Maximum length of rafters. Rafter leg: design and installation. Features of installation of rafter legs
The content of the articleThe construction of a house always ends with the construction of a roof, which requires the installation of a rafter system. This design includes rafter legs, Mauerlat, tie-downs, struts, extensions, trusses, racks, sheathing and other elements that ensure the strength and rigidity of the entire system.
IN different designs A roof rafter leg can be called an ordinary rafter or a diagonal (sloping) rafter leg; it requires strength calculations. The calculation of the rafter system is based on the collection of permanent and temporary loads that will act on the roof.
Constant loads:
- the weight of all elements of the rafter structure;
- weight of vapor and waterproofing materials;
- weight roofing material;
- weight finishing materials ceiling, in the presence of attic rooms.
Live loads:
Characteristics of rafter legs
Based on the obtained load value, the calculation of the rafter leg, its length and cross section, depending on the selected material, type of roof and type of rafters - layered or hanging. Some types complex roofs may contain both.
And in hip roofs, except rafter legs Shortened rafters are also used, which are called sprigs and also require their own calculations. In addition, everyone needs calculation additional elements rafter system, such as ties, struts, racks and crossbars, since they bear a certain load transmitted from the rafters.
The length of the rafter leg depends, first of all, on the size of the building, as well as on the slope of the roof slopes, which is obtained from the chosen roof shape. Usually, they try to make the length of the rafters no more than 6 m, so all lumber that goes on sale has exactly this maximum length. But it happens that the size of the house requires longer rafters, in which case they are extended. Basically, long rafter legs are found in slanted (diagonal) rafters when constructing hip or half-hip roofs.
The choice of rafter leg section is influenced by multiple factors:
- permanent and temporary loads;
- type of roofing material;
- slope of slopes;
- roof type;
- house dimensions;
- climatic conditions;
- quality of material for making rafter legs.
Wood is used to construct the roof coniferous species. But, when choosing, you need to make sure that you don’t come across boards or beams with blue spots or a lot of large knots.
The moisture content of the wood should be no more than 20-22%, since wood that is too wet will change in size as it dries, and this in turn can lead to a violation of the roof’s tightness and other negative consequences.
It is best if the calculation of the rafter system is carried out by a specialist. Currently, there are enough companies that offer such services.
You can independently calculate the rafter legs, dimensions and length if you use ready-made calculators on the Internet. You just have to enter it into the program required sizes, and the program itself will produce the finished result of the section, length and pitch of the rafters.
In the construction of private residential buildings, as a rule, boards with a cross section of 50x150 mm are used in the manufacture of roof rafters of any configuration. The pitch of the rafter legs is approximately 1 meter, depending on the type of roofing material chosen, the amount of snow in winter time and roof slope.
So, for roofs with a slope of more than 45 degrees, the rafter pitch is selected within the range of 1.2-1.4 m, and for regions with large snow loads this distance will be 0.6-0.8 m.
You should also pay attention to the type of roofing material. The heaviest is considered natural tiles. The cross-section of the rafter legs will increase accordingly if the length of the rafter legs and their pitch are large.
Features of installation of rafter legs
Attaching the rafter legs to the mauerlat is the most crucial moment in the entire roof construction. The strength of the entire roof structure depends on the correct connection of the rafters and the Mauerlat.
There are two methods of fastening - sliding and rigid, each of which is suitable for a certain type of rafters - hanging or layered.
Rigid fastening eliminates any movement, rotation or bending of the rafters. This is achieved by making cuts on the rafter itself and then securing the rafter leg with the Mauerlat using metal staples, wire or long nails, as well as using metal corners.
A sliding joint, or as it is often called a “hinged” joint, can have two degrees of freedom. This connection is often used in construction wooden houses to give the roof freedom to gradually settle on the frame, which may shrink over several years. In this case, the connection of the rafter legs at the ridge is not made rigid. The rafter leg itself, when slidingly mated, is connected to the mauerlat by means of a gash and reinforced on the sides with two nails driven diagonally, in relation to each other, or by driving one nail from top to bottom into the rafter leg, penetrating into the mauerlat.
Other methods are the use of metal plates that have holes for nails or connecting the rafters and the mauerlat with metal staples.
During construction hip roof the diagonal rafter leg is often more than 6 meters in length, and therefore requires extension.
This is achieved by pairing two boards, which are used to construct conventional rafters. Diagonal rafters are always longer than ordinary rafters; moreover, they experience a load one and a half times greater than that of ordinary rafters, since the slanting legs also rest on them.
Any rafter system is formed from numerous rafters, for the creation of which timber or boards are used. Most often, boards are chosen due to their affordable cost, but their strength is considered not too high compared to timber.
Important! The durability of the roof and the safety of living in the house depend on the quality of the selected lumber.
Requirements for rafter boards
Roof rafters can withstand significant impacts from snow, wind and roofing covering, therefore, in the process of their creation, certain rules must be taken into account that they must comply with.
Important! During the formation of rafters, not only right choice their size and cross-section, but also the material used to create them.
It is considered optimal to choose timber for rafters , but this material is expensive, so boards are often purchased to reduce costs. Only high-quality wood is selected, and often the choice falls on pine needles or larch.
When searching for boards used to create rafters, the basic requirements for them are taken into account:
Boards are purchased only from trusted sellers who provide buyers with information about the product. To do this, there must be special accompanying documentation, and it contains information:
- type of wood used to make boards;
- name and number of the product standard;
- the name of the production organization involved in its production;
- number of units in one package;
- date of release of boards;
- dimensions of lumber, as well as its moisture content.
Important! Wood is natural material, therefore, various biological influences lead to its destruction, so it is important to choose the boards wisely, as well as protect them with special protective compounds.
Rafter boards must be treated before use different compositions:
- treatment with high-quality antiseptics that will prevent the material from rotting;
- impregnation with fire retardants that protect wood from fire;
- treatment with pest and insect repellents.
Only with the right choice of boards and after their high-quality processing is it possible to make rafters that not only high quality, but also resistance to various influences. 
What size should the rafters be?
Once the optimal board for the rafters has been selected, you can begin to create a special drawing and diagram of the future rafter system. To do this, determine the cross-section, length, width and other parameters of the rafters, which, after manufacturing, will be connected to each other in the correct sequence.
The size of the rafters can vary significantly, since this parameter is influenced by many factors. These include the dimensions of the house and roof, the chosen design of the rafter system, possible wind loads and other similar influences. It would be best to use the following recommendations:
- the minimum size is 50x150 mm;
- if significant spans are created, then the size is selected 150x150 or 250x100;
- Larger rafters are often used if it is planned to build a large building shopping pavilion or other large building.
Important! To accurately know the optimal dimensions of the rafters for the roof, you need to correctly calculate this indicator. 
For the calculation, it is important to determine what load will affect the roof as much as possible, which allows you to select the cross-section and other parameters of the rafters. Additionally, it is possible to use special standard values, but they do not take into account certain climatic conditions different regions, so experts prefer to carry out correct calculations, materials on the topic: , .
Correct determination of rafter dimensions
When determining the optimal sizes of rafters, it is important to consider how thick the board used to create the rafters should be.
Important! The thickness of the board has a direct impact on the strength of the products created from it.
For these purposes, it is advisable to use a board whose thickness varies from 4 to 6 cm. If a rafter system is being erected in small buildings intended for economic use, then to reduce costs it is allowed to use boards 3.5 cm thick. For a residential building, it is recommended to choose products , the thickness of which will not be less than 5 cm. 
When choosing the width of the board, you must take into account the length of the opening that overlaps the rafters. The longer the rafters should be, the wider the board used to create them:
- if the length of the rafters is approximately 6 m, then it is advisable to use a board whose width is approximately 15 cm;
- if the length of the legs exceeds 6 m, then the width of the board should be 18 cm;
- if you want to get an even longer rafter leg, then building up the elements is used, and the places where there is an overlap should be located next to the ridge part of the roof.
The cross section of the rafters is calculated depending on the specific optimal distance between them, and the length of the elements is also taken into account. During work, it is necessary to decide what constant loads will affect the roof from wind and snow. The weight of the created rafter structure, the angle of inclination of the slope, as well as the length of the opening that needs to be covered are taken into account. When calculating, it is additionally taken into account how wide the structure is.
Important! To facilitate calculations, it is recommended to use special computer programs, which are freely available on the Internet, and with their help they not only ensure quick results, but also guarantee the accuracy of the values.
After determining the cross-section of the rafters, you need to decide at what distance from each other they will be installed. directly related to their cross-section, so if these parameters are not correctly determined, this may negatively affect the reliability and durability of the roof. 
Important! It is allowed to reduce the cross-section of the rafters if special struts are used.
Basic rules for choosing boards
The rafter board must meet numerous requirements and conditions described above. To really choose quality material, it is recommended to use certain expert advice. These include:
Important! If material is purchased from high rate humidity, then it is not allowed to use it for the construction of a rafter system, since it will be fragile, and there is also a danger for living in a house with such a roof, since after a short period of time the geometry of the structure will be disrupted.
If several unsuitable elements are found in one batch of boards, it is not recommended to use them to create important parts of the roof, so they are used for additional elements.
Do you want to calculate the rafter system quickly, without studying theory and with reliable results? Take advantage online calculator Online!
Can you imagine a person without bones? In the same way, a pitched roof without a rafter system is more like a structure from a fairy tale about the three little pigs, which can easily be swept away by natural elements. Strong and reliable system rafters are the key to the durability of the roof structure. In order to design a high-quality rafter system, it is necessary to take into account and predict the main factors affecting the strength of the structure.
Take into account all roof bends, correction factors for uneven distribution snow on the surface, snow drift by the wind, the slope of the slopes, all aerodynamic coefficients, forces affecting structural elements roofs and so on - calculating all this as close as possible to the real situation, as well as taking into account all the loads and skillfully assembling their combinations is not an easy task.
If you want to understand it thoroughly, a list of useful literature is given at the end of the article. Of course, a strength of strength course for a complete understanding of the principles and impeccable calculation of the rafter system cannot be fit into one article, so we will present the main points for simplified versioncalculation.
Load classification
Loads on the rafter system are classified into:
1) Basic:
- permanent loads: the weight of the rafters and roof themselves,
- long-term loads- snow and temperature loads with a reduced design value (used when it is necessary to take into account the influence of load duration when testing endurance),
- variable short-term influence- snow and temperature effect at full calculated value.
2) Additional- wind pressure, weight of builders, ice loads.
3) Force majeure- explosions, seismic activity, fire, accidents.
To carry out the calculation of the rafter system, it is customary to calculate the maximum loads in order to then, based on the calculated values, determine the parameters of the elements of the rafter system that can withstand these loads.
Calculation of the rafter system pitched roofs produced according to two limit states:
a) The limit at which structural failure occurs. The maximum possible loads on the structural strength of the rafters should be less than the maximum permissible.
b) Limit state at which deflections and deformations occur. The resulting deflection of the system under load should be less than the maximum possible.
For more simple calculation Only the first method applies.
Calculation of snow loads on the roof
To count snow load use the following formula: Ms = Q x Ks x Kc
Q- weight of snow cover covering 1 m2 of flat horizontal surface roofs. Depends on the territory and is determined by the map in Figure No. X for the second limit state- calculation of deflection (when the house is located at the junction of two zones, a snow load with a large value is selected).
For strength calculations according to the first type, the load value is selected according to the area of residence on the map (the first digit in the indicated fraction is the numerator), or is taken from table No. 1:
The first value in the table is measured in kPa, in parentheses the desired converted value is in kg/m2.
Ks- correction factor for the roof slope angle.
- For roofs with steep slopes with an angle of more than 60 degrees, snow loads are not taken into account, Ks=0 (snow does not accumulate on steeply pitched roofs).
- For roofs with an angle from 25 to 60, the coefficient is 0.7.
- For others it is equal to 1.
The angle of the roof can be determined online roof calculator the appropriate type.
Kc- coefficient of wind removal of snow from roofs. Assuming a flat roof with a slope angle of 7-12 degrees in areas on the map with a wind speed of 4 m/s, Kc is taken = 0.85. The map shows zoning based on wind speed.
Drift factor Kc is not taken into account in areas with January temperatures warmer than -5 degrees, since an ice crust forms on the roof and snow does not blow off. The coefficient is not taken into account if the building is blocked from the wind by a taller neighboring building.
The snow falls unevenly. Often on the leeward side a so-called snow bag, especially at joints and kinks (valley). Therefore, if you want strong roof, keep the pitch of the rafters to a minimum in this place, and also pay close attention to the recommendations of the roofing material manufacturers - snow can break off the overhang if it is of the wrong size.
We remind you that the calculation given above is presented to your attention in a simplified form. For a more reliable calculation, we recommend multiplying the result by the load safety factor (for snow load = 1.4).
Calculation of wind loads on the rafter system
We've sorted out the snow pressure, now let's move on to calculating the wind influence.
Regardless of the angle of the slope, the wind has a strong impact on the roof: it tries to throw off a steeply pitched roof, more flat roof- lift from the leeward side.
To calculate the wind load, its horizontal direction is taken into account, while it blows bidirectionally: on the facade and on the roof slope. In the first case, the flow is divided into several - part goes down to the foundation, part of the flow tangentially from below vertically presses on the roof overhang, trying to lift it.
In the second case, acting on the roof slopes, the wind presses perpendicular to the slope, pressing it in; a vortex is also formed tangentially on the windward side, going around the ridge and turning into a lifting force on the leeward side, due to the difference in wind pressure on both sides.
To calculate the average wind load use the formula
Mv = Wo x Kv x Kc x strength factor,
Where Wo- wind pressure load determined from the map
Kv- wind pressure correction factor, depending on the height of the building and the terrain.
Kc- aerodynamic coefficient, depends on the geometry of the roof structure and wind direction. Values are negative for the leeward side, positive for the windward side
For pitched roof it is necessary to take the coefficient from the table for Ce1.
To simplify the calculation, it is easier to take the maximum value of C, equal to 0.8.
Calculation of own weight, roofing pie
To calculate permanent load you need to calculate the weight of the roof ( roofing pie-see Figure X below) per 1 m2, the resulting weight must be multiplied by a correction factor of 1.1 - the rafter system must withstand this load throughout its entire service life.
The weight of the roof consists of:
- the volume of wood (m3) used as sheathing is multiplied by the density of the wood (500 kg/m3)
- rafter system weight
- weight of 1m2 roofing material
- weight 1m2 of insulation weight
- weight of 1m2 of finishing material
- weight 1m2 of waterproofing.
All these parameters can be easily obtained by checking this data with the seller, or looking at the main characteristics on the label: m3, m2, density, thickness - perform simple arithmetic operations.
Example: for insulation with a density of 35 kg/m3, packed in a roll 10 cm or 0.1 m thick, 10 m long and 1.2 m wide, weight 1 m2 will be equal to (0.1 x 1.2 x 10) x 35 / (0.1 x 1.2) = 3.5 kg/m2. The weight of other materials can be calculated using the same principle, just do not forget to convert centimeters to meters.
More often the roof load per 1 m2 does not exceed 50 kg, therefore, when making calculations, this value is used, multiplied by 1.1, i.e. use 55 kg/m2, which itself is taken as a reserve.
More data can be taken from the table below:
10 - 15 kg/m² |
|
Ceramic tiles | 35 - 50kg/m² |
Cement-sand tiles | 40 - 50 kg/m² |
Bituminous shingles | 8 - 12 kg/m² |
Metal tiles | |
Corrugated sheet | |
Subfloor weight | 18 - 20 kg/m² |
Sheathing weight | 8 - 12 kg/m² |
Rafter system weight | 15 - 20 kg/m² |
Collecting loads
According to the simplified version, now it is necessary to add up all the loads found above by simple summation, we will get the final load in kilograms per 1 m2 of roof.
Calculation of the rafter system
After collecting the main loads, you can already determine the main parameters of the rafters.
falls on each rafter leg separately, convert kg/m2 to kg/m.We calculate using the formula: N = rafter spacing x Q, Where
N - uniform load on the rafter leg, kg/m
rafter pitch - distance between rafters, m
Q - final roof load calculated above, kg/m²
It is clear from the formula that by changing the distance between the rafters, you can regulate the uniform load on each rafter leg. Typically, the pitch of the rafters is in the range from 0.6 to 1.2 m. For a roof with insulation, when choosing a pitch, it is reasonable to focus on the parameters of the insulation sheet.
In general, when determining the installation pitch of the rafters, it is better to proceed from economic considerations: calculate all the options for the location of the rafters and choose the cheapest and optimal in terms of quantitative consumption of materials for the rafter structure.
- Calculation of the cross-section and thickness of the rafter leg
In the construction of private houses and cottages, when choosing the section and thickness of the rafters, they are guided by the table below (the cross section of the rafters is indicated in mm). The table contains average values for the territory of Russia, and also takes into account the sizes building materials presented on the market. IN general case, this table is enough to determine what cross-section of timber you need to purchase.
However, we should not forget that the dimensions of the rafter leg depend on the design of the rafter system, the quality of the material used, constant and variable loads exerted on the roof.
In practice, when building a private residential building, boards with a cross section of 50x150 mm (thickness x width) are most often used for rafters.
Independent calculation of rafter cross-section
As mentioned above, rafters are calculated based on maximum load and deflection. In the first case, the maximum bending moment is taken into account, in the second, the section of the rafter leg is checked for resistance to deflection over the longest section of the span. The formulas are quite complex, so we have chosen for you simplified version.
The section thickness (or height) is calculated using the formula:
a) If the roof angle< 30°, стропила рассматриваются как изгибаемые
H ≥ 8.6 x Lm x √(N / (B x Rben))
b) If the roof slope is > 30°, the rafters are flexural and compressive
H ≥ 9.5 x Lm x √(N / (B x Rben))
Designations:
H, cm- rafter height
Lm, m- working section of the longest rafter leg
N,
kg/m- distributed load on the rafter leg
B, cm- rafter width
Rizg, kg/cm²- bending resistance of wood
For pine and spruce Rizg depending on the type of wood is equal to:
It is important to check whether the deflection exceeds the permitted value.
The deflection of the rafters should be less L/200- length of the thing being checked longest span between supports in centimeters divided by 200.
This condition is true if the following inequality is satisfied:
3,125 xNx(Lm)³ / (BxH³) ≤ 1
N (kg/m) - distributed load on linear meter rafter leg
Lm (m) - working section of the rafter leg of maximum length
B (cm) - section width
H (cm) - section height
If the value is greater than one, it is necessary to increase the rafter parameters B or H.
Sources used:
- SNiP 2.01.07-85 Loads and impacts with latest changes 2008
- SNiP II-26-76 “Roofs”
- SNiP II-25-80 “Wooden structures”
- SNiP 3.04.01-87 “Insulating and finishing coatings”
- A.A. Savelyev “Rafter systems” 2000
- K-G. Götz, Dieter Hoor, Karl Möhler, Julius Natterer “Atlas of wooden structures”
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The rafter system is a structure that provides the strength of the roof and serves as the basis for laying roofing material. It is shown in the photo.
The roof is a load-bearing structure that performs following functions:
- makes the building beautiful appearance;
- takes over external loads;
- protects the attic from the outside world;
- transfers the load from the sheathing and the material on it to the building walls and internal supports.
The main elements of the roof include sheathing, rafters and mauerlat. The supporting structure also includes additional fastening elements - crossbars, racks, rafter struts, spacers, etc. The reliability and strength of the roof is most influenced by the rafter system. Rafters are the main load-bearing part of the roof. The rafter system bears the weight of not only the roof covering, but also the snow cover and wind pressure. It must withstand all these impacts, so the calculation is made taking into account the type of roofing material and the climatic characteristics of the region.
Rafter system design
Connecting the rafters to each other gives rigidity roof frame, and the result is a durable truss structure. The load on the rafters can be quite significant, for example, during strong winds, so the frame is tightly tied to the frame of the building.
In the construction of private houses and cottages, wooden rafter systems are usually used, which are easy to manufacture and install. If mistakes were made during the construction of the walls, these products can be easily processed: shortened, extended, hemmed, etc.
During installation, fasteners of the rafter system are used: bolts, screws, clamps, nails, staples. They are also used to strengthen the carrier roofing structure. Interconnected roof elements create roof truss, which is based on triangles, which are the most rigid geometric figure.
When choosing a material for the manufacture of a rafter system, it is necessary to take into account the structural and architectural nuances of the project. Do not forget about antiseptic and fireproof impregnation for them, as this affects the durability of the roof.
The system consists of rafter legs. The rafters are installed at the angle of the roof slopes. The lower sections of the rafter legs rest on external walls using a Mauerlat that promotes uniform load distribution. The upper ends of the rafters rest on a beam under the ridge or on intermediate supports. Using a system of racks, the load is transferred to load-bearing internal walls.
Types of rafters
The design transmits a significant horizontal bursting force to the walls. In order to reduce the load, a stretcher is used to connect the rafter legs. It is done either at the base of the rafters or at a greater height. The brace at the base of the rafters is also a floor beam - this is important when creating mansard roofs. When increasing the height of the brace, it is necessary to increase its power and ensure that it is securely attached to the rafters.
Part layered rafters includes: rafter leg, mauerlat, headstock, strut, tightening. This type of rafters is installed in buildings that have a middle load-bearing wall or intermediate supports in the form of pillars. Elements of this design work only in bending, performing the function of headstocks. The weight of the layered rafter system is less, and less materials are also required, so it is cheaper than a hanging system.
Installation of a layered system is done if the supports are no more than 6.5 meters from each other. If there is an additional support, the rafters sometimes cover a width of 12 meters, and if there are two supports, up to 15 meters.
Rafter legs most often rest not on the walls of the building, but on a special beam - a mauerlat. This element can be located along the entire length of the house or placed only under the rafters. If the structures are wooden, for the mauerlat they take a log or timber, which is the upper crown of the log house.
At brickwork wall mauerlat is installed flush with inner surface walls are timber fenced from the outside with a masonry projection. A layer of waterproofing is laid between this element and the brick - for example, you can put roofing felt in two layers.
If the width of the rafters is small, they may sag over time. To prevent this from happening, use a grid consisting of a rack, a crossbar and struts. At the top of the structure, a purlin is laid that connects the rafters or trusses. This is done regardless of the type of roof. Subsequently, the roof ridge is made on this run. In places where there are no load-bearing walls, the heels of the rafters rest against the side purlins - longitudinal beams significant power. The dimensions of these parts depend on the expected load.
When building private houses, rafters made of logs are used - they are lighter. To create roofs on multi-storey residential buildings and industrial buildings metal rafters are used.
Installation of rafter systems
The slope angles of the slopes are selected based on the type of building and purpose attic space. The amount of slope is also influenced by the material chosen to create the roofing.
If roll products are to be laid, the angle of inclination should be 8-18 degrees. For tiles, the required angle is 30-60 degrees, for roofing steel or asbestos cement sheets - 14-60 degrees.
The installation of the rafter system begins after the construction of the load-bearing walls of the house (more details: ""). The design of the rafters of a log house differs significantly from systems for houses made of foam aerated concrete, brick, frame wooden or panel houses. The differences are significant even with the same shape, type and type of roof. As for how to treat the rafter system, it is necessary to use antiseptic and fire-fighting agents so that the roof lasts a long time.
Main elements load-bearing structure- and the sheathing. The roof is outer part roof, which is laid on a supporting structure consisting of sheathing and rafters.
To produce rafters, material of a certain size is taken. Thus, the thickness of the rafters (section) is most often 150x50 and 200x50 millimeters. For lathing, beams and boards measuring 50x50 and 150x25 millimeters are usually used. The distance between the rafter legs is on average 90 centimeters. If the roof slope is more than 45 degrees, this step is increased to 100-130 centimeters, and if a huge amount of snow falls in the region, then it is reduced to 60-80 centimeters.
To produce more accurate calculations Regarding the gap between the construction legs, you need to take into account their cross-section, the step between the supports (struts, ridge purlin, uprights), and the type of roofing material.
The floating rafter system is attached using special brackets, allowing the rafters to “sit down” along with the shrinkage of the gables and not hang over the ridge log.
In mountainous areas, the chalet rafter system is popular (more details: ""). A feature of this design is the significant protrusion of the roof beyond the load-bearing walls. Sometimes such a protrusion reaches two to three meters, and the angle of the roof slope is small. Snow does not linger on such a roof, so it lasts a long time. But the best option is a roof overhang of 1-1.5 meters (read also: “Characteristics and design of roofs: rafter systems”).
The installation of the rafter system must be carried out in strict compliance with all requirements. If you have no experience in construction, it is better to entrust the construction of the roof to specialists, since this is not an easy task, and the slightest mistakes may lead to its collapse.
The construction of a rafter system may seem quite simple, but it requires precise mathematical calculations. Correct sizes elements of the supporting structure will not allow the roof to be fragile and will save the owner of the house from excessive spending.
Calculation of rafter system parameters
The rafter system is formed not only by the rafter legs. The design includes a Mauerlat, racks, struts and other elements, the dimensions of which are strictly standardized. The fact is that the components of the rafter system are supposed to withstand and distribute certain loads.
The elements of the rafter system of a simple gable roof are rafters, purlin (ridge board), racks, beds, mauerlat and rafter legs (struts)
This is a structure of four beams connecting brick, concrete or metal walls houses with a wooden load-bearing roof structure.
The Mauerlat beam should occupy 1/3 of the space at the top of the wall. The optimal cross-section of this lumber is 10x15 cm. But there are others suitable options, for example, 10x10 or 15x15 cm.
The main thing is not to take beams less than 10 cm wide to create a mauerlat, as they will seriously let you down in terms of strength. But lumber with a width of more than 25 cm will not raise doubts about its reliability, but it will put pressure on the house so that it will soon begin to collapse.
The Mauerlat must be narrower than the walls, otherwise it will put excessive pressure on the walls
The ideal length of the beam for the base under the rafter system is equal to the length of the wall. It is not always possible to meet this condition, so the Mauerlat can also be constructed from segments that are completely or at least approximately equal in length.
The bed acts as an element of the rafter system, which is in a lying position and serves as the basis for the rack (headstock) of the supporting roof structure.
A beam of the same cross-section as the Mauerlat is usually taken as a beam. That is optimal size horizontal element on the inside load-bearing wall- 10x10 or 15x15 cm.
The size of the bench is no different from the Mauerlat
Ridge beam
Due to the size of the ridge beam, into which the rafters rest at the upper end, the weight of the roof should not exceed acceptable limits. This means that for the ridge it is necessary to take a beam that is quite strong, but not heavy, so that other elements of the supporting structure of the roof do not bend under its pressure.
The most suitable pine lumber for the roof ridge is a beam with a section of 10x10 cm or 20x20 cm, like the racks of the structure.
The ridge purlin should not be thicker than the rack of the rafter system
filly
A fillet is a board that extends the rafter if it is unacceptably short.
When using fillies, the rafter legs are cut flush with outer wall. And the boards that extend them are selected in such a way that they form the necessary overhang of the roof and are no thicker than the rafters themselves.
An extra 30–50 cm must be added to the length of the filly, which will be used to combine the rafters with the additional board and make the connection between the frame and the roof overhang as strong as possible.
The thickness of the filly is inferior to the rafter leg
Racks
The post is the same as the center support. The height of a vertical beam in a rafter system is usually found using the formula h = b 1xtgα – 0.05. h is the height of the rack, b 1 is half the width of the house, tgα is the tangent of the angle between the rafters and the mauerlat, and 0.05 is the approximate height of the ridge beam in meters.
The main requirement for racks is stability, so they choose beams as thick as a bench
A strut is an element of the rafter system, which is mounted at an angle of at least 45° (relative to the horizontal cut of the walls) with one end on the rafter, and the other on a tie laid in the direction from one wall of the house to the other, close to the vertical post.
The length of the strut is determined by the cosine theorem, that is, by the formulaa² =b² +c² - 2xbxcxcosα for a plane triangle. a denotes the length of the brace, b is part of the length of the rafter, c is half the length of the house, and α is the angle opposite to side a.
The length of the strut depends on the length of the rafters and the house
The width and thickness of the struts should be identical to the same dimensions of the rafter leg. This will greatly facilitate the task of securing the element to the roof frame.
The tie is installed at the base of the rafter system and plays the role of a floor beam. The length of this element is determined by the length of the building, and its cross-section does not differ from the parameter of the rafter legs.
The tightening can also be called a ceiling joist
A sliding support or element of the rafter system, allowing it to adapt to changes in configuration, must be characterized by the following parameters:
- length - from 10 to 48 cm;
- height - 9 cm;
- width - 3–4 cm.
Size sliding support should allow the rafters to be well fixed to the roof base
Boards or beams for rafters
The size of the boards that will become the rafters of the roof with symmetrical slopes is not difficult to determine. The formula from the Pythagorean theorem will help with this: c² = a²+ b², where c acts as the required length of the rafter leg, a denotes the height from the base of the roof to the ridge beam, and b is ½ part of the width of the building.
The parameters of rafters that differ in asymmetry are also recognized using the Pythagorean formula. However, the indicator b in this case will no longer be half the width of the house. This value will have to be measured separately for each slope.
Using the Pythagorean formula, you can calculate both the length of the rafters and the height of the rack
Rafters are usually boards with a thickness of 4 to 6 cm. Minimum parameter Ideal for commercial buildings, such as garages. And the rafter system of ordinary private houses is created from boards 5 or 6 cm thick. The average width of the main elements of the supporting roof structure is 10–15 cm.
With a large pitch and a significant length, the cross-section of the rafters will certainly be increased. Let’s say that when the distance between the legs of the supporting roof structure reaches 2 m, a section of 10 × 10 cm is chosen for the rafters.
The length of the rafters is influenced by the degree of roof slope and the length of the space between the walls located opposite each other. As the roof slope increases, the length of the rafter leg increases, as does its cross-section.
The size of the rafters is determined by the size of the gap between them
Table: correspondence of the length of the rafter leg to its thickness and pitch
| Rafter leg length (m) | Space from one rafter to another (m) | |||||||
| 1,1 | 1,4 | 1,75 | 2,13 | |||||
| Rafter thickness (mm) | ||||||||
| Bruschi | Logs | Bruschi | Logs | Bruschi | Logs | Bruschi | Logs | |
| Until 3 | 80×100 | Ø100 | 80×130 | Ø130 | 90×100 | Ø150 | 90×160 | Ø160 |
| From 3 to 3.6 | 80×130 | Ø130 | 80×160 | Ø160 | 80×180 | Ø180 | 90×180 | Ø180 |
| From 3.6 to 4.3 | 80×160 | Ø160 | 80×180 | Ø180 | 80×180 | Ø180 | 100×200 | Ø180 |
| From 4.3 to 5 | 80×180 | Ø180 | 80×200 | Ø200 | 100×200 | Ø200 | - | - |
| From 5 to 5.8 | 80×200 | Ø200 | 100×200 | Ø220 | - | - | - | - |
| From 5.8 to 6.3 | 100×200 | Ø200 | 120×220 | Ø240 | - | - | - | - |
Rafter angle
The value of the rafter angle is determined by the formula α = H / L, where α is the angle of inclination of the roof, H is the height of the ridge beam, and L is half the span between the opposite walls of the house. The resulting value is converted into percentages according to the table.
How the rafters will be inclined depends on two indicators - the height of the ridge and the width of the house
Table: determining the rafter angle as a percentage
Video: calculating the size of rafter legs
For each element of the rafter system, there are averaged size data. You can use them as a guide, but it is better to calculate the parameters of the racks, struts and other components of the supporting structure of the roof in special programs on a computer or using complex geometric formulas.
