What step should be between the farms. At what distance are rafters placed for a gable and single-pitch roof - calculation for different types of roofing: corrugated sheets, metal tiles, ondulin, etc. It is manufactured in several stages
The quality and durability of the roof largely depend on correct calculation pitch of rafters. It affects the installation of insulation, fastening of roofing material and installation of additional parts. Pay attention only to the requirements for the distance under roofing sheets, then problems may arise with the insulation boards. Conversely, adjusting the insulation to fit the dimensions can make the frame too weak, and in winter there is a risk of collapse. How to correctly calculate the distance between roof rafters? We'll talk about this in our article.
As a rule, the pitch between the rafters depends on several factors. But usually the distance ranges between 0.6 and 1.2 meters. These figures are rounded; in practice, the distance may differ by several centimeters downward. For precise definition the following calculations must be performed:
- Determine the length of the slope by measuring along the eaves of the house. Let's assume that it turns out to be 17.8 meters.
- Divide the resulting figure by the planned pitch of the rafters. If it was decided that the distance between the beams would be 0.8 meters, then 17.8/0.8 = 22.25.
- Next, round the result up and add one: 23 + 1 = 24. At this stage, the number of required rafters is determined.
- Now we calculate the distance between the axes of the beams. To do this, the length of the slope must be divided by the number of rafters: 17.8/24 = 0.74 m.
So, you will determine the actual distance at which the rafters should stand for a roof made of metal tiles or some other material.
Often, when planning a roof frame, specialized calculators are used. They enter basic data and get the finished result. However, you should not completely trust the program, since some conditions can only be taken into account by a person. When calculating, you can understand how the system works, what loads are applied. If necessary, adjustments are made to strengthen or lighten the structure. Self-calculation will help you more accurately determine the required pitch of gable rafters and pitched roof.
Calculation of pitch depending on roofing material
For each coating, its own standards and dimensions for the rafter system are developed. The main factors are the strength of the material, its weight and resistance to external loads. Let's look at the main types of coatings.
Corrugated sheet
The pitch of the rafters under the corrugated sheet can vary from 0.6 to 0.9 m. The final result depends on additional conditions, which we will discuss below. If for some reason the step should be larger, add a cross-section board with a larger cross-section. In this case, the rafters themselves are usually chosen with dimensions of 50x100 or 50x150 mm.
In addition to the rafters, 30x100 mm sheathing is used. The gap between the boards should be about 0.5 meters. It can be more, it all depends on the type of corrugated sheet and the height of the wave. The sheathing boards protruding beyond the boundaries of the cornice should be 1.5 cm thicker than usual. This is done based on the need to attach ventilation, chimney or drainage.
Ceramic tiles
The main difficulty of design rafter frame under ceramic tiles is a large weight of roofing material. It is 10 times heavier than corrugated sheeting and square meter accounts for 40 to 60 kg.
Dry beams with a moisture content of only 15% are used for this coating. Their cross-section should be either 50x150 or 60x180 mm. With such indicators, the maximum permissible distance between the rafters can be 1.3 meters. The minimum permissible step through which rafters can be placed is 0.8 m. The final decision is influenced by the slope of the roof: at 15° the step is 0.8 m, at 75° - 1.3 m.
The length of the rafter leg is also taken into account; the shorter it is, the larger the gap, and vice versa. In addition, if the slope is less than 45°, and movement on the surface is planned, the beams are placed in increments of no more than 0.85 m.
The distance between the sheathing boards is selected so that each intersection of the tiles has its own base. The standard length of the material is 400 mm, and the amount of overlap during installation ranges between 55 and 90 mm. It turns out that the distance between the central axes of the sheathing can be either 310 or 345 millimeters.
Metal tiles
In recent years, the question has become particularly acute: how to determine the distance between the rafters for metal tiles? It imitates a more expensive one ceramic material, but at the same time has positive properties corrugated sheets Installation of such a coating is simple and does not require large financial costs. All this makes the material attractive for the roofs of private houses.
The mass of metal tiles is less than ceramic tiles, so a simpler frame is required. The cross-section of the beams is reduced to 50x150 mm and the pitch between the sheathing is increased. The pitch of the rafters for metal tiles varies between 0.6 and 0.95 m.
Ondulin
The permissible step for ondulin coating is 0.6-0.9 meters. The standard rafter section is 50x200 millimeters. These dimensions will allow you to withstand the created load. roofing pie on a gable roof.
A lathing made of bars measuring 40x50 millimeters is attached on top of the counter-lattice. The pitch between the central axis is 600 mm.
Additional factors
When calculating the pitch of the rafters, attention is paid not only to the type of roofing. Many other points are taken into account, data on which can be found in reference books and SNiPs. Here are some of the factors:
- Snow and wind loads. The more snow falls in winter and the stronger the winds blow, the smaller the distance between the beams should be. But, if you increase the roof pitch above 45°, then you are allowed to increase the pitch.
- Choice of insulation. To reduce the cost of insulation boards, it is recommended to take into account their standard sizes. Mats are produced in widths of 600, 800 and 1200 mm. If this condition is neglected, then a large number of undercuts will appear, cold bridges will appear and construction will be delayed.
- Quality of lumber. Here the type of wood, grade and section are taken into account. As mentioned above, strength is affected by how well the beam was dried. When purchasing wood, pay attention to the evenness and presence of defects in the form of knots and traces of pests.
- Floor beams and tie rods. If you are installing under a gable roof attic floor, then the maximum distance between the rafters should be 0.75 meters.
The calculation of the distance between the beams for a gable and pitched roof is different. Even if he makes several slopes, for each it is necessary to make the calculation individually. This especially applies to buildings with different lengths of eaves.
Taking into account all these factors, you can determine at what distance to install the rafters. Although all calculations can be done independently, since reference data is freely available, it is better to turn to professionals. They have experience in design and will quickly determine the required distance between the rafters. Yes, you will avoid possible errors and you will be sure of safety.
The construction of the roof truss system and the subsequent roofing are the most important stages in any construction. This is a very complex matter, involving comprehensive preparation, which includes the calculation of the main elements of the system and the acquisition of materials of the required cross-section. Not every novice builder will be able to design and renovate a complex structure.
However, often in the construction of house buildings, utility or utility structures, garages, sheds, gazebos and other objects, the special complexity of the roof is not required at all - simplicity of design comes first, minimal amount costs for materials and speed of work, which are quite feasible for independent execution. It is in such situations that the rafter system becomes a kind of “lifesaver”
In this publication, the main emphasis is on calculations of a pitched roof structure. In addition, the most typical cases of its construction will be considered.
The main advantages of pitched roofs
Despite the fact that not everyone likes the aesthetics of a building over which a pitched roof is installed (although the question itself is ambiguous), many owners of suburban areas, when constructing buildings, and sometimes even a residential building, choose this option, guided by a number of advantages similar design.
- Very little materials are required for a single-pitch rafter system, especially if it is being built over a small outbuilding.
- The most “rigid” flat figure is a triangle. It is this that underlies almost any rafter system. In a single-slope system, this triangle is rectangular, which greatly simplifies the calculations, since all geometric relationships are known to everyone who has completed high school. But this simplicity does not in any way affect the strength and reliability of the entire structure.
- Even if the presenter self-construction the owner of the site has never encountered the construction of a roof before, the installation of a lean-to rafter system should not cause him excessive difficulties - it is quite understandable and not so complicated. Often, when covering small outbuildings or other adjacent structures, it is quite possible to do without not only 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 structure from the vagaries of the weather as quickly as possible. In terms of this parameter, the pitched roof is clearly the “leader” - its design contains practically no complex connecting units that take a lot of time and require high-precision adjustment.
How significant are the disadvantages of a lean-to rafter system? Alas, they exist, and they also have to be taken into account:
- An attic with a pitched roof is either not intended at all, or it turns out to be so small that one has to forget about its wide functionality.
- Based on the first point, there are certain difficulties in ensuring sufficient thermal insulation of rooms located under a pitched roof. Although, of course, this can be corrected - nothing prevents you from insulating the roof slope itself or 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 consequences. Firstly, 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 with such slopes, the influence of wind pressure on the roof is significantly reduced, especially if the slope is positioned correctly - in the windward direction, in accordance with the prevailing winds in a given area of the area.
- Another drawback, perhaps, can be classified as very conditional and subjective - this is the appearance of a pitched roof. It may not be to the liking of lovers of architectural delights, they say, it greatly simplifies the appearance of the building. This can also be objected to. First, the simplicity of the system and the cost-effectiveness of construction often play a decisive role in the construction of auxiliary structures. And three times - if you look at the overview of residential building projects, you can find very interesting design options in which the emphasis is placed specifically on the pitched roof. So, as they say, there is no arguing about tastes.
How is a lean-to rafter system calculated?
General principles of system calculation
In any case, a shed roof system is a structure of layered rafter legs installed parallel to each other. The name itself, “layered”, means that the rafters rest (lean) on two rigid support points. For ease of perception, let us turn to a simple diagram. (By the way, we will return to this same diagram more than once – when calculating the linear and angular parameters of the system).
So, two points of support for the rafter leg. One of the points (IN) located above the other (A) on specific value exceeding (h). Due to this, a slope of the slope is created, which is expressed by the angle α.
Thus, as already noted, the basis for constructing the system is a right 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 hypotenuse becomes the length of the rafter leg between the support points - L. Base angle (α) determines the steepness of the roof slope.
Now let's look at the main aspects of choosing a design and carrying out calculations in a little more detail.
How will the required slope of the slope be created?
The principle of arranging the rafters - parallel to each other with a certain pitch, with the required slope angle of the slope - is general, 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 set in excess 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 for this is already ready.
- 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 project development stage, it is built into it, then the vertical posts of the frame on one side are higher by the same amount h compared to the opposite.
In the illustrations presented above and in those that will be placed below, the diagrams are made with simplification - the Mauerlat running along the upper end of the wall is not shown, or the strapping beam - on frame structure. This does not change anything fundamentally, but in practice it is impossible to do without this element, which is the basis for installing the rafter system.
What is a Mauerlat and how is it attached to the walls?
The main task of this element is to uniformly distribute the load from the rafter legs to the walls of the building. Read the rules for selecting materials for the walls of the house in a special publication on 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 posts of the required height h.
The solution is simple, but the design turns out, at first glance, to be somewhat unstable - each of the “rafter triangles” has a certain degree of freedom to the left and to the right. This can be easily eliminated by attaching the transverse beams (boards) of the sheathing and covering the rectangular gable part of the roof on the front side. The remaining gable 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 to install a roof using single-pitch trusses. This method is good because it is possible, after making calculations, to ideally assemble and fit one truss, and then, taking it as a template, make the required number of exactly the same structures on the ground.
This technology is convenient to use in cases where, due to their large length, they require a certain amplification (this will be discussed below).
The rigidity of the entire rafter system is already inherent in the design of the truss - it is enough to install these assemblies on the mauerlat with a certain pitch, secure it to it, and then connect the trusses with strapping or transverse sheathing beams.
Another advantage of this approach is that the truss serves as both a rafter leg and a floor beam. Thus, the problem of thermal insulation of the ceiling and lining of the flow is significantly simplified - everything for this will be immediately ready.
- Finally, one more case - it is suitable for the situation when a pitched roof is planned over an extension being built near the house.
On one side, the rafter legs rest on the frame posts or the wall of the extension being built. On the opposite side there is the main wall of the main building, and the rafters can rest on a horizontal purlin fixed on it, or on individual fastenings (brackets, embedded bars, etc.), but also aligned horizontally. The attachment line for this side of the rafter legs is also made in excess h.
Please note that although there are differences in installation approaches single slope system, in all options there is the same “rafter triangle” - this will be important for calculating the parameters of the future roof.
In which direction should the roof slope be provided?
It would seem like an idle question, however, it needs to be decided 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 to ensure the free flow of storm water and melted snow.
A free-standing building already has certain options to choose from. Of course, the option is rarely considered in which the rafter system is positioned in such a way that the direction of the slope falls on the façade (although such a solution is not excluded). Most often, the slope is organized backwards or to one side.
Here you can already take the external one as the selection criteria design decoration of the building under construction, features of the site, ease of laying communications for the storm water collection system, etc. But you should still keep in mind certain nuances.
- The optimal location of a pitched roof is in the windward direction. This allows us 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 rip the roof upward. It is for pitched roofs that this is of utmost importance. If there is wind blowing into the roof, especially at small slope angles, the wind impact will be minimal.
- The second aspect of choice is the length of the slope: in case of a rectangular building, it can be placed along it or across it. It is important to take into account here that the length of the rafters without reinforcement cannot be unlimited. In addition, the longer the rafter span between the support points, the thicker the cross-section of the lumber used to make these parts should be. This dependence will be explained a little later, during the calculations of the system.
However, the rule of thumb is that the free length of the rafter leg should usually not exceed 4.5 meters. As this parameter increases, additional structural reinforcement elements must be provided. Examples are shown in the illustration below:
So, if the distance between opposite walls is from 4.5 to 6 meters, it will be necessary to install a rafter leg (strut), located at an angle of 45°, and resting from below on a rigidly fixed support beam (bench). At distances of up to 12 meters, you will have to install a vertical post in the center, which should rest either on a reliable ceiling, or even on a solid partition inside the building. The stand 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 standard length lumber usually does not exceed 6 meters, and the rafter leg will have to be made composite. So in any case it will not be possible to do without additional support.
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 pitch of no more than 6 meters, supported by capital walls, and with the connection of these racks with contractions, with the installation of the same struts both on each rack and on both external walls.
Thus, you should think carefully about where it would be more profitable to orient the direction of the roof slope, also for reasons of simplifying the design of the rafter system.
wood screws
What slope angle 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 explained by a number of reasons, and the most important of them has already been mentioned - the strong vulnerability of the lean-to structure to wind loads from the façade side. It is clear that, following the recommendations, the direction of the slope is oriented to the windward side, but this does not mean that the wind from the other side is completely excluded. The steeper the slope, the greater the lifting force created, and the greater the load on the roof structure will experience.
In addition, pitched 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...
A slope that is too gentle, with a slope angle of up to 10 degrees, is also not very desirable, for the reason that the load on the rafter system from snow drifts increases sharply. In addition, with the beginning of snow melting, it is very likely that ice will appear along the lower edge of the slope, impeding the free flow of melt water.
An important criterion for choosing the slope angle is what is planned. It's no secret that for different roofing materials there are certain “frames”, that is, the minimum permissible roof slope angle.
The slope angle itself can be expressed not only in degrees. Many masters find it more convenient to operate with other parameters - proportions or percentages (even in some technical sources you can find a similar measurement system).
Proportional calculus is the ratio of the span length ( d) to the height of the slope ( h). It can be expressed, for example, by the ratio 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 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 diagram 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 low slope, from 30 to 45° - medium slope, and above 45 - steep slopes.
Blue arrows and their corresponding numerical designations (in circles) show the established lower limits for the use of a particular roofing material.
 |
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|---|---|---|---|
| № | Slope amount | Type of acceptable roof covering ( minimum level slope) | Illustration |
| 1 | from 0 to 2° | Absolutely flat roof or with an inclination angle of up to 2°. At least 4 layers of roll bitumen coating, applied using “hot” technology, with the obligatory top coating of fine gravel embedded in molten mastic. | |
| 2 | ≈ 2° 1:40 or 2.5% | The same as in point 1, but 3 layers of bitumen material will be enough, with mandatory topping | |
| 3 | ≈ 3° 1:20 or 5% | At least three layers of bitumen roll material, but without gravel backfill | |
| 4 | ≈ 9° 1:6.6 or 15% | When using roll bituminous materials– at least two layers glued to the mastic using a hot method. The use of certain types of corrugated sheets and metal tiles is allowed (according to manufacturer's recommendations). | |
| 5 | ≈ 10° 1:6 or 17% | Asbestos-cement corrugated slate sheets with reinforced profile. Euroslate (odnulin). | |
| 6 | ≈ 11÷12° 1:5 or 20% | Soft bitumen shingles | |
| 7 | ≈ 14° 1:4 or 25% | Flat asbestos-cement slate with reinforced profile. Corrugated sheeting and metal tiles - practically without restrictions. | |
| 8 | ≈ 16° 1:3.5 or 29% | Sheet steel roofing with seam connection of adjacent sheets | |
| 9 | ≈ 18÷19° 1:3 or 33% | Asbestos-cement wavy slate of regular profile | |
| 10 | ≈ 26÷27° 1:2 or 50% | Natural ceramic or cement tiles, slate or composite polymer tiles | |
| 11 | ≈ 39° 1:1.25 or 80% | Roofing made of wood chips, shingles, natural shingles. For lovers of special exoticism - reed roofing | |
Having such information and having outlines for the future roofing covering, it will be easier to determine the slope angle.
metal tiles
How to set the required slope angle?
Let's turn again to our basic “rafter triangle” diagram posted above.
So, to set the required slope angle α , it is necessary to ensure that one side of the rafter leg is raised by the amount h. Parameter Relationships right triangle are known, that is, determining this height will not be difficult:
h = d × tg α
The tangent value is a tabular value that is easy to find in reference books or in tables published on the Internet. But in order to simplify the task as much as possible for our reader, below is a special calculator that will allow you to perform calculations in just a few seconds.
In addition, the calculator will help solve, if necessary, the inverse problem - by changing the slope angle in a certain range, select optimal value exceeding when this particular criterion becomes decisive.
Calculator for calculating the excess of the upper installation point of the rafter leg
The distance between the rafters is one of the key parameters affecting the strength of the structure. Proper calculation of the installation pitch of the rafters allows you to build a roof that is resistant to high operating loads.
Roof loads and calculation of the rafter system
Development of a project for a single-pitched or gable roof begins with choosing the type of rafter system, the angle of inclination of the slopes (roof height) and materials for constructing the structure. Calculation of distance between rafter legs is carried out taking into account the loads that the roof will experience during operation. In number permanent loads included:
- the weight of the materials from which the rafter system is made;
- roofing weight;
- weight of roofing materials (waterproofing, vapor barrier, insulation);
- weight of finishing elements of a residential attic or attic.
In addition to permanent loads, the roof also experiences temporary loads, which include:
The pitch refers to the distance between the rafters of one slope. When calculating a single-pitch, gable or complex roof, we usually adhere to the following scheme:
- the length of the future roof slope is measured;
- the resulting value is divided by the optimal numerical value of the rafter pitch;
- one is added to the resulting value, the result is rounded;
- the length of the roof slope is divided by the rounded result.
The final result will allow you to determine at what distance the rafter legs should be placed. Determining the pitch cannot be extremely accurate, since it is necessary to take into account a number of additional factors, including the width of the insulation, the features of installing the sheathing for various types of roofing material. If a roof with a chimney is being designed, the pitch can be adjusted taking into account its location, so that you do not have to subsequently remove part of the rafters and install a supporting structure, such as a special rafter system.
Distance between rafters under slate
Slate is a traditional roofing material. Its advantages include characteristics such as resistance to external influences(excluding mechanical) and low cost. Slate allows you to create a roofing covering, the repair of which can be reduced to replacing individual elements. Slate is heavy and requires the installation of a fairly powerful rafter system. Calculation of the distance at which it is necessary to place 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 a system under slate, in which the gap between the rafters should 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 sheathing is made of timber or boards with a cross-section of at least 30 mm. When calculating a rafter system for slate, you should read that this material has quite large restrictions on the choice of the angle of inclination of the slopes.
Rafters for metal tiles
Metal tiles are actively used as a practical and aesthetic roofing material when arranging a pitched, gable, hip or complex roof. The frame for metal tiles is built according to standard principles. To calculate at what distance it is better to install the rafters, it is necessary to take into account the loads and the angle of the roof. Metal tiles are characterized by their relatively low weight, thanks to which they can serve as a replacement for old slate or roofing coverings. ceramic tiles. In this case, the rafters will not need to be strengthened or the pitch of their installation changed.
The standard pitch 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 metal tiles. But if you plan to use insulation, the layer of which in areas with low winter temperatures should be 200 mm, for rafters under metal tiles it is recommended to use 200x50 timber so as not to install additional system holding the insulation. It is better to adjust the gap between the rafters for metal tiles to the width of the sheet or roll insulation.
Corrugated sheeting: rafters and sheathing
Corrugated sheeting is a lightweight and easy-to-use roofing material. Galvanized or coated with a decorative protective layer, corrugated sheets can be used for installing a pitched roof utility room or a garage, or for a gable roof of a residential building. How to calculate the distance required to install rafters under corrugated sheeting?
To ensure the necessary structural rigidity, it is enough to install the rafters under the corrugated sheets in increments of 600-900 mm. In this case, you need to pay attention to the angle of the roof. Calculations show that under high external loads, it is better to lay corrugated sheets on the system with a minimum step. But if the distance between the rafters under the corrugated sheet is close to the maximum, and the roof slope angle is small, then the structure is strengthened by using more frequent lathing. In this case, the pitch of the sheathing under the corrugated sheet should be about 50 mm, the width of the elements should be at least 100 mm.
Rafter system for soft roofing
Soft roofs include bitumen and bitumen-polymer roll materials, roofing membranes, as well as soft tiles. Soft roofing is characterized by relatively low weight and the absence of the need to install a massive rafter structure.
The minimum distance between the rafter legs 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: smaller angle, the smaller the distance between the supports for a continuous sheathing should be. The choice of step is also influenced by the thickness of the material for the sheathing - the thicker the sheet of plywood or OSB, the larger the installation step of the rafter legs can be.
Ondulin: calculation of rafters
Ondulin (bitumen slate) is laid on a flat, continuous sheathing made of sheet material. This allows the roofing covering to successfully resist wind and snow loads. The sheathing for 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 pitched roof.
Rafters for ondulin are made of timber with a section of 200×50 mm. When choosing at what distance to place the rafter legs for the structure under the ondulin, it is recommended to take into account the width insulation material to make it easier to install. This calculation allows us to reduce financial expenses for the installation of the roof.
Sandwich panel roof
Sandwich roofing is most often erected on houses made of sip panels or hangar-type buildings. A special 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 top of the wall (or the distance between supporting structures pitched roof) allow the installation of a sandwich without additional supports.
If the span exceeds 4 meters, additional purlins 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 at larger intervals - they serve as a support for the purlins. The distance between the rafter legs is selected based on the length of the available material for the purlins and the length load-bearing walls. The technical parameters of the sandwich allow the roof to withstand high operating loads.
Polycarbonate: construction of a supporting structure
Polycarbonate has recently been actively used as a roofing material. First of all, polycarbonate is in demand in the construction of gazebos, canopies, winter gardens. The lathing and rafter system for polycarbonate can be made of wood or metal.
Polycarbonate varies in weight depending on the thickness of the sheet. It is recommended to perform lathing under polycarbonate in increments of 600-800 mm. The lathing (wooden or metal) is mounted on rafters, which can be straight or arched. Typically, the gap between rafters for polycarbonate is from 1500 to 2300 mm. In order to correctly calculate at what distance it is better to lower the rafters, it is necessary to take into account the glazing area, the size and thickness of the sheets, and take into account that polycarbonate is attached with gaps for thermal expansion.
The distance between the rafters of a gable and pitched roof
Find out what distance between the rafters needs to be made for a gable and shed roof. The maximum distance between rafters for metal tiles, ondulin and soft roofing.
Distance between rafters: principles and examples of calculating the pitch of a rafter system
The task of correctly calculating the distance between the rafters is a very important one. How seriously you begin to solve this problem will determine not only the reliability and durability of the roof, but also all subsequent work on it: laying insulation, installing roofing, installing additional elements. If you manipulate the pitch of the rafters under the roofing sheets, as many do, then it is not a fact that insulation will then fit between the rafters. If you focus only on insulation, the very first winter with its abundant Russian snow will crush the rafter system. That’s why the whole point is to choose the optimal rafter pitch for all slopes, and this is the skill we’ll teach you now.
What determines the pitch of the rafters?
So, the distance between the rafters is determined by the following important factors:
- Roof shape (gable, single-slope or multi-slope).
- Roof angle.
- Parameters of the timber used to make rafters (width, thickness).
- The design of the rafter system (sloping, hanging or sliding).
- The totality of all loads on the roof (weight of covering, atmospheric phenomena, etc.).
- Lathing material (20x100 or 50x50) and its parameters (solid, with gaps of 10 cm, 20 cm or solid plywood)
And each of these parameters needs to be taken into account, which is exactly what this article is about.
Decorative rafters: 0% load
First of all, decide on the most important point: the type of roof and its purpose. The fact is that the roof of a residential building in winter can withstand a large cap of snow, a constant wind at height, and it is also often insulated from the inside, but to the rafter system small gazebo, hidden under the canopy of trees, have completely different requirements.
For example, if you are building a pergola in its classical sense, then it does not matter at all what exactly the distance between the rafters will be - this is a purely aesthetic factor:
The illustration above shows that even such a building has its own pitch of rafters. After all, here it provides both the aesthetic factor and the rigidity of the structure itself. But they choose a step arbitrarily.
Functional rafters: detailed calculation
We come to the main question: what distance should be between the rafters of the roof of a residential building? Here, be patient and carefully study all the nuances.
Point 1. Wall length and choice of rafter spacing
The first step in installing rafters on the roof of a residential building is usually chosen structurally based on the size of the building, although taking into account many other factors.
For example, the easiest way is to install rafters in 1-meter increments, so for a wall 6 meters long, 7 rafters are installed as a standard. At the same time, you can save money by placing them at a distance of 1 and 2 meters, and you will get exactly 5 rafters. It can also be placed at a distance of 2 and 3 meters, but reinforced with lathing. But it is extremely undesirable to make the rafter spacing more than 2 meters.
Point 2. The influence of snow and wind loads on the shape of the roof
So, we settled on the fact that the average distance between the rafters of a regular roof is 1 meter. But, if the area has a significant snow or wind load, or the roof is more or less flat or simply heavy (for example, covered with clay tiles), then this distance must be reduced to 60-80 cm. But on a roof with a slope of more than 45 degrees it can be even increase to a distance of 1.2 m-1.4 m.
Why is this so important? Let's figure it out. The fact is that air flow On its way, it collides with a wall under the roof of a building, and turbulence occurs there, after which the wind hits the eaves of the roof. It turns out that the wind flow seems to bend around the slope of the roof, but at the same time trying to lift it. And at this moment forces arise in the roof that are ready to tear it off or overturn it - these are two windward sides and one lifting one.
There is another force that arises from wind pressure and acts perpendicular to the slope, trying to press the roof slope inward. And the greater the angle of inclination of the roof slope, the more important are the safe wind forces and the less tangential ones. And the greater the angle of the slope, the less frequently the rafters need to be installed.
This map of the average wind load will help you understand whether to make a high roof or a flat one:
The second point: in the Russian region the standard roof of a house is constantly exposed to such atmospheric phenomenon like snow. Here too, you need to take into account that the snow bag usually accumulates more on one side of the roof than on the other.
That is why in places where such a bag is possible, you need to insert paired rafter legs or make a continuous sheathing. The easiest way to identify such places is by the wind rose: single rafters are placed on the windward side, and paired rafters are placed on the leeward side.
If you are building a house for the first time, then you won’t have to decide on your own worldview, but rather determine the average snow load for your area according to official data:
Point 3. The issue of insulation and standard width of mats
If you will be insulating the roof, then it is advisable to set the pitch of the rafters to the standard sizes of insulation boards, which are 60, 80 cm and 120 cm.
Modern insulation materials are now sold in standard widths, usually at the same standard rafter spacing. If you then take them and adjust them to existing parameters, then there will be a lot of waste, cracks, cold bridges and other problems.
Point 4. Quality and strength of the lumber used
It is also of great importance what kind of material you use to build the rafter system. Thus, for each type of wood there is its own regulatory documentation that concerns its load-bearing capacity:
Because For the manufacture of roof truss systems in Russia, pine and spruce are most often used; their bending strength and features of use have long been prescribed. If you use wood of other species, you can derive a correction factor.
In addition, if the rafters have sections, notches or holes for bolts, in this place the load-bearing capacity of the beam must be calculated with a coefficient of 0.80.
Point 5. Distance between tie rods and floor beams
One more point: if the roof is built with interconnected trusses, and their bottom belt is used simultaneously as floor beams, then the distance between the trusses must be made within 60-75 cm to take into account the design of the future floor.
Point 6. Loads on rafters
So, here are the main loads that act on the roof truss system:
- Static, which includes the weight of the rafter system itself, the weight of the roof, the snow lying on the roof and additional elements.
- Dynamic, which includes wind force, unexpected damage to the roof, the weight of a person and equipment for repairs, and similar factors.
And all these factors are capable of influencing the roof simultaneously at a certain moment, and therefore there is such a thing as critical value. This is exactly the load value at which the roof cannot withstand and becomes deformed.
Therefore, if a building is being built with significant spans, then steel roof trusses. The fact is that there is no longer any tension in such rods, and the entire load falls on the nodes - here they are affected by compressive and tensile forces. And the distance between such trusses is calculated depending on the type of roof and the design of the roof itself.
Typically, a unified truss is installed with a span that is a multiple of six, and therefore there is a distance between the nodes of the truss that is a multiple of one and a half meters.
Point 7. Weight of the rafter system and roofing pie
Do not forget that the main purpose of the rafters is to support the entire roof, and its weight is crucial:
Point 8. Ease of installation of roofing
The distance between the rafters is also influenced by factors such as the selected roofing covering. The higher the roof slope, the more roofing materials will be used. And the heavier they are, the more often you will have to put rafters under them. But what about continuous lathing? The fact of the matter is that it also has its own weight:
Each type of roof has its own optimal rafter pitch. After all, many standard sheets at the edges need to be fastened directly into the rafters or sheathing, and it is important that they coincide. Otherwise, the work of covering the roof will easily turn into a living hell at altitude, believe me.
That is why, even before starting installation, you must make a layout and check everything several times. And know some important subtleties for each type of coating.
Determination of the totality of loads on the roof as a whole and the rafters separately
So, we have determined that, in addition to other design factors, a whole set of loads simultaneously acts on the roof rafter system: the weight of the rafter system, the snow cap, wind pressure. After you add all the loads together, be sure to multiply them by a factor of 1.1. So you will all count on unexpected favorable conditions, that is, provide an additional 10% percentage strength.
Now all you have to do is divide the total load by the planned number of rafters and see if each of them can cope with its task. If it seems that the structure will be frail, feel free to add 1-2 rafters to the total, and you will have peace of mind for your home.
You need to make calculations for destruction, i.e. for the full load acting on the roof. All these loads are determined according to the technical characteristics of the materials and SNiPs.
The standard roof structure consists of rafters, lattice purlins, and each of these elements responds only to the load that puts pressure on it, and not on common roof generally. Those. Each individual rafter is subject to its own load, total, but divided by the number of rafter legs, and by changing the step of their location, you change the area where the load is collected on the rafters - reducing it or increasing it. And, if changing the pitch of the rafters is inconvenient for you, then work with the parameters of the cross-section of the rafter legs, and the overall load bearing capacity the roof will increase significantly:
When making this calculation, try to ensure that the longest rafter in your project is no more than six and a half meters, otherwise, splice along the length. Now let's explain in more detail. So, on roofs with slopes up to 30 degrees, the rafters have so-called “bending elements”. Those. they work specifically for bending, and there are certain requirements for them. And the possibility of rafter deflection is calculated using a special formula, and if the result exceeds the norm, then the rafters are increased in height and a new calculation is made again.
But on a roof with a warehouse slope of more than 30 degrees, which rafters are already considered “bending-compressed” elements. That is, they are affected not only by a uniformly distributed load, which causes the rafters to bend, but also by forces that are already acting along the axis of the rafters. Speaking in simple language, here the rafters not only bend a little under the weight of the roof, but are also compressed from the ridge to the mauerlat. In addition, the crossbar, which usually holds the two rafter legs, must also be checked for tension.
As you can see, even a person far from construction can handle such calculations. The main thing is to take everything into account, be attentive and ready to spend a little more time on design, so that then all the work goes like clockwork!
Distance between rafters: what step should be between rafters
Detailed instructions on how to correctly calculate the distance between roof rafters different designs, taking into account the sum of the loads and the material used.
A gable roof is formed on the basis of a frame that combines the simplicity of the device and unsurpassed reliability. But the roof skeleton of two rectangular slopes can boast of these advantages only if the rafter legs are carefully selected.
Parameters of the gable roof rafter system
You should start making calculations if you understand that the rafter system gable roof- this is a complex of triangles, the most rigid elements of the frame. They are assembled from boards, the size of which plays a special role.
Rafter length
The formula will help determine the length of durable boards for the rafter systema²+b²=c², derived by Pythagoras.
The length of the rafter can be found by knowing the width of the house and the height of the roof
The parameter “a” indicates the height and is independently selected. It depends on whether the under-roof space will be residential, and also has certain recommendations if an attic is planned.
Behind the letter "b" is the width of the building, divided in two. And “c” represents the hypotenuse of the triangle, that is, the length of the rafter legs.
Let’s assume that the width of half the house is three meters, and it was decided to make the roof two meters high. In this case, the length of the rafter legs will reach 3.6 m (c=√a²+b²=4+√9=√13≈3.6).
You should add 60–70 cm to the figure obtained from the Pythagorean formula. The extra centimeters will be needed to carry the rafter leg behind the wall and make the necessary cuts.
The six-meter rafter is the longest, so it is suitable as a rafter leg
The maximum length of a beam used as a rafter leg is 6 m. If a durable board of greater length is required, then they resort to the fusion method - nailing a section from another beam to the rafter leg.
Section of rafter legs
For various elements rafter systems have their own standard sizes:
- 10x10 or 15x15 cm - for mauerlat timber;
- 10x15 or 10x20 cm - for the rafter leg;
- 5x15 or 5x20 cm - for purlin and bracing;
- 10x10 or 10x15 cm - for a stand;
- 5x10 or 5x15 cm - for a bed;
- 2x10, 2.5x15 cm - for laths.
Thickness of each part load-bearing structure roofing is determined by the load that it will experience.
A beam with a section of 10x20 cm is ideal for creating a rafter leg
The cross-section of the rafter legs of a gable roof is affected by:
The most significant effect on the cross-section of the rafter legs is the pitch of the rafters. An increase in the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafter legs.
Table: rafter cross-section depending on length and pitch
Variable impact on the rafter system
The pressure on the rafter legs can be constant or variable.
From time to time and with varying intensity, the supporting structure of the roof is affected by wind, snow and precipitation. In general, the roof slope is comparable to a sail, which can break under the pressure of natural phenomena.
The wind tends to overturn or lift the roof, so it is important to make all the calculations correctly
The variable wind load on the rafters is determined by the formula W = Wo × k x c, where W is the wind load indicator, Wo is the value of the wind load characteristic of a certain area of Russia, k is a correction factor determined by the height of the structure and the nature of the terrain, and c is the aerodynamic factor coefficient.
The aerodynamic coefficient can vary from -1.8 to +0.8. A negative value is typical for a rising roof, while a positive value is typical for a roof on which the wind presses. At simplified calculation with a focus on improving strength, the aerodynamic coefficient is considered equal to 0.8.
Calculation of wind pressure on the roof is based on the location of the house
The standard value of wind pressure is determined from map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient taking into account the change in wind pressure with height is also standardized.
Table: standard value of wind pressure
Table: k coefficient value
It's not just the terrain that affects wind loads. Great importance has a residential area. Behind a wall of tall buildings there is almost no threat to the house, but in an open space the wind can become a serious enemy for it.
The snow load on the rafter system is calculated using the formula S = Sg × µ, that is, the weight of the snow mass per 1 m² is multiplied by a correction factor, the value of which reflects the degree of roof slope.
The weight of the snow layer is indicated in SNiP “Rafter Systems” and is determined by the type of terrain where the building is built.
The snow load on the roof depends on where the house is located
The correction factor, if the roof slopes tilt less than 25°, is equal to one. And in the case of a roof slope of 25–60°, this figure decreases to 0.7.
When the roof is sloped more than 60 degrees, the snow load is discounted. Still, snow rolls off a steep roof quickly, without having time to negative influence to the rafters.
Constant loads
Loads acting continuously are considered to be the weight of the roofing pie, including sheathing, insulation, films and Decoration Materials for arranging an attic.
The roofing pie creates constant pressure on the rafters
The weight of the roof is the sum of the weight of all materials used in the construction of the roof. On average it is 40–45 kg/sq.m. According to the rules, per 1 m² of rafter system there should not be more than 50 kg of roofing material weight.
To ensure that there is no doubt about the strength of the rafter system, it is worth adding 10% to the calculation of the load on the rafter legs.
Table: weight of roofing materials per 1 m²
| Type of roofing finish | Weight in kg per 1 m² |
| Rolled bitumen-polymer sheet | 4–8 |
| Bitumen-polymer soft tiles | 7–8 |
| Ondulin | 3–4 |
| Metal tiles | 4–6 |
| Corrugated sheeting, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tiles | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| Slate roofing | 40–50 |
| Copper | 8 |
| Green roof | 80–150 |
| Rough flooring | 18–20 |
| Lathing | 8–10 |
| The rafter system itself | 15–20 |
Number of beams
How many rafters will be needed to arrange the frame of a gable roof is determined by dividing the width of the roof by the pitch between the beams and adding one to the resulting value. It indicates an additional rafter that will need to be placed on the edge of the roof.
Let's say it was decided to leave 60 cm between the rafters, and the length of the roof is 6 m (600 cm). It turns out that 11 rafters are needed (including the additional timber).
The rafter system of a gable roof is a structure made from a certain number of rafters
The pitch of the beams of the supporting roof structure
To determine the distance between the beams of the supporting roof structure, you should look close attention for moments such as:
- weight of roofing materials;
- the length and thickness of the beam - the future rafter leg;
- degree of roof slope;
- level of wind and snow loads.
It is customary to place rafters at 90–100 cm intervals when choosing a lightweight roofing material
A normal step for rafter legs is 60–120 cm. The choice in favor of 60 or 80 cm is made in the case of constructing a roof inclined at 45˚. The same small step should be taken if you want to cover wooden frame roofs with heavy materials such as ceramic tiles, asbestos-cement slate and cement-sand tiles.
Table: rafter pitch depending on length and cross-section
Formulas for calculating the rafter system of a gable roof
Calculation of the rafter system comes down to establishing the pressure on each beam and determining the optimal cross-section.
When calculating the rafter system of a gable roof, proceed as follows:
- Using the formula Qr = AxQ, they find out what the load per linear meter of each rafter leg is. Qr is the distributed load per linear meter of a rafter leg, expressed in kg/m, A is the distance between the rafters in meters, and Q is the total load in kg/m².
- Proceed to determining the minimum cross-section of the rafter beam. To do this, study the data from the table included in GOST 24454–80 “Lumber coniferous species. Dimensions".
- Focusing on standard parameters, select the section width. And the section height is calculated using the formula H ≥ 8.6 Lmax sqrt(Qr/(BRbend)), if the roof slope is α< 30°, или формулу H ≥ 9,5·Lmax·sqrt(Qr/(B·Rизг)), когда уклон крыши α >30°. H is the section height in cm, Lmax is the working section of the rafter leg maximum length in meters, Qr - distributed load per linear meter of rafter leg in kg/m, B - section width cm, Rbend - bending resistance of wood, kg/cm². If the material is made from pine or spruce, then Ri can be equal to 140 kg/cm² (grade 1 wood), 130 kg/cm² (grade 2) or 85 kg/cm² (grade 3). Sqrt is the square root.
- Check whether the deflection value complies with the standards. It should not be greater than the figure obtained by dividing L by 200. L refers to the length of the working section. Correspondence of the deflection value to the ratio L/200 is feasible only if the inequality 3.125·Qr·(Lmax)³/(B·H³) ≤ 1 is true. Qr denotes the distributed load per linear meter of the rafter leg (kg/m), Lmax is the working section of the rafter leg maximum length (m), B is the section width (cm), and H is the section height (cm).
- When the above inequality is violated, the indicators B and H increase.
Table: nominal dimensions of thickness and width of lumber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
Example of load-bearing structure calculation
Let us assume that α (roof inclination angle) = 36°, A (distance between rafters) = 0.8 m, and Lmax (working section of the rafter leg of maximum length) = 2.8 m. First grade pine material is used as beams , which means that Rben = 140 kg/cm².
Cement-sand tiles were chosen to cover the roof, and therefore the weight of the roof is 50 kg/m². The total load (Q) experienced by each square meter is 303 kg/m². And for the construction of the rafter system, beams 5 cm thick are used.
The following computational steps follow from this:
- Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
- H ≥ 9.5·Lmax·sqrt(Qr/B·Rben).
- H ≥ 9.5 2.8 sqrt(242/5 140).
- 3.125·Qr·(Lmax)³/B·H³ ≤ 1.
- 3.125·242·(2.8)³ / 5·(17.5)³= 0.61.
- H ≥ (approximate height of the rafter section).
In the table of standard sizes, you need to find a section height of the rafters that is close to 15.6 cm. A suitable parameter is 17.5 cm (with a section width of 5 cm).
This value fully corresponds to the deflection indicator in regulatory documents, and this is proven by the inequality 3.125·Qr·(Lmax)³/B·H³ ≤ 1. Substituting the values (3.125·242·(2.8)³ / 5·(17.5)³) into it, we find that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the rafter system
Calculating the rafter system of a gable roof is a whole complex of calculations. In order for the beams to cope with the task assigned to them, the builder needs to accurately determine the length, quantity and cross-section of the material, find out the load on it and find out what the pitch between the rafters should be.
The roof frame must be reliable and durable. But without proper calculations this is difficult to achieve. In the process of carrying out calculations, they determine at what distance to place the rafters on the roof.
What can result from an incorrect or inaccurate calculation of the loads to which the truss structure will be subjected? The most negative consequences, ranging from deformation of the rafter legs and damage to the roof covering and ending with the collapse of the base of the roof frame. Therefore, when designing buildings, the list of mandatory calculations contains data on what the distance between the roof rafters should be. There is a certain technique that allows you to calculate this value.
Method for calculating the distance between rafters
The distance between the rafters on the roof is called the rafter pitch. As a rule, the pitch of the rafter legs in a roof structure usually exceeds one meter, and the minimum gap ranges between 60 centimeters.
Calculation required quantity rafters for a roof of a certain length and the pitch of the rafters are made as follows:
Rafter structure for metal tiles
When constructing suburban private real estate, metal tile roofing can be found most often. This roofing material is similar to clay tile flooring, but has a number of advantages over it. Sheet metal tiles are easy to install, so you can build a roof in a shorter time; the rafter system for metal tiles is also not complicated.
Metal tiles are lighter than ceramic products; the difference in weight sometimes reaches 35 kilograms per square meter, depending on the thickness of the products (read also: ""). Thanks to a significant reduction in the weight of the roof deck, it becomes possible to reduce the thickness of the elements of the rafter structure and the cross-sectional dimensions of the sheathing bars, and to increase the installation pitch of the rafters.
Under the metal tile covering, rafter legs are mounted at a distance of 600 to 950 millimeters, while the cross-section of the construction material is 150 by 50 millimeters. According to experts, in this case, if you place 150 millimeters thick insulation between the rafters, then such thermal insulation will create comfortable conditions for staying in the attic room. At the same time, for greater reliability, it is advisable to choose 200 mm insulation.
When installing rafters, to ensure ventilation of the space filled with insulation, holes with a diameter of 10-12 millimeters are drilled in the rafters near the upper roof.
The technology for creating a rafter system for metal tiles does not differ significantly from designs for other types of roofing materials. The only peculiarity is that the upper support of the rafters is mounted on the ridge girder from above, and not on the side ridge beam. The presence of a free zone between the rafters ensures air circulation under the roof deck, and this, due to the use of metal material, reduces the risk of condensation.
Rafter system of a pitched roof, main advantages and characteristics ").
The distance between the rafters of a gable roof is made taking into account the size of the heat insulator that is laid between them. The approximate step between the rafter legs is 1-1.2 meters (read: ""). Rafters regulate the amount of roof overhang.
