sheathing should be calculated taking into account the length of the gable and eaves overhangs
Any adjustment to the project requires a careful recalculation of the parameters of the rafter system, taking into account the changes that have arisen.
Video: calculation of the attic roof
Knots of the rafter system of the attic roof
The nodes of the roof truss system represent the junction of individual elements into a single structure, which allows you to evenly distribute the load on the walls of the building. The connection is made with nails, self-tapping screws or bolts using overhead wooden elements or metal squares and plates, as well as by connecting to a groove. To construct a gable roof, the following main components are used:
- A ridge unit that provides a connection between the rafter legs and the ridge purlin.
- Places where the crossbar connects the rafters to give greater strength and rigidity to the trusses.
- Attachment points for struts and posts that provide additional support to the rafters.
- A cornice assembly in which the rafters are attached to a tie rod or mauerlat, forming a cornice overhang.
Nodal connections of the rafter system must be made in such a way as to ensure the most rigid fastening of the elements to each other
A characteristic feature of a gable sloping roof is the knot where the upper and lower rafter joists, the vertical post, the crossbar and the purlin are connected. Such a complex connection requires the use of mortises, bolts, steel plates and construction staples.
In the most complex unit of a broken mansard roof, five rafter elements are connected
The most complex component of a hip mansard roof is the junction of the side or diagonal rafters with the mauerlat. The side rafter in the lower part rests on the corner beam of the mauerlat and on the embedded beam; in another option, a vertical post or truss is placed between the embedded beam and the rafter leg. The upper part of the hip rafters is attached to the ridge purlin using bolts or nails.
The corner rafters of a hip roof bear the greatest load, so their connection to the Mauerlat must be thought out most carefully
The described units are used most often when installing rafter systems of various designs and allow you to install load-bearing elements on your own. For efficient and high-quality assembly, drawings and production of templates with verified angles of joints and inserts are required.
Video: rafter system components
Calculation of the rafter system of the attic roof
The rafter system is the basis of the roof, so it is important to choose the design that is most suitable for the climatic conditions of the region and the existing requirements for the size of the residential attic. After selecting the design, the angle of inclination of the slopes and the height of the ridge are calculated for the required dimensions of the attic room. Calculations are made taking into account the size of the cornice using the following formulas:
The meaning of trigonometric functions can be found in reference tables.
Table: values of trigonometric functions for different slope angles
The most difficult thing when designing mansard roofs is counting lumber. Calculating the number of rafters required and matching them with a standard length of 6 m can sometimes be quite difficult. Let’s assume that we have chosen the hip roof, the most difficult to calculate, measuring 10X13 m, taking into account the eaves overhangs 80 cm long and the slope angle of 45 degrees. Then the side rafters will have a length of 5 / sin 45 o = 7.04 m. Therefore, the standard six-meter beam will have to be lengthened. Typically, for rafters a little more than 6 m long, a 100X200 mm beam or a 50X250 mm board is used.
If the building is large, it requires rafters longer than the standard size of 6 m, so the beam has to be extended
As for the horizontal floor beam, since the width of the building is 10 m, the tie rods should consist of two parts, which either rest on the internal wall of the building, or are joined together by reinforcing elements and rest on the purlin. For tightening and purlins, use timber with a cross-section of at least 50x200 mm. Along the perimeter of the building there is a mauerlat, for which timber 150X150 mm or 200X200 mm is used. According to the scheme we have chosen, the perimeter of the building is 39.6 m, so installing the Mauerlat will require seven six-meter beams. The dimensions of all other elements of the rafter system will not exceed 6 m.
The weight of the lumber of the rafter system is calculated by summing the lengths of all elements with a certain cross-section and converting their quantity into cubic meters. This is necessary to determine the mass of the entire roof, and is also necessary when purchasing and transporting material. The calculation is made according to the table, and then the obtained values are multiplied by the weight of 1 m 3 of lumber.
Table: calculation of the amount of lumber in 1 m3 and the volume of one unit of material
Pine lumber weighs 505 kg/m 3 at 12% humidity, and 540 kg/m 3 at transport humidity 25%. Here are some examples of calculations:
- If 1 m 3 of material with a cross section of 50X200 mm contains 16.6 boards, then the weight of one board will be 540 / 16.6 = 32.5 kg.
- If 25 m 3 of lumber is purchased, then it will weigh 25 ∙ 540 = 13,500 kg.
- If 100 boards 25X200 are required, then you need to buy 100 / 33.3 = 3 m 3 of wood, which will weigh 3 * 540 = 1,620 kg.
It is important to note that it is advisable to purchase edged lumber with the lowest moisture content so that after installation it does not warp or crack, especially for large-section timber. For the construction of a rafter system, the wood moisture content should not exceed 18%.
Installation of attic rafter system
Installation of a rafter system with a residential roof space requires careful preparation. It is necessary to install convenient scaffolding, decking and ladders, as well as provide workplaces with safety ropes. Workers must be provided with protective clothing, protective equipment and working equipment. It is necessary to select a flat place on the ground for pre-assembling the trusses, marking the corners and making templates. All wooden elements must be treated with antiseptic and fire-retardant compounds.
After this, you can begin work, which takes place in the following sequence:
- On walls with walled studs, a mauerlat is mounted around the perimeter. If there is a load-bearing wall inside the building, we lay a beam or purlin on it that is the same height as the mauerlat.
If the house is being built from building blocks, then the Mauerlat is most conveniently laid on threaded rods, walled into the wall during its laying.
- Cable ties with cornice extensions are attached to the Mauerlat parallel to the short wall.
- Vertical posts are placed on the tie rods, delimiting the attic space.
- The racks are connected to a tie that serves as the ceiling of the attic space. The trusses installed in this way are connected to each other by horizontal girders.
Vertical posts, top ties and horizontal purlins form the frame of the attic space
- The lower and then the upper rafters are installed, which are attached to the ridge part.
- To strengthen the lower and upper rafters, struts, headstocks and ribs are used.
- The sheathing and front board are installed on the eaves overhang.
After installing all the rafter joists, all that remains is to lay the sheathing and nail the front board
We looked at the assembly of the rafter system using the example of a broken mansard roof. The construction of other structures consists of similar operations and mainly consists of performing work according to the design installation diagram, which reflects the methods of connecting the elements of the rafter system. With carefully calculated drawings, a team of four people is able to install a roof with a rafter system of any complexity.
Video: installation of an attic roof
We examined the rafter group of the attic roof, its design, calculations, as well as a diagram and description of the main components. They offered an option for step-by-step installation of load-bearing structures of the attic, attached illustrations and videos that explain the procedure for assembling the structural elements of the attic roof. Now its successful construction depends only on the careful fulfillment of the requirements of instructions and technologies and the availability of certain skills for the performers to carry out construction work. We wish you success.
A house with an attic is not only an additional living space, but also a respectable appearance for the entire building. Even if the room under the roof is made unheated and is used only in the summer, it still creates a powerful “air cushion” that helps retain heat inside the entire building.
And about that - read on our portal.
Attic project
When drawing up a diagram for the construction of an attic, it is best to do this in different projections in order to see and understand the placement of all elements of the rafter system. It is very important to correctly calculate the height of the roof ridge, since the size of the area under it will directly depend on it.
When drawing up a design diagram for the construction of an attic roof, you need to calculate the height of the ridge, ceiling and total area of the room.
The minimum height from the floor to the ridge should be 2.5-2.7 m, but if this distance is less, then the room is not an attic, it can only be called an attic. This parameter is established by SNIP standards.
In order for all the elements to be drawn accurately and have the desired location in the overall system, you need to start from a figure with right angles, that is, a rectangle or square - a section of the attic room being created. Based on the sides (height and width of the future room), it will be almost impossible to make a mistake with the magnitude of the angles at which the roof slopes are located, with the location of the ridge, rafters and all supporting elements. When determining these parameters, they must immediately be entered into the drawing.
First you need to find the middle of the width of the front wall. Starting from this point, the parameters of the height of the ridge, the future ceiling of the attic, the location of the wall studs and the size of the eaves overhang are determined.
Due to the fact that each of the structures has a certain number of connecting nodes, which have different configurations, it would be a good idea to draw each of these connections separately in order to understand their features of interconnecting all the elements connecting at this point.
Any rafter system consists of basic elements and additional ones, which may not be present in every structure. The main components of an attic roof include:
- Floor beams, which are the basis for the remaining elements of the rafter system. They are laid on the main walls of the building.
- A rafter leg, straight in a gable roof system or consisting of two sections - in a broken pattern. In this case, the top rafter is called the ridge rafter, since it forms the highest point of the roof - and the rafters that form the walls of the attic are called side rafters.
- A ridge board or beam is a mandatory element for a gable roof, but is not always used when installing a broken roof model.
- Mauerlat is a powerful beam attached to the main side walls of the building. Rafter legs are installed on this element.
- Racks are the supporting elements necessary to strengthen a gable and broken structure. In the latter case, the ridge and side rafters are attached to it, and in the first, the stand is a reliable support for a long rafter. In addition, the racks serve as a frame for insulating and covering the walls of the attic.
- Diagonal bracing members or bevels additionally secure posts or longitudinal beams and rafters, making the structure more durable.
- Attic floor beams are used in all versions of the attic - they connect the racks, and they also serve as the frame for the ceiling.
- Inter-rafter purlins are installed in a broken roof for structural rigidity.
To be sure that the prepared project is developed correctly, you need to show it to a specialist. Only he will be able to determine whether the attic parameters are correctly selected for the width and length of the walls of the building.
Video: professional calculation of a mansard roof using special software
Material parameters for the construction of an attic roof
If the graphic design is ready, then, based on the dimensions marked on it, you can calculate the amount of materials required for the construction of the attic roof. Materials must be selected according to their characteristics, which must meet fire and environmental safety requirements. For wood, it is necessary to provide special treatment with fire retardants, which will reduce the flammability of the material. So, for construction you will need:
- Boards for rafter legs. Their cross section is selected based on the results of special calculations - this will be discussed in more detail below.
- A beam having a cross-section of 100×150 or 150×200 mm is for floor beams, depending on the chosen rafter system and the width between the load-bearing walls, as well as for purlins, diagonal legs or valleys - if they are provided for in the design.
- Beam with a cross section of 100×150 mm or 150×150 mm for laying the Mauerlat.
- For racks, timber 100 × 100 or 150 × 150 mm is usually used.
- Unedged board for laying the subfloor and some fasteners.
- Annealed steel wire with a diameter of 3-4 mm - for fastening some parts together.
- Nails, bolts, staples of various sizes, angles of various configurations and other fasteners.
- A metal sheet with a thickness of at least 1 mm is for cutting out overlays.
- Lumber for sheathing and counter-lattens for roofing material - depending on the type of roof chosen.
- – for thermal insulation of the roof.
- Waterproofing and vapor barrier membranes.
- Roofing material and fastening elements for it.
What section of rafters are required?
Rafters are roofing elements that will bear the main external loads, so the requirements for their cross-section are quite special.
The size of the required lumber will depend on many parameters - on the step between the rafter legs, on the length of these legs between the support points, on the snow and wind load that falls on them.
The geometric parameters of the rafter system design are easy to determine in the drawing. But with the remaining parameters, you will have to refer to the reference material and make some calculations.
Snow load is not the same for different regions of our country. The figure below shows a map on which the entire territory of Russia is divided into zones according to the intensity of the snow load.
There are eight such zones in total (the last, eighth, is rather extreme and cannot be considered for the construction of an attic roof).
Now you can accurately determine the snow load, which will depend on the angle of the roof slope. For this there is the following formula:
S = Sg × μ
Sg– table value – see the map and the table attached to it
μ — correction factor depending on the steepness of the roof slope.
- If the slope angle is me 25°, then μ=1.0
- With a slope from 25 to 60° - μ=0.7
- If the roof is steeper than 60°, then it is considered that snow does not linger on it, and the snow load is not taken into account at all.
It is typical that if the attic roof has a broken structure, then for different sections of it the load can have different values.
The slope angle of the roof can always be determined either with a protractor - according to the drawing, or by a simple ratio of the height and base of the triangle (usually half the span width):
Wind load also mainly depends on the region in which the building was built and on the characteristics of its surroundings and the height of the roof.
And again, for the calculation, the initial data on the map and the table attached to it are first determined:
The calculation for a specific building will be carried out according to the formula:
Wp = W × k × c
W– table value, depending on the region
k– coefficient taking into account the height of the building and its location (see table)
The following zones are indicated by letters in the table:
- zone A - open areas, steppes, forest-steppes, deserts, tundra or forest-tundra, windswept sea coasts, large lakes and reservoirs.
- zone B – urban areas, wooded areas, areas with frequent wind obstacles, relief or artificial, at least 10 meters high.
- zone IN– dense urban development with an average building height above 25 meters.
With– coefficient depending on the predominant wind direction (wind rose of the region) and on the angle of inclination of the roof slopes.
With this coefficient the situation is somewhat more complicated, since the wind can have a dual effect on the roof slopes. So, it has a direct, overturning effect directly on the roof slopes. But at small angles, the aerodynamic effect of the wind takes on special importance - it tries to raise the slope plane due to the resulting lift forces.
The drawings, diagrams and tables attached to them indicate areas of the roof exposed to maximum wind loads, and indicate the corresponding coefficients for calculation.
It is characteristic that at slope angles of up to 30 degrees (and this is quite possible in the area of ridge rafters), the coefficients are indicated both with a plus sign and negative, that is, directed upward. They somewhat dampen the frontal wind load (this is taken into account in calculations), and in order to neutralize the effect of lifting forces, it will be necessary to very carefully secure the rafter system and roofing material in this area, using additional connections, for example, using annealed steel wire.
Once the wind and snow loads have been calculated, they can be summed up and, taking into account the design features of the system being created, the cross-section of the rafter boards can be determined.
Please note that the data is given for the most commonly used coniferous material (pine, spruce, cedar or larch). The table shows the maximum length of the rafters between the support points, the section of the board depending on the grade of the material, and on the pitch between the rafters.
The value of the total load is indicated in kPa (Kilopascals). Converting this value into more familiar kilograms per square meter is not difficult. With completely acceptable rounding we can accept: 1 kPa ≈ 100 kg/m².
The dimensions of the board along its cross-section are rounded up to standard lumber sizes.
| rafter section (mm) | Distance between adjacent rafters (mm) | ||||||
|---|---|---|---|---|---|---|---|
| 300 | 600 | 900 | 300 | 600 | 900 | ||
| 1.0 kPa | 1.5 kPa | ||||||
| higher | 40×89 | 3.22 | 2.92 | 2.55 | 2.81 | 2.55 | 2.23 |
| 40×140 | 5.06 | 4.60 | 4.02 | 4.42 | 4.02 | 3.54 | |
| 50×184 | 6.65 | 6.05 | 5.28 | 5.81 | 5.28 | 4.61 | |
| 50×235 | 8.50 | 7.72 | 6.74 | 7.42 | 6.74 | 5.89 | |
| 50×286 | 10.34 | 9.40 | 8.21 | 9.03 | 8.21 | 7.17 | |
| 1 or 2 | 40×89 | 3.11 | 2.83 | 2.47 | 2.72 | 2.47 | 2.16 |
| 40×140 | 4.90 | 4.45 | 3.89 | 4.28 | 3.89 | 3.40 | |
| 50×184 | 6.44 | 5.85 | 5.11 | 5.62 | 5.11 | 4.41 | |
| 50×235 | 8.22 | 7.47 | 6.50 | 7.18 | 6.52 | 5.39 | |
| 50×286 | 10.00 | 9.06 | 7.40 | 8.74 | 7.66 | 6.25 | |
| 3 | 40×89 | 3.06 | 2.78 | 2.31 | 2.67 | 2.39 | 1.95 |
| 40×140 | 4.67 | 4.04 | 3.30 | 3.95 | 3.42 | 2.79 | |
| 50×184 | 5.68 | 4.92 | 4.02 | 4.80 | 4.16 | 3.40 | |
| 50×235 | 6.95 | 6.02 | 4.91 | 5.87 | 5.08 | 4.15 | |
| 50×286 | 8.06 | 6.98 | 6.70 | 6.81 | 5.90 | 4.82 | |
| total snow and wind load | 2.0 kPa | 2.5 kPa | |||||
| higher | 40×89 | 4.02 | 3.65 | 3.19 | 3.73 | 3.39 | 2.96 |
| 40×140 | 5.28 | 4.80 | 4.19 | 4.90 | 4.45 | 3.89 | |
| 50×184 | 6.74 | 6.13 | 5.35 | 6.26 | 5.69 | 4.97 | |
| 50×235 | 8.21 | 7.46 | 6.52 | 7.62 | 6.92 | 5.90 | |
| 50×286 | 2.47 | 2.24 | 1.96 | 2.29 | 2.08 | 1.82 | |
| 1 or 2 | 40×89 | 3.89 | 3.53 | 3.08 | 3.61 | 3.28 | 2.86 |
| 40×140 | 5.11 | 4.64 | 3.89 | 4.74 | 4.31 | 3.52 | |
| 50×184 | 6.52 | 5.82 | 4.75 | 6.06 | 5.27 | 4.30 | |
| 50×235 | 7.80 | 6.76 | 5.52 | 7.06 | 6.11 | 4.99 | |
| 50×286 | 2.43 | 2.11 | 1.72 | 2.21 | 1.91 | 1.56 | |
| 3 | 40×89 | 3.48 | 3.01 | 2.46 | 3.15 | 2.73 | 2.23 |
| 40×140 | 4.23 | 3.67 | 2.99 | 3.83 | 3.32 | 2.71 | |
| 50×184 | 5.18 | 4.48 | 3.66 | 4.68 | 4.06 | 3.31 | |
| 50×235 | 6.01 | 5.20 | 4.25 | 5.43 | 4.71 | 3.84 | |
| 50×286 | 6.52 | 5.82 | 4.75 | 6.06 | 5.27 | 4.30 | |
Tools
Naturally, during work you cannot do without tools, the list of which includes:
- Electric drill, screwdriver.
- Building level and plumb line, tape measure, square.
- Axe, chisel, chisel, hammer
- Circular saw, jigsaw, hacksaw.
- Carpenter's knife.
Installation will be accelerated if the tools for the work are of high quality, and the work will be carried out with competent mentors and assistants, carefully and step by step.
Installation stages
It is necessary to strictly follow the sequence of work - only under this condition the structure will be reliable and durable.
Mounting the Mauerlat
Installation of any rafter system begins with securing a powerful supporting structure to the end of the side walls of the building. timber - mauerlat, on which it will be convenient to install rafter legs. The Mauerlat is made from high-quality timber with a cross-section of at least 100 × 150 mm. It must be laid on roofing felt waterproofing laid along the upper end of the wall (regardless of the material).
Due to the Mauerlat, the load will be evenly distributed over the walls and transferred to the foundation of the building.
The Mauerlat is secured to the wall using metal pins, which are pre-embedded in a concrete belt or crown running along the upper edge of the wall, or with anchor bolts with a diameter of 12 mm. They must go into the wall at least 150 — 170 mm. If the Mauerlat is installed on a wooden wall, then the beams are attached to it using wooden dowels.
Installation of truss structure
- Installation of the rafter system begins with the installation of floor beams. They can be attached to the mauerlat from above if the beams are planned to be moved outside the perimeter of the building and thereby increase the area of the attic. In this design, the rafter legs are fixed to the floor beams.
Floor beams fixed on top of the Mauerlat (Fig. A) - In another case, they can be stacked on waterproofed walls and fastened with corners or brackets to the inner edge of the Mauerlat. This option is used when the rafter legs are planned to be attached directly to the mauerlat.
Another option is that only the rafter legs are attached to the mauerlat - Next, you need to find the middle of the floor beam, since this mark will become a guideline for determining the location of the support posts and the ridge.
- The racks should be located at the same distance from the marked middle of the floor beam. They will subsequently determine the location of the walls of the attic room, that is, its width.
- The bars for the racks must have a cross-section equal to the size of the floor beams. The constructions are attached to the beams using special corners and wooden overlays. However, to begin with, they are first nailed, then carefully leveled using a building level and a plumb line, and only then are they permanently secured, taking into account future loads.
- When the first pair of racks is installed, they are fastened together from above with a bar, which is called a tie. This tightening is also connected to the racks using special metal corners.
- After securing the tie, you will get a U-shaped structure. Layered rafters are installed on its sides, the second end of which is attached to the floor beam or placed on the mauerlat.
- A special recess (groove) is cut into the installed supports for the timber or in the rafters. With its use The rafters are tightly installed on the Mauerlat beam and secured with metal brackets.
- To provide rigidity to the structure, additional struts can be installed from the base of the rack to the middle of the installed side rafters. If this does not seem enough, and saving material is not in the foreground, then you can strengthen the overall structure with additional racks and contractions (they are indicated in the drawing, Fig. A, with translucent lines).
- Next, while tightening, the middle is calculated - the headstock will be attached to this place, supporting the ridge connection of the upper hanging subsystem of the rafters.
- The next step is to install the ridge rafters, which can be fastened together with different connections - this can be a metal plate or powerful bolts with metal plates or washers.
- After installing them, the headstock is attached to the ridge and the middle of the tightening.
- Having completed work on one part of the rafter system, you need to make all the rest according to the same principle. The distance between adjacent rafters in such a system should be no more than 900 — 950 mm, but the optimal interval would probably still be 600 mm - this will provide the necessary rigidity and stability of the structure, and will be convenient for insulation using standard mineral wool mats. True, this makes the structure heavier and will require more materials.
- First, the side parts of the system assembly are installed, and then the intermediate parts. They are connected to each other by purlins, which are installed between the upper ends of the racks and act as spacers. Thus, you will get a rigid structure of the attic rafters, in which the frame for wall cladding will already be ready.
Prices for various types of fasteners for rafters
Rafter fasteners
Waterproofing attic roof
When the rafter system is built, you can proceed to finishing it and accompanying materials.
- The first coating that should be fixed directly on top of the rafters will be a waterproofing and windproof film. it is attached to the rafters using staples and a stapler, starting from the cornice. The canvases are laid with an overlap of 150 — 200 mm, and then the joints are glued together with waterproof tape.
- On top of the waterproofing, a counter-lattice is placed on the rafters, which will more reliably fix the film on the surface and create the necessary ventilation distance between the windproof and roofing material. The counter-lattice is usually made of boards 100 wide — 150 mm and thickness 50 — 70 mm.
- The sheathing is fixed perpendicular to the counter-lattice, on which the roofing material will then be laid. The pitch between the slats must be calculated depending on the type and size of the sheet roofing material, taking into account the overlap required for it
- If a soft roof is chosen, then plywood sheets are most often fixed to the counter-lattice.
Roofing installation
The roofing material is attached to the prepared sheathing or plywood. Its installation usually starts from the roof eaves and proceeds in rows, from one of the edges - depending on the type of roof. Roofing sheets are mounted with an overlap. If a metal profile or metal tile is used for the coating, then such material is secured with special self-tapping screws with elastic gaskets. Fastening elements are usually matched in color to the roofing material.
The most difficult thing in covering an attic sloping roof is the transition from layered side rafters to hanging ridge rafters. There may be certain difficulties if the roof has projections for installing roofs over balconies or windows.
In addition, if a chimney pipe goes onto the roof, it requires a separate design of the hole inside the rafter system and an insulating layer, and on the roof, reliable waterproofing must be installed around the pipe.
You can find out in detail how and what is the best way to cover a roof on our portal; there is a whole section where you can find answers to many questions, including recommendations for reliable insulation of an attic room.
Prices for popular types of corrugated sheets
Corrugated sheet
Video: detailed video tutorial on building a mansard roof
It should be noted that the work of constructing any roof, and especially one as complex as an attic roof, is not only responsible, but also quite dangerous and requires special, increased safety measures. If you have no experience in carrying out such construction processes, then it is better to entrust their implementation to professionals or perform all actions under the supervision of an experienced craftsman, and with the utmost care and precision.
Mansard roof rafter system
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.
Types of rafter systems
At the design stage of a house, the roof structure must be calculated. Calculations using complex formulas should give a value for 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 the structural elements of a mansard roof:
- 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.
Elements 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.
Gable roof rafter system with attic 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.
Gable mansard roof design Be sure to 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.
Advice!
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. Coniferous wood for timber must be well dried. The Mauerlat beams are fixed with anchors or studs to the walls in increments of approximately two meters. Waterproofing is placed under the Mauerlat, usually of a roll type (roofing felt).
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.
Intermediate supports of the roof truss system 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.
The purlin beam forms a cantilever to support the diagonal roof rafters 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.
Roof truss system 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.
Rafter legs 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.
Fastening the rafter system For strength, the lower rafters should be strengthened with struts (board 50x150 mm). After securing the struts, the temporary stops can be removed.
Often elements are connected not only with carpentry knots, but also with fasteners Installation of rafters
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.
Lathing is an important part of the roof structure 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
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 (openings) above the attic floor. Thermal insulation should be done only inside the attic.
Insulation of the attic roof 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.
Ksenia Skvortsova. Chief Editor. Author.
Planning and distribution of responsibilities in the content production team, working with texts.
Education: Kharkov State Academy of Culture, specialty “Culturologist.” Teacher of history and cultural theory." Experience in copywriting: From 2010 to the present. Editor: since 2016.
The installation of an attic roof allows you to significantly increase the usable area and rationally organize the space of a low-rise building. However, its construction often frightens home craftsmen because the process is too complex and labor-intensive.
There is no need to be afraid, because the result will provide a beautiful roof and comfortable additional rooms. And in order for the result of the work to please the owner and household members, you need to know what rules are used to construct the rafter system of an attic roof, and what is the easiest and best way to arrange it.
When we mention mansard roofs, we immediately recall a pentagonal gable structure of impressive size over a log house, concrete or brick walls. Visual memory suggests that its slopes must certainly have a different slope, i.e. the bottom of the roof simply has to be much steeper than the top. Due to the difference in the angles of inclination, a convex fracture is formed, which gave the roof the popular name “broken”. The term has justifiably migrated to the technical definitions of attic structures. It reflects the essence of the usual standard in the device, but often has nothing in common with the configuration. Despite the fact that the design of all mansard roofs necessarily includes two parts, their presence cannot always be determined visually.
Purely based on external indicators, the predominant number of attic structures can be divided into:
- Triangular roofs, the lower and upper parts of which have an equal slope. Outwardly, they resemble traditional gable structures without kinks in the plane of the slopes.
- Pentagonal roofs with slopes having convex corners. This category clearly demonstrates the presence of two joined parts in the design.
In both of these varieties, the rafter system consists of two tiers stacked on top of each other. The lower structure forms the useful space of a residential attic with a height of 2 to 2.5 m, so that moving inside it is not difficult. The second tier creates the shape of the roof top and is allowed to be of arbitrary height.
By varying the angle of inclination of the upper and lower rafter legs, you can obtain the optimal roof shape in your own opinion. It is believed that a pentagonal attic, the corners of which are in contact with an imaginary circle, looks best.
Note that the principle of constructing a sloping roof is suitable not only for gable rafter systems. Interpreting the basic method, the attic can be organized in hip, single-pitched, hipped and other roofing structures.
Sometimes an existing structure is converted into an attic, the construction of which did not use “broken” technology. However, these roofs cannot a priori be classified as attic. True, if the rafter legs have sufficient power, no one bothers to use the crossbars of pitched rafter systems as ceiling beams, and the supports of additional purlins as beams for cladding the attic.
We found out that the main feature of an attic roof is the presence of two adjacent rafter structures connected into a triangle or pentagon of a shape that is attractive to the owner. In their construction, standard ones are used:
- Layered, according to which the lower tier of the attic is built and used in the construction of the upper part.
- Hanging. In accordance with it, only the upper part of the structure is built.
If, to simplify, the section of the attic roof is divided into two halves, you will get a trapezoid at the bottom and a triangle at the top. The inclined sides of the trapezoid are allowed to be exclusively layered, and the sides of the triangle are layered and hanging.
Basic diagrams of rafter systems
A “classic of the genre” is rightfully considered to be the pentagonal diagram of the rafter system of an attic roof with support posts forming the walls of the interior. Its section is conventionally divided into simple geometric shapes. In the center there is a rectangle, on the sides of which there are two mirrored rectangular triangles, and an equilateral triangle on top.
Standard attic construction
The layered rafters of the lower part of the structure rest at the bottom on the mauerlat, and with the upper heel on the right or left purlin. The part of the mansard roof frame that crowns the structure is made up of hanging rafter arches. They are supplemented with a suspension headstock in the middle if they are intended to cover a span of more than 3 m. The headstock cannot be connected to the arch tightening with a notch, like a support post. Its job is to prevent the tie from sagging - it is not a support, but a suspension.
The supports-racks of the layered rafters of the lower part rest through the beam on the ceiling. If it is necessary to increase stability, struts are installed under the supports. The posts are connected to the frame and purlins by notches, the joints are duplicated with metal corners and toothed plates. If the floor is concrete, bitumen waterproofing is laid under the floor. The bed can be laid not on the ceiling, but on brick pillars or on leveling boards. When constructing an attic on a wooden floor, you can do without the beams altogether and embed the racks directly into the beams.
The rather steep lower parts of the slopes of attic roofs are practically not affected by the snow load; precipitation does not linger on them. However, steeply installed rafters have another problem - gusty winds will tend to overturn and tear off the roof. Therefore, attaching the system to the Mauerlat must be taken very seriously. In an attic situation, each rafter is tied to the walls with twists, and not through one, as in conventional pitched structures.
Method of moving rafters beyond the wall line
It often happens that the planned attic structure creates too narrow an internal space. It can be expanded by moving the rafter legs outside the walls. Those. the rafter leg will rest not on the mauerlat, but on the beams of the upper floor. This case, in theory, does not need a Mauerlat at all. But reinforcing struts in the scheme with the removal of rafters are used without question, because there is no support at all under the extreme part of the side triangles.
The installation of the Mauerlat can be eliminated, but pouring a monolithic reinforced concrete belt for attaching beams to brick walls is highly desirable. The floor beams are attached to the monolithic belt with anchors, and support posts are driven into them to a maximum of 1/3 of the thickness of the beam. An important point: moving the rafters outside the wall is simply required to form a cornice for wooden houses with a width of at least 0.5 m, for concrete and stone houses at least 0.4 m.
Technology for constructing a rafter structure with the rafter leg extending beyond the wall:
- We install the outermost floor beams that define the contour of the eaves overhangs. Because the ceiling will be loaded, the section of the beams is taken from 150x200mm. If, when laying the starting beam, it turns out that the walls do not form an ideal rectangle, we try to correct the flaws by changing the position of the beams.
- We lay and fix the remaining bars along the laces stretched between the fixed outer beams. We control the height and pitch of the beams before fastening. The distance between the floor elements is equal to the step between the rafter legs. For insulated roofs, the optimal rafter installation pitch is 0.6 m, because it is equal to the width. If the rafters will be installed with a similar frequency, they can be made from 50x150mm boards.
- From the left and right edges we set aside a distance equal to the length of the short leg of the right triangle. At the marked points, carefully use a chisel to select nests a third of the height of the beam under the outer supports.
- Let's make supports by cutting out tenons. They need to be made according to the size of the selected nests. For the manufacture of corner supports, a beam with a cross-section of 100×150 mm is suitable, and two load-bearing supports for the gable sides of the roof should be made from it. For ordinary racks, 50×100 mm timber is sufficient. The material for the supporting elements should be longer than the design height by the length of the tenon, but it is better by 10 cm in case of errors during cutting.
- We install the corner posts and fasten them with temporary spacers. We connect the posts with lace.
- We use a plumb line to check the points in the beams for selecting nests for row supports and select the indicated holes.
- We install row posts and two load-bearing supports in the centers of the attic gables.
- On the installed supports we lay purlins - boards with a section of 50x150mm. We fasten the purlins with corners. It is not necessary to use as many nails as in the corners of the holes. Two or three are enough for each plane. As a result of laying the boards, the frame of the walls of the future attic is obtained.
- We connect the supports installed opposite each other with bars, attaching them to the purlins with corners. These elements will serve as tensile crossbars. Therefore, for their manufacture you will need 1st grade lumber with a cross-section of 100×150mm. For each installed crossbar, a temporary support from a 25x150mm inch is needed.
- We temporarily fasten the crossbars on top with the same inch, retreating 20-30cm from the edges of the frame. Temporary thin flooring of one, two or three boards is needed for ease of installation of the upper part of the rafter system.
- We make a template for the rafters of the bottom row from an inch. To do this, we apply a blank board to the end of the purlin and beam. Then we outline the lines of the grooves along which the excess will be sawed off. We try it on and trim the excess if necessary.
- We make rafter legs using the template. If there are doubts about the flawlessness of the construction, then it is better to cut out only the upper groove first. By placing the rafter in its proper place, you can then adjust the lower groove after the fact without unwanted damage to the material.
- We install the end rafter legs, which will need to be connected again with lace.
- Using the lace as a guide, we install the rafter legs of the lower tier of the attic.
- Similarly, we make a template for the upper part of the rafter system. In order to find the line of the top cut, we temporarily sew a board onto the gable support.
- Let's make a mirror version of the previous template. The rafters of the upper tier will rest against each other.
- Let's try both templates on the roof. If everything is normal, we use them to make the required number of upper rafters from 50x150mm boards.
- We are constructing the upper tier of the rafter system.
- To prevent the crossbars from sagging, we install headstocks of the required size to each upper truss. We firmly sew them only to the ridge area; the bottom should not be rigidly fixed.
Next, the rafter legs are screwed to the walls with wire ties. Then the pediment frame is installed, along which it needs to be sheathed. Finally, the sheathing is installed at a pitch corresponding to the characteristics of the roofing material.
Method with frame modules
The technology differs from the previous method in that it is not individual supports that are installed on the floor, but modules-blocks of the side walls of the future attic that are completely prepared for fastening.
The block method of constructing a rafter system allows you to optimize the construction of an attic roof, because the construction of modular elements is carried out on the ground. In quiet conditions without a sense of altitude, it is easier to achieve precision node connections.
The process of installing a block mansard roof:
- Based on a pre-made design, we manufacture the frames of the attic walls. Using this method, longitudinal beams play the role of purlins and beds. We lay them out together with the racks on a flat area and use a square to mark the sockets for the supports of the side walls. We make cuts along the measured lines.
- We cut out spikes on the racks, the size of which must correspond to the size of the sockets.
- We connect the longitudinal beam with vertical posts, we get two modular frames - these are the walls of the attic.
- We lift the frames up and install them in the intended place. We temporarily fix the position of the walls with spacers, then attach them to the floor beams with brackets.
- Using a chisel, we select sockets at the edges of the beams for installing the lower row of rafters. They need to be placed in one line. To maintain the geometry, it’s easier to first mark them with a chainsaw, then modify them with a chisel.
- We perform the upper rafter tier of the attic on the ground, having previously fitted the blanks to the installed elements. To ensure an accurate fit, we temporarily nail a board to the end of the future roof so that one of its edges clearly follows the central axis of the rafter system. The base of the upper attic triangle serves as a stretcher. Its length is equal to the distance between the outer vertical planes of the installed frames. We select nests along the edges of the guy wires, and spikes on the lower heels of the rafters.
- We assemble the trusses of the upper tier, for reliability we install an additional crossbar, and we reinforce the ridge assembly with a triangular wooden overlay.
- Before we move to the roof, we make preparations for the rafter legs. We try them on to the frames laid out on the ground. It is more convenient to “cut” them in one fell swoop, grabbing several pieces with a clamp. We cut out only the upper bevel, taking into account the fact that it will rest partly on the wall post, partly on the stretch of the upper rafter trusses.
- We try on the bottom rafter to the end. In the area of its lower heel we draw the shape of a spike, repeating the configuration of the nest in the beam. We cut out the thorns.
- We move the trusses of the upper tier and the rafters of the lower tier to the roof. We first install the trusses, attaching them to the upper frame of the walls with staples, then the rafters of the lower part, attaching them to the floor beams with the same staples.
The subsequent stages of roof construction are carried out according to standard rules. Drawings for a mansard roof, clearly representing the structure, will introduce you in detail to the described principles of constructing a rafter system. Thanks to the production of joints by cutting half a tree, the strength and rigidity of the frame as a whole increases, which will eliminate the need to install additional struts.
The disadvantage of this method is that the finished modules are quite difficult to transport to the roof. To transfer the assembled blocks there without the use of lifting equipment, a minimum of 4 people will be required.
Board and nail rafter system
It is impractical to build a powerful attic over small country houses, but you still want to save space on a small plot. For owners of small buildings there is an excellent option - a lightweight board and nails layered structure. The method should appeal to those who love saving, because the construction does not use solid timber.
For the manufacture of each of the supporting elements, two boards are used, between which spacer sections of the bar are installed. The cavity formed by the bars explains why the system is lightweight compared to its solid counterparts. To ensure spatial rigidity, wind braces are installed connecting the supports to the rafter legs. The lathing, in turn, will make its contribution to strengthening the structure.
The popular way to develop a layout
For a successful work result, a project is very desirable. It is not a fact that the presented drawings with dimensions are suitable for furnishing a particular home. Typology in construction is now completely unwelcome. If there is no documentation at all, it is better to make at least a sketch of the future roof, not forgetting the height of the ceilings in the attic. Wherein:
- Proportions must be observed, because an attic that is too large can turn a small house into an awkward, mushroom-like structure.
- It must be remembered that the lower part of the attic roof is built using layered rafter legs, and they optically lower the overhang and overlap the upper part of the high windows. There will be no noticeable overhanging effect when constructing an attic according to the scheme with the rafters removed.
- Do not forget that the height of the attic room must ensure freedom of movement. It is this landmark that is required to correctly determine the height of the racks of the attic walls.
You can select the best roof proportions using the traditional template-layout method. According to it, bars or boards are laid out on a flat, spacious area, repeating the contour of the building in real size. By changing angles and moving components, you can achieve the optimal configuration. The elements need to be fixed with nails and immediately measure the lengths of beams, rafters, tie rods, and posts. The resulting dimensions will help in making templates.
The video will demonstrate the calculations and layout of the rafter system for an attic roof:
The basic options and diagrams for the installation of an attic truss structure that we have given will help you decide on the choice of the optimal type of truss structure.
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.
Definitions
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 |
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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.
Fastening 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 width of the rafter. 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 heads 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!