The design of a gable roof for private houses is chosen most often, as it is the best option for ease of installation and ease of use, for strength and reliability, as well as for style. A gable roof is also called a gable roof and has two slopes that can be located relative to each other at the same or different angles, that is, it can be an isosceles or scalene triangle. The latter option can be increasingly found among new buildings, as it is becoming a fashionable style solution. And, in addition to originality, such a roof has some positive features that simplify its operation.
This type of truss system can be used for all types of roofing materials, but the crate fixed to the rafters has its own parameters for each of the coatings.
Gable roof: the truss system under which has its own characteristics must be carefully studied and calculated ahead of time, before buying materials and carrying out its installation. Let's try to understand these issues.
Varieties of designs of gable roofs
First you need to figure out what designs of gable roofs are, as you will have to choose the most suitable for a particular building.
- Simple equilateral gable construction
This version of the gable design can be called the traditional and most used, as it is easy to install and maintain, and is also durable and reliable.
Symmetry in this system helps to achieve uniform load on the Mauerlat and load-bearing walls. With the right choice of the cross-section of the beam for arranging the rafter system and the Mauerlat, these parts will provide the margin of safety necessary for long-term operation of the roof. Properly installed racks, struts and puffs will give additional reliability to the structure.
The disadvantage of this system for arranging rooms inside the attic space is that after installing the walls and ceiling, a large area is occupied by blind zones in the corner areas of the structure, which remain unused.
- Simple asymmetrical gable design
The asymmetric gable design differs from the traditional system in that its slopes are located at different angles. So, one of them is usually more than 45 degrees, which allows you to increase the useful area of \u200b\u200bthe attic space, in which it is quite possible to equip a living space, of course, with proper insulation.
Another important advantage of such a design can be a smaller slope, which is recommended to be planned on the leeward side of the building, where a large amount of snow always collects on the roof. Small in size, but with a large slope, the slope will not hold large snowdrifts on its surface.
The disadvantage of an asymmetric design is its more complex calculation to achieve an even distribution of the load on the walls of the house.
- Bent gable construction
This gable rafter system can be called a rarity, although due to this arrangement of slopes in the attic space, a fairly large room is formed that can be used as a residential or utility room.
In addition to such a rafter system, an attic version of the installation of rafters can also be attributed to a broken gable structure.
Both slopes are "broken" - a clear gain in the usable area of the attic 3 - Rack mounted on a bed.
4 - Rafter.
5 - Lathing.
The layered system differs from the hanging system in that it is installed on a structure that has internal capital partitions. The main internal walls are used to fix a bed on them, on which racks are installed, supporting the ridge run, with which the upper ends of the rafter legs are fastened. Then the batten boards are fixed to the rafters.
This design is more popular than hanging, as it is reliable and easy to install.
fasteners for rafters
Hanging truss system
The scheme of the hanging truss system looks like the one shown in the illustration, and includes the following main elements:
1 - Bearing walls.
2 - Mauerlat.
3 - Rafter.
4 - Lathing.
5 - Puff (bolt).
The hanging rafter system is mounted on two external load-bearing walls, on which the Mauerlat is preliminarily fixed. This roof option can only be used if the distance between the load-bearing walls is no more than 7000 mm, since apart from them there is no additional support for the roof truss structure. Such a system is usually equipped with puffs, reinforced with slopes - these elements will remove part of the load from the walls of the building.
In addition to layered and hanging systems, there are combined options that include individual elements of both designs.
When the truss system is selected, it is recommended that before buying materials, draw up a detailed drawing of the roof with dimensions - it will be easier to calculate the amount of everything you need and the amount for their purchase. In addition, such a scheme will greatly help to carry out installation work. But in order to draw up a drawing, you will have to spend some
How to calculate the parameters of a gable truss system
It is very important to correctly calculate the parameters of the elements for installation work. To do this, it is first recommended to make a list of everything you need and calculate step by step. All calculations should be made with a margin of 10 ÷ 15%, avoiding excessive savings, which will be at the expense of the quality and strength of the structure.
If you decide to do this part of the work yourself, you must definitely check with the technical documents developed by specialists, for example, posted in SNiP.
The main areas of calculation will be three interrelated quantities - the steepness of the slope, the height of the ridge above the ceiling and the length of the rafter legs. Further, having linear parameters, it will be necessary to determine the cross section of the material for the rafters. But it already, in turn, depends on the loads that fall on the rafter system.
Loads on the truss system
Loads on the truss system are divided into three categories:
- Constant loads. This category includes those that will constantly keep the truss system in tension - insulation, if provided, roofing, windproof, hydro- and vapor barrier film, fasteners, finishing materials for the inside of the attic. The weight of all the elements and materials required for the roofing "pie" is summed up, and on average, the optimal value should be 40-45 kg / m². It is advisable to calculate the materials in such a way that the weight of 1 m² does not exceed 50 kg/m², especially if a hanging rafter roof system is used.
- short term loads. Such loads occur periodically and can have different effects on the structure. These include the following impacts:
— the weight of people during repair work;
— climatic temperature effects;
— possible ice loads.
These external loads are highly dependent on the characteristics of the region of construction. In addition, their value directly depends on the steepness of the slopes. So, for example, on gentle slopes, the snow load will play a decisive role. With an increase in the steepness of the roof, the influence of snow pressure decreases, but the dependence on wind action increases. On slopes with a steepness of over 60 degrees, the snow load is completely written off, but the windage of the roof increases significantly, and the wind becomes the dominant external influence.
The data for calculations can be found in SNiP 2.01.07-85* “Loads and impacts” in the sections “Snow loads” and “Wind loads”. At the same time, it is necessary to take into account not only the region where the house is located, but also the place of its construction - a lowland or a hill, a separate building or surrounded by other buildings.
A convenient algorithm for calculating loads will be given below.
- Special loads. This category includes factors such as seismic effects, hurricane winds, deformation processes due to subsidence of the soil, which are usually called force majeure. It is impossible to foresee everything, and in order for the roof to withstand all these tests, it is recommended to lay an additional margin of safety when purchasing materials and installing the structure.
If the roof is to be installed on an old building, the bearing capacity of the foundation and walls must be calculated, as the new roof may have more weight than the old one. Only specialists can professionally carry out such calculations, but such calculations must be made without fail, otherwise you will have to not only change the roof, but also repair the entire building. At the same time, specialists will need to provide a roof project, in which all its parameters will be indicated.
The angle of inclination of the slopes of the truss system and the height of the ridge
The angle of the roof slopes is determined by the choice, since each of them requires a personal calculation. Often the manufacturer of a particular brand of coating himself gives the necessary recommendations, but if we talk about general requirements, for example, for our case - metal tiles, then for it the slope angle should be at least 20 degrees.
Increasing the slope angle will significantly expand the attic space, but the construction of such a roof will require more building materials and, of course, the cost of construction will increase significantly.
So, any gable truss system, whether symmetrical or not, can be represented as a triangle.
Its tops:
- dot "a"- this is the external point of intersection. Angle "A" is adjacent to this vertex, which determines the steepness of the roof slope.
- dot « b"- the top of the skate.
- dot "with"- crossing a plumb line from the ridge with an overlap or just the upper level of the wall.
Known initial value - « D" the length of the base of the triangle. For a symmetrical roof, this is half the span. For asymmetric options - it may be different, it is easy to determine.
"N"- the height of the ridge above the base (overlap);
« L"- the length of the rafter leg, which, if desired, can be increased by an amount "m" for the formation of a cornice overhang.
According to the well-known trigonometric relations:
H =D×tgA
Thus, it is possible either to determine the height of the ridge from the given value of angle A, or, conversely, by planning in advance a certain height of the attic, determine the slope of the slope.
All this makes it easy to make the calculator below. By changing the value of the angle " BUT" you can come to the optimal value of the height " H".
The rafter is one of the individual elements of the supporting structure of the roof, with the help of which its slope is formed. In the design, the rafter is attached with its upper end to the ridge, and the lower end rests on the Mauerlat, with a straight slope, or on a rack (with a sloping roof). Rafters are made from edged boards with a section of 150x60 mm or a beam of 150x100 mm. The distance between the rafters at which they are located from each other after installation is called a step, which can be in the range of 600 - 1200 mm.
Rafter systems their types
Depending on the type of installation of rafter legs, systems are divided into 3 types:
What may determine the installation step of the rafters
The step of the roof rafters depends on such factors, as:
Taking into account all these indicators when calculating, you can get a solid and reliable roof structure, which for a long time will be a high-quality support for the roof.
It is the calculation, subject to the basics of regulatory loads, allows you to choose the right distance between the rafters. As a rule, the standard values can be determined from the Sheaves, and the calculated ones are derived on their basis separately for each structure.
At the same time, it is customary to use rafter legs with a section of 150x50 mm with an optimal distance between the rafters within 0.8 - 1.8 m / p. but it should be remembered that when the slope of the roof slope changes, the distance between the rafters also changes.
Calculation of the truss structure
The strength and stability of the entire roof directly depends on the quality of the calculation for the selection of the section of the rafter legs and the distance between them. Regardless of which type of coating is chosen: profiled sheet, slate or metal tile, the initial calculation should remain unchanged. After all, each calculation is based on structural strength under mechanical loads, and other effects are no longer important.
When calculating the choice of the optimal distance between the rafters for the roof, are based on the following parameters:
- Type of roofing.
- View of the truss system and design features of the roof.
- feasibility and economy.
For a small house, the design calculation for the roof can be done by yourself. True, the method of calculating the elements for the roof truss system is rather complicated, and it is recommended to do it using programs specially created for this. Especially if you need to calculate a complex sloping roof with a large area, most likely, you will not be able to do without a specialist. At the same time, the calculation of the step of the rafters will also be based on the standards - the minimum step is 0.6 m, the maximum step is 1.2 m.
Method of calculation
Produced in this way.
- The length of the building is measured along the eaves.
- The resulting length is divided by the estimated distance between the rafters. For example, the estimated pitch of the rafters will be 0.8 m / p. (mean distance is considered to be 950 mm).
- After performing this action, one should be added to the result obtained, and the resulting amount should be rounded. Thus, the required number of rafters is obtained on one side of the slope. After that, the length of the building must be divided by the number of rafters received, and as a result, the exact axial pitch of the rafters is determined.
Example, - building length 26.5 m / p. the distance between the rafters is supposed to be 0.8 m. So:
- 26.5 m ˸ 0.8 m = 33.1 33.1+1 = 34.1. As a result, after rounding, it turns out that 34 rafters need to be installed on one slope.
26.5 m/s ˸ 34 art. = 0.77 m - this value is the distance between the rafters along their central axes.
But this is only a general calculation method, which does not take into account the peculiarity of the planned roofing. Therefore, it is recommended by experts to calculate the step between the rafters for a certain roofing material and insulation, for example, for the most popular roofing metal tile today.
Roof structure for metal roofing
metal tile visually imitates ceramic roof tiles. It is made from thin sheet steel by cold stamping. Due to the polymer coating, it has a high resistance to weathering and a rather attractive visual appearance, it is not afraid of sudden temperature changes.
The advantage of metal tiles
Often used in the construction of most mansard roofs.
The cross section of wooden rafters under a metal roof is usually standard 150-50 mm, but the distance between them can be from 600 mm, but not exceed 900 mm (depending on their angle of inclination, which can vary between 22 - 45 degrees). This limitation of the pitch of the rafters is due to the fact that the crate under the metal tile is mounted with a distance from each other within 300 mm. The standard timber, which is used for the crate, has a section of 30x50 mm or 50x50 mm. And this means that each rafter is subjected to additional load.
The stability of the roof structure against various mechanical loads depends on four factors:
An important factor in the calculation of the truss structure, is the estimated maximum load on the roof, the formation of which includes:
- The weight of the entire truss structure.
- The weight of the crate under the cover.
- Weight of insulation and roofing.
- Snow load (determined according to a special, unique for each region, reference book).
- Wind load (also according to a special guide for the region).
- The weight of a person with a tool (repair work, estimated weight - 175 kg / m²).
When installing the rafter system, the distance of the rafter legs should not exceed an average value of 0.9 m / p. except for certain, foreseen in advance cases.
If, when calculating the loads, any inaccuracy is made in the selection of material for the roof and the location of the rafters, it may occur its deformation and destruction of the roofing. A reliable roof structure will be guaranteed only with the correct calculation of the cross-section of the rafters and their installation step.
It should be remembered. There is no universal value for the calculation of truss structures. Each building requires an individual calculation.
Rafters for a shed roof
Shed roofs can often be found on small outbuildings. They can also be used in private residential buildings, but quite rarely. In such cases, the angle of inclination of the roof is rather small, and with such an arrangement of the load-bearing floor beams, they are under a lot of pressure, especially in winter.
Therefore, for a shed roof, load-bearing floor beams are installed from a beam of large section from 60x150 to 100x220 mm, depending on the width of the overlapped span. At the same time, the distance at which the rafters are laid from each other should be within 400 - 800 mm, depending on the angle of the roof.
For pitched roof no complex rafter construction required, they can simply be laid on the walls, without even using a Mauerlat. In regions where winters are quite snowy with a lot of snow, it is recommended to build a roof slope at a maximum angle of 35 ⁰ and place the roof in the “wind”. This reduces windage and leads to its self-cleaning.
Gable roof
Represents a structure assembled from rafters interconnected in the form of a triangle. The upper part, which rests on the ridge, and the lower part on the Mauerlats, located parallel to each other on opposite walls. In simple words, this is a roof consisting of two opposite slopes, connected by a ridge.
Double pitched roof structure, depending on its area, is mounted from separate rigid elements that enhance the strength of the roof. These include racks that support rafters, puffs that connect rafters to each other, jibs, girders, support beams, etc.
For a gable roof, often the rafters are mounted taking into account the insulation in increments of 0.9 - 1.2 m / p. In this case, the strength of the mounted structure will be the highest if the resulting triangle is isosceles. In regions with strong winds, it is recommended to mount the rafters with a slope of approximately 20⁰, and in snowy areas, the optimal angle should be 45⁰.
Although the gable roof is considered classic, it has several alternative "related" types.
mansard roof
For mansard roof, for the calculated parameter for determining the step between the rafters and their number, a load is taken in the range of 40-60 kg for every 1 m / p. rafters, and the maximum deflection from its length is 1/250. Usually, with a properly selected section, this distance along the centers of the rafters is, as for a gable roof, 0.6 - 1.2 m / p.
It should be noted that the average load on the attic is approximately 200 kg / m2. So, with the standard calculation of the section of the rafters, it is recommended to add a small percentage of the margin of safety.
hip roof
Among all roof structures, it is considered one of the most difficult . It's almost a hipped roof., while the rafters of the end slopes are attached with their upper ends to the corner bowstrings, and not to the ridge. Therefore, certain requirements may be imposed on this type of roof during construction. In this case, the rafters are installed similarly to a gable roof at a distance of 60 cm - 1.2 m / p.
Attic rooms under such a roof are made in rare cases, since its slopes “eat up” some area of the attic room, especially in height.
The distance between the rafters is one of the key parameters that affect the strength of the structure. Competent calculation of the installation step of the rafters allows you to build a roof that is resistant to high operational loads.
Roof loads and rafter system calculation
The development of a single-pitched or dual-pitched roof project begins with the choice of the type of truss system, the angle of inclination of the slopes (roof height) and materials for the construction of the structure. The calculation of the distance between the rafter legs is carried out taking into account the loads that the roof will experience during operation. Permanent loads include:
- the weight of the materials from which the truss system is made;
- roofing weight;
- weight of roofing cake materials (waterproofing, vapor barrier, insulation);
- the weight of the finishing elements of a residential attic or attic.
In addition to permanent loads, the roof also experiences temporary ones, which include:
A step is the distance between the rafters of one slope. When calculating a shed, gable or complex roof, the following scheme is usually followed:
- the length of the future roof slope is measured;
- the resulting value is divided by the optimal numerical value of the pitch of the rafters;
- one is added to the obtained value, the result is rounded off;
- the length of the roof slope is divided by the rounded result.
The final result will determine how far the rafter legs should be placed. The determination of the step cannot be extremely accurate, since it is necessary to take into account a number of additional factors, including the width of the insulation, the particular installation of the crate for various types of roofing material. If a roof with a chimney is being designed, the pitch can be adjusted according to its location so that it is not necessary to remove part of the rafter in the future and install a supporting structure, such as a special rafter system.
The distance between the rafters under the slate
Slate is a traditional roofing material. Its advantages include such characteristics as resistance to external influences (excluding mechanical ones) and low cost. Slate allows you to create a roofing, the repair of which can be reduced to the replacement of individual elements. Slate is heavy and requires the installation of a sufficiently powerful truss system. The calculation of the distance at which it is necessary to put the rafters under the slate is carried out taking into account the cross section of the beam for the manufacture of rafter legs.
The optimal solution is to install the system under the slate, in which the gap between the rafters must be at least 800 mm. In order for the slate structure to withstand not only the weight of the material, but also increased external loads, the crate is made of timber or boards with a cross section of at least 30 mm. When calculating the truss system for slate, it should be read that this material has rather large restrictions on the choice of the angle of inclination of the slopes.
Rafters for metal roofing
Metal tiles are actively used as a practical and aesthetic roofing material when arranging a shed, gable, hip or complex roof. The frame for the metal tile is built according to standard principles. To calculate at what distance it is better to place the rafters, it is necessary to take into account the loads and the angle of the roof. The metal tile is characterized by relatively low weight, so it can serve as a replacement for the old roofing of slate or ceramic tiles. In this case, the rafters do not need to be strengthened or the pitch of their installation changed.
The standard step of rafters for metal tiles is 600-900 mm. The cross section of the elements can be 50-150 mm - this is enough to create a reliable frame for a metal tile. But if it is planned to use insulation, the layer of which in areas with low winter temperatures should be 200 mm, it is recommended to use a 200x50 beam for rafters under metal tiles so as not to mount an additional system that holds the insulation. The gap between the rafters under the metal tile is best adjusted to the width of the sheet or roll heat insulator.
Decking: rafters and crate
The professional flooring belongs to easy and convenient roofing materials in application. Galvanized or coated with a decorative and protective layer, corrugated board can be used both for mounting a shed roof of a utility room or garage, and for a gable roof of a residential building. How to calculate the distance required to install the rafters under the corrugated board?
To ensure the necessary rigidity of the structure, it is enough to install the rafters under the corrugated board in increments of 600-900 mm. In this case, it is necessary to pay attention to the angle of the roof. The calculation shows that with high external loads, it is better to lay the corrugated board on the system with a minimum step. But if the distance between the rafters under the corrugated board is close to the maximum, and the angle of the roof slope is small, then the structure is strengthened by using a more frequent crate. In this case, the step of the crate under the corrugated board should be about 50 mm, the width of the elements should be at least 100 mm.
Rafter system for soft roofing
Soft roofing includes bituminous and bitumen-polymer roll materials, roofing membranes, as well as soft tiles. A soft roof is characterized by a relatively low weight and no need to install a massive truss structure.
The minimum distance between the rafters is 600 mm, the maximum is 1500 mm. When installing a support under a soft roof, it is important to take into account the angle of inclination of the slopes: the smaller the angle, the smaller the distance between the supports for a continuous crate should be. The choice of step is also influenced by the thickness of the material for the crate - the thicker the sheet of plywood or OSB, the greater the installation step of the rafter legs can be.
Ondulin: calculation of rafters
Ondulin (bituminous slate) is laid on a flat, continuous crate made of sheet material. This allows the roofing to successfully resist wind and snow loads. The crate under the ondulin rests on the rafters, which should be located in increments of 600 - 1000 mm, depending on the angle of inclination of the gable or shed roof.
Rafters for ondulin are made of timber with a section of 200 × 50 mm. When choosing at what distance to put the rafter legs for the structure under the ondulin, it is recommended to take into account the width of the insulation material in order to simplify its installation. This calculation allows you to reduce the financial costs of arranging the roof.
Sandwich panel roof
Sandwich roofing is most often erected on sip-panel houses or hangar-type buildings. A feature of the sandwich is its bending rigidity, which makes it possible to dispense with the installation of traditional rafters. Small spans from the ridge of a gable roof to the upper part of the wall (or the distance between the supporting structures of a shed roof) allow the installation of a sandwich without additional supports.
If the span exceeds 4 meters, additional girders must be installed. To install a sandwich roof on a residential building, a traditional rafter system is often installed, but in this case the rafters can be positioned with a large step - they serve as a support for the girders. The distance between the rafter legs is selected based on the length of the available material for the runs and the length of the load-bearing walls. The technical parameters of the sandwich allow the roof to withstand high operating loads.
Polycarbonate: erection of the supporting structure
Polycarbonate has recently been actively used as a roofing material. First of all, polycarbonate is in demand in the construction of gazebos, sheds, winter gardens. The crate and truss system for polycarbonate can be made of wood or metal.
The construction of the roof truss system and the subsequent roofing are the most important stages in any construction. This matter is very complicated, associated with comprehensive preparation, which includes the calculation of the main elements of the system and the acquisition of materials of the desired section. Not every novice builder will be able to design and sanitize a complex structure.
However, often in the construction of adjoining buildings, utility or auxiliary buildings, garages, sheds, gazebos and other objects, the special complexity of the roof is not required at all - the simplicity of design, the minimum amount of material costs and the speed of work, which are quite feasible, come first. for independent performance. It is in such situations that the rafter system becomes a kind of "lifesaver"
In this publication, the main focus is on the calculations of a shed roof structure. In addition, the most typical cases of its construction will be considered.
The main advantages of shed roofs
Despite the fact that not everyone likes the aesthetics of a building over which a pitched roof is mounted (although the question itself is ambiguous), many owners of suburban areas, when building buildings, and sometimes even a residential building, choose this option, guided by a number of advantages similar design.
- Materials for a shed truss system, especially if it is being built over a small outbuilding, will require very little.
- The most "rigid" flat figure is a triangle. It is he who underlies almost any truss system. In a shed system, this triangle is rectangular, which greatly simplifies the calculations, since all geometric relationships are known to everyone who graduated from high school. But this simplicity does not affect the strength and reliability of the entire structure.
- Even if the owner of the site, who is leading an independent construction, has never previously encountered the construction of a roof, the installation of a shed truss system should not cause him excessive difficulties - it is quite understandable, not so complicated. Often, when blocking small outbuildings or other adjoining structures, it is quite possible to do not only without calling a team of specialists, but even without inviting assistants.
- When erecting a roof structure, the speed of work is always important, naturally, without loss of quality - you want to protect the building from the vagaries of the weather as quickly as possible. According to this parameter, the shed roof is clearly the "leader" - in its design there are practically no complex connecting nodes that take a lot of time and require high-precision adjustment.
How significant are the shortcomings of a single-pitched truss system? Alas, they exist, and they also have to be reckoned with:
- An attic with a shed roof is either not supposed at all, or it turns out to be so small that you have to forget about its wide functionality.
- Based on the first point, there are certain difficulties in ensuring sufficient thermal insulation of the premises located under a pitched roof. Although, of course, this can be corrected - nothing prevents the roof slope itself from being insulated or from placing an insulated attic floor under the rafter system.
- Shed roofs, as a rule, are made with a slight slope, up to 25 ÷ 30 degrees. This has two implications. First, not all types of roofing are suitable for such conditions. Secondly, the significance of the potential snow load increases sharply, which must be taken into account when calculating the system. But on the other hand, with such slopes, the influence of wind pressure on the roof is significantly reduced, especially if the slope is correctly positioned - to the windward side, in accordance with the prevailing winds in this area.
- Another drawback, perhaps, can be attributed to very conditional and subjective - this is the appearance of a pitched roof. It may not appeal to lovers of architectural delights, they say, it greatly simplifies the appearance of the building. This can also be objected to. Firstly, the simplicity of the system and the cost-effectiveness of erection often play a decisive role in the construction of auxiliary structures. And three times - if you look at the overview of projects of residential buildings, you can find very interesting design options, in which the emphasis is on a shed roof. So, as they say, tastes differ.
How is a shed truss system calculated?
General principles of system calculation
In any scenario, a shed roof system is a structure of rafter legs installed parallel to each other. The name itself - “layered” indicates that the rafters rest (lean) on two rigid points of support. For ease of perception, we turn to a simple scheme. (By the way, we will return to the same scheme more than once - when calculating the linear and angular parameters of the system).
So, two points of support of the rafter leg. One of the points (AT) located above the other (BUT) to a certain excess value (h). Due to this, a slope of the slope is created, which is expressed by the angle α.
Thus, as already noted, the construction of the system is based on a right-angled triangle ABC, in which the base is the horizontal distance between the support points ( d) - most often this is the length or width of the building being built. Second leg - excess h. Well, the length of the rafter leg between the fulcrum becomes the hypotenuse - L. Base angle (α) determines the slope of the roof slope.
Now let's consider the main aspects of choosing a design and performing calculations in more detail.
How will the required slope of the slope be created?
The principle of the location of the rafters - parallel to each other with a certain step, with the required slope angle of the slope - is common, but this can be achieved in various ways.
- The first is that even at the stage of developing a building project, the height of one wall (shown in pink) is immediately laid in excess of h relative to the opposite (yellow). The two remaining walls, running parallel to the roof slope, are given a trapezoidal configuration. The method is quite common, and although it somewhat complicates the process of building walls, it extremely simplifies the creation of the roof truss system itself - almost everything is already ready for this.
- The second method can, in principle, be considered a variation of the first. In this case, we are talking about frame construction. Even at the stage of project development, it is laid in it, then the vertical racks of the frame on one side are higher by the same amount h compared to the opposite.
In the illustrations above and in those that will be placed below, the diagrams are made with simplification - the Mauerlat passing along the upper end of the wall, or the strapping beam - on the frame structure is not shown. This does not fundamentally change anything, but in practice, this element, which is the basis for the installation of the truss system, cannot be dispensed with.
What is a Mauerlat and how is it attached to the walls?
The main task of this element is to evenly distribute the load from the rafter legs to the walls of the building. Rules for the selection of material and on the walls of the house - read in a special publication of our portal.
- The following approach is practiced when the walls are of equal height. The excess of one side of the rafter legs over the other can be ensured by installing vertical racks of the required height h.
The solution is simple, but the design is, at first glance, somewhat unstable - each of the "rafter triangles" has a certain degree of freedom to the left - to the right. This is quite simply eliminated by fastening the transverse bars (boards) of the crate and sewing the rectangular gable part of the roof from the front side. The remaining pediment triangles on the sides are also sewn up with wood or other material convenient for the owner.
rafter mount
- Another solution to the problem is the installation of a roof using shed trusses. This method is good because, after carrying out the calculations, it is possible to perfectly assemble and fit one truss, and then, taking it as a template, make the required number of exactly the same structures on the ground.
It is convenient to use such technology when, due to their large length, they require a certain amplification (this will be discussed below).
The rigidity of the entire truss system is already incorporated in the design of the truss - it is enough to install these assemblies on the Mauerlat with a certain step, gain a foothold on it, and then connect the trusses with a strapping or transverse bars of the crate.
Another advantage of this approach is that the truss performs both the role of the rafter leg and the floor beam. Thus, the problem of thermal insulation of the ceiling and filing of the flow is greatly simplified - everything for this will be ready right away.
- Finally, one more case - it is suitable for the situation when a shed roof is planned over an extension being built near the house.
On the one hand, the rafter legs rest on the racks of the frame or on the wall of the extension being built. On the opposite side is the main wall of the main building, and the rafters can rest on a horizontal run fixed on it, or on individual fasteners (brackets, embedded bars, etc.), but also aligned horizontally. The attachment line of this side of the rafter legs is also made in excess h.
Please note that despite the differences in approaches to the installation of a shed system, the same “rafter triangle” is present in all options - this will be important for calculating the parameters of a future roof.
In which direction should the roof slope be provided?
It would seem - an idle question, however, it is necessary to decide on it in advance.
In some cases, for example, if there are no special options, the slope should be located only in the direction from the building in order to ensure free flow of storm water and melted snow.
On a stand-alone building, there are already certain choices. Of course, the option is rarely considered in which the truss system is positioned in such a way that the direction of the slope falls on the front part (although such a solution is not excluded). Most often, the slope is organized back or to one side.
Here it is already possible to take the external design of the building under construction, the features of the territory of the site, the convenience of laying communications of the storm water collection system, etc. as selection criteria. But you should still keep in mind certain nuances.
- The optimal location of a shed roof is to the windward side. This allows you to minimize the wind effect, which can work with the lifting application of the force vector, when the slope turns into a kind of wing - the wind tries to tear the roof up. It is for pitched roofs that this is of paramount importance. When the wind blows into the roof, especially at small angles of steepness of the slopes, the value of the wind effect will be minimal.
- The second aspect of the choice is the length of the slope: with a rectangular building, it can be placed along it or across it. It is important to consider here that the length of the rafters without reinforcement cannot be unlimited. In addition, the longer the span of the rafters between the support points, the thicker the lumber used for the manufacture of these parts should be in cross section. This dependence will be explained a little later, already during the calculations of the system.
However, they practice the rule that the free length of the rafter leg should usually not exceed 4.5 meters. With an increase in this parameter, additional elements of structural reinforcement are necessarily provided. Examples are shown in the illustration below:
So, with a distance between opposite walls from 4.5 to 6 meters, it will already be necessary to install a rafter leg (strut), located at an angle of 45 °, and resting from below on a rigidly fixed support beam (lying). At distances up to 12 meters, it will be necessary to install a vertical rack in the center, which should be based either on a reliable ceiling, or even on a major partition inside the building. The rack also rests on the bed, and in addition, a strut is also installed on each side. This is all the more relevant due to the fact that the standard length of lumber usually does not exceed 6 meters, and the rafter leg will have to be made composite. So without additional support to do in any case will not work.
A further increase in the length of the slope leads to an even greater complication of the system - it becomes necessary to install several vertical racks, with a step of no more than 6 meters, relying on capital walls, and linking these racks with contractions, installing the same struts on each rack, and on both outer walls.
Thus, you should think carefully about where it will be more profitable to orient the direction of the roof slope also for reasons of simplifying the design of the truss system.
wood screws
What angle of slope will be optimal?
In the vast majority of cases, when it comes to a pitched roof, an angle of up to 30 degrees is chosen. This is due to a number of reasons, and the most important of them has already been mentioned - the strong vulnerability of a shed structure to wind load from the front side. It is clear that, following the recommendations, the direction of the slope is oriented to the windward side, but this does not mean at all that the wind on the other side is completely excluded. The steeper the angle of inclination, the greater the resulting lifting force becomes, and the greater the shear load will be experienced by the roof structure.
In addition, shed roofs with a large angle of inclination look somewhat awkward. Of course, this is sometimes used in bold architectural and design projects, but we are talking about more “mundane” cases ...
Too gentle slope, with a slope angle of up to 10 degrees, is also not very desirable, for the reason that the load on the truss system from snow drifts increases sharply. In addition, with the onset of snowmelt, it is very likely that ice will appear along the lower edge of the slope, making it difficult for the free flow of melt water.
An important criterion for choosing the slope angle of the slope is the intended one. It is no secret that for various roofing materials there are certain "frameworks", that is, the minimum allowable roof slope angle.
The slope angle itself can be expressed not only in degrees. It is more convenient for many masters to operate with other parameters - proportions or percentages (even in some technical sources you can find a similar system of measurements).
Proportional calculus is the ratio of the span length ( d) to the slope height ( h). It can be expressed, for example, in a ratio of 1:3, 1:6, and so on.
The same ratio, but in absolute terms and reduced to percentages, gives a slightly different expression. For example, 1:5 - this will be a slope of 20%, 1:3 - 33.3%, etc.
To simplify the perception of these nuances, below is a table with a graph-diagram showing the ratio of degrees and percentages. The scheme is fully scaled, that is, it can be easily converted from one value to another.
The red lines show the conditional division of roofs: up to 3 ° - flat, from 3 to 30 ° - roofs with a small slope, from 30 to 45 ° - medium steepness, and above 45 - steep slopes.
The blue arrows and their corresponding numerical designations (in circles) show the established lower limits for the use of a particular roofing material.
 |
|||
|---|---|---|---|
| № | Slope | Type of acceptable roofing (minimum slope) | Illustration |
| 1 | 0 to 2° | Completely flat roof or sloped up to 2°. At least 4 layers of rolled bituminous coating applied using a "hot" technology, with a mandatory top dressing of fine gravel embedded in molten mastic. | |
| 2 | ≈ 2° 1:40 or 2.5% | Same as in point 1, but 3 layers of bituminous material will be enough, with obligatory sprinkling | |
| 3 | ≈ 3° 1:20 or 5% | At least three layers of bituminous roll material, but without gravel backfill | |
| 4 | ≈ 9° 1:6.6 or 15% | When using rolled bituminous materials - at least two layers glued to the mastic in a hot way. It is allowed to use some types of corrugated board and metal tiles (according to the manufacturer's recommendations). | |
| 5 | ≈ 10° 1:6 or 17% | Asbestos-cement slate corrugated sheets of reinforced profile. Euroslate (single line). | |
| 6 | ≈ 11÷12° 1:5 or 20% | Soft bituminous tile | |
| 7 | ≈ 14° 1:4 or 25% | Flat asbestos-cement slate with reinforced profile. Decking and metal tiles - practically without restrictions. | |
| 8 | ≈ 16° 1:3.5 or 29% | Sheet steel roofing with folded connection of adjacent sheets | |
| 9 | ≈ 18÷19° 1:3 or 33% | Slate asbestos-cement corrugated regular profile | |
| 10 | ≈ 26÷27° 1:2 or 50% | Natural ceramic or cement tiles, slate or composite resin tiles | |
| 11 | ≈ 39° 1:1.25 or 80% | Roofing from wood chips, shingles, natural shingle. For lovers of special exotics - reed roofing | |
Having such information and having outlines for the future roofing, it will be easier to determine the slope angle of the slope.
metal tile
How to set the desired slope angle?
Let's turn again to our basic "rafter triangle" scheme, posted above.
So, to set the required slope angle α , it is necessary to ensure the elevation of one side of the rafter leg by an amount h. The ratios of the parameters of a right-angled triangle are known, that is, it will not be difficult to determine this height:
h = d × tg α
The value of the tangent is a tabular value that is easy to find in reference literature or in tables published on the Internet. But in order to simplify the task for our reader as much as possible, a special calculator is placed below, which will allow you to perform calculations in just a few seconds.
In addition, the calculator will help to solve, if necessary, the inverse problem - by changing the slope angle in a certain range, select the optimal value of the excess when this criterion becomes decisive.
Calculator for calculating the excess of the upper point of the installation of the rafter leg
As you know, the roof of any building is its upper part, which can combine protective and decorative functions. It protects the roof mainly from atmospheric precipitation getting inside the building from above, at the same time, with its appearance, material and color of the roof, it can emphasize the architectural features of the building.
The wooden beams that make up the rigid roof frame are called rafters; the selected roofing material is already mounted directly on them.
As buildings carry different functional contents (for example, residential buildings or industrial and technological buildings), so the roofs of various buildings differ from each other. Their shape can directly depend on climatic conditions: on the wind load or the amount of snowfall. It is difficult to clean the roof from the latter if its slope is 30 0 or less, and the large "sail" of a high roof can be a serious problem with wind gusts of more than 18 m / s.
Among the huge variety of roofs, most usually consists of a roof and a set of building structures that hold this roof.
One of the main elements of these structures are, as a rule, wooden beams, on which the roofing is mounted. These beams are called rafters or trusses. They are also the stiffening elements that determine the mechanical strength of the roof, as well as those guides that determine the angle of inclination of the roofing.
The rafters can be located either from one to the other outer wall of the building, with a certain slope, or from the center (ridge) of the roof to the outer wall. According to the first method, single-pitched roofs are arranged, according to the second - gable roofs.
It can be assumed that the closer these truss trusses are located to each other, the more reliable the base for roofing will be.
However, the excessive use of materials makes the structure heavier and leads to higher construction costs. Therefore, the question of how to install rafters is one of the fundamental when designing a roof.
There are two types of rafters: the so-called "hanging", resting with their ends only on the outer load-bearing walls, and those that rest with one of their ends on the inner load-bearing wall of the building or the inner column. Farms of the latter type are called "layered".
Proper placement and fastening of these elements of the building is the basis for the fact that its upper part is not deformed under the influence of possible loads.
How to properly install rafters
General provisions
When designing the roof of a building, determining the number of trusses and the distance between them, they must take into account the required section of the beam used for the construction of the rafters, determine its material and the optimal length of the rafter. Usually, a bar made of coniferous trees is used for the device of rafters, with a section of 50x150 mm (considered the most used) or more.
The length of the trusses directly depends on the size of the building box, the type of roof, and also on its height. The cross section of the timber used and the distance between the rafters determine the strength of the supporting structure for the roof. The distance between the axes of adjacent trusses is called and calculated when designing the roof. In practice, the applied pitch can take a value from 600 to 2000 mm. The specified step is interconnected with the length of the trusses: the shorter they are, the greater the distance between them can be installed.
There is a generalized method for calculating the specified distance. It lies in the fact that the table determines the preliminary step of the rafters. Having then measured the length of the roof overhang of one slope along the lower edge, the resulting distance must be divided by the step determined from the table. The result obtained and the unit added to it, after rounding up, will correspond to the number of rafters required for one slope of the projected roof.
The exact distance between the axes of the "legs" of neighboring farms will be obtained by dividing the length of one roof slope by the number of rafters calculated for it.
In this way, it is possible to determine at what minimum distance the rafters can be installed so that the roof supporting structure meets the design load requirements.
However, this method does not take into account possible additional loads on the structure associated with the use of various types of roofing, from slate to ondulin. Does not take into account the need to organize free space between trusses to accommodate sheets or slabs of roofing insulation used.
In the case when it is planned to use insulating agents, the width of which is known to be the width of the canvases or panels, you can immediately determine at what distance the rafters should be installed. It is recommended in such cases to equate the step to the width of the insulation, minus 1.5 to 2 mm.
Recommendations for choosing a rafter pitch for different roofing
For corrugated roofing, the pitch is selected in the range from 600 to 900 mm. At the same time, the beam is recommended with an optimal section - 50x150 mm.
A heavy roof made of ceramic tiles is characterized by an increased load on the rafters, about 60 - 70 kg / m 2. The step is recommended in the range from 800 to 1300 mm. Moreover, it can increase in proportion to the increase in the angle of inclination of the roof. For example, the distance between trusses should be no more than 800 mm if the angle of inclination of the roof does not exceed 15 0. By increasing the specified angle to 70 0 the step can be increased to the maximum. The cross section of the timber for such a roof is recommended from 50x150 to 60x180 mm.
The device of the supporting structure of the roofing for metal tiles is not much different from the standard one. The material, in comparison with ceramics, is almost twice as light: the load per 1 m 2 does not exceed 30 kg. A bar with dimensions of 50x150 mm is recommended for use. Some features of fastening the upper ends of the rafters are associated with the provision of ventilation of the metal roof to prevent condensation.
Slate roofing is the optimal solution for numerous buildings, despite the fact that this material is recognized as harmful and banned for use in European countries.
Recommendations for the installation of rafters for corrugated slate roofing are typical: they are placed in intervals from 600 to 800 mm, they can be 50x100 or 50x150 mm.
For roofing from ondulin, it is proposed to perform according to the recommendations in force for slate roofing. The modern innovative material ondulin looks like slate, but it is five times lighter than the last one.
The determination of the inter-rafter distance for multi-pitched (tented) roofs is made separately for each slope. For buildings in which the “box” is assembled from logs or timber, the lower end of the rafters is attached directly to the upper part of the outer load-bearing wall, and not to a special timber laid along the perimeter of the upper part of the building (Mauerlat). This installation method makes the price of an error especially high when determining the pitch of the rafters, since it can be very difficult to eliminate such an error.
Bearing truss structure for a mansard roof
For such roofs, the supporting structures for the roof are usually made of timber. The pitch of the rafters for a slope no longer than 15 m can be selected in the range from 800 to 1000 mm. For attics with slopes longer than 15 m, it is recommended to use metal rafters.
It should be noted that for all types of roofs, when determining the pitch of the rafters, the presence of existing vertical structural elements of the building passing through the attic and roof should be taken into account. These elements include chimneys and air ducts. If the design point of the truss installation coincides with the passage of the existing pipe or other building elements that cannot be transferred to another part of the attic, the rafter placement plan should be changed accordingly.
If for some reason it is not advisable to change the specified plan, it is recommended that the rafter, which coincides in place with the building element, be arranged so that it is interrupted at the place where the pipe passes. Moreover, the ends of this truss, cut off before and after the passed pipe, must rest on the corresponding jumpers connecting adjacent rafters.
It must be borne in mind that the nodes of such an "interception" of the truss should be performed with the necessary reliability and quality, which allows it to correspond to the calculated reliability of the supporting structure of the roofing.
It should be noted that the installation of rafters is part of a whole complex of very serious and very important construction work on the roof of a building. As a structural element of the load-bearing roofing system of a building, rafters are specified in the roof design plan, which reflects the results of calculations of various possible loads.
Such calculations should take into account all kinds of factors that affect the designed structure in the complex:
- necessary and sufficient height and slope of the roof;
- optimal material for roofing;
- the parameters of its placement on the required crate and the total weight of the roofing;
- the necessary bearing capacity of the truss structure in general and the corresponding parameters of the rafters in particular;
- method of attaching the roof to the walls of the building and the condition of the walls.
And other equally important data, without taking into account which the constructed building and its roof may not withstand various loads.
Therefore, in order not to have distressing consequences as a result of inept actions, it is better to entrust issues related to the design and construction of buildings to professional professionals with the necessary experience and knowledge. At least in the part that concerns the calculation of the load on the truss structures.
