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.
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, the above 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.
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.
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:
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 specialists with the necessary experience and knowledge. At least in the part that concerns the calculation of the load on the truss structures.
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.
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.
How significant are the shortcomings of a single-pitched truss system? Alas, they exist, and they also have to be reckoned with:
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 (IN) 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.
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.
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 solution is simple, but the design turns out, 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
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.
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.
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 for the storm water collection system, etc. as selection criteria. But you should still keep in mind certain nuances.
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
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 the single-slope structure to the 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.
 |
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|---|---|---|---|
| № | 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
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.
Before installing the truss system, the question arises with what step to put the floor beams. Correctly calculating the distance between the rafters means preventing later deformation or even destruction of the roof frame. After reading the article to the end, readers will find out what should be the distance between the rafters for various types of roofs and how to correctly calculate.
Even at the stage of designing a house made of wood, you need to make all load calculations. This also applies to the truss system. This is especially important in wooden housing construction, since the upper link is often used instead of the Mauerlat. Correcting errors in such a design is subsequently difficult. For the correct calculation of the distance between the beams, there is a technique.
The span of the roof frame for the construction of a house from a bar standardly exceeds 1 m, and the smallest allowable value is 60 cm, such indicators are indicated in GOSTs (see figure). Correctly calculate the length of the rafters, their step can be the following option:
Using a tape measure, the length of the roof slope is measured, the result is divided by the step size of the roof frame. For example, if the distance between the rafters is 1 m, then you need to divide by 1, if 70 cm, then by 0.7. The resulting figure is summed from 1 and rounded up to the nearest higher number. So you can determine the number of beams for one roof slope.
By the result you need to divide the length of the future slope. The result is the distance between each rafter.
For example, consider a roof whose slope is 25.5 m and a step of 0.6 m. You need to calculate the following indicators: 25.5: 0.6 \u003d 42.5, to 42.5 + 1 \u003d 43.5. We round this figure to the nearest larger integer, we get 44. This is the number of rafters per 1 slope of the future roof.
Now we calculate the span between the rafters: 25.5:44 \u003d 0.58 m. It turns out that you need to put the legs of the frame after 58 cm. So you can easily calculate the step of any roof frame, shed or complex, without taking into account the roof. But professionals recommend their calculations for a certain type of roof.
Since each forging material has its own characteristics and features. Among the most common are:
The step size for the most common types of coverage will be discussed below.
Envy the distance between the roof beams on the size of the sheet that the corrugated board has. The step of the roof frame beams under the corrugated board is taken as a standard not less than 60 cm and not more than 90 cm.
If this distance is greater, then boards with a large section are attached between the rafters. The cross section of the rafter legs for corrugated board is selected 50x100 or 150 mm.
The most important thing to consider when planning to fix corrugated board is the crate. They make it from a board with a section of 30x100mm, you need to mount it with a span of 50 cm or more. It depends on the brand and thickness that the corrugated board and the slope of the roof have.
So a roof of 15º grade C 10 corrugated board is mounted on a solid crate, C 21 corrugated board is mounted on a crate with a span of 30 cm. The largest C 44 corrugated board is mounted on a crate in increments of 50 cm, up to 1 m. passage of a chimney, ventilation hood, etc.
The specificity of the installation of a truss system for laying ceramic tiles is related to the weight of this roof. Ceramic tiles are made from clay, and this roof has a weight greater than that of a metal tile by 9-10 times. The calculation of the load on the roof system for ceramic tiles is 40-60 kg / m 2.
Beams are made for the roof truss system under ceramic tiles only from dried material. The cross section is suitable for 50x150 or 60x180 mm. The standard distance between the rafter legs when roofing tiles is 80-120 cm. The span depends on what kind of slope the roof has. At an angle of 15º, the span between the rafters is 80 cm, at 750 after 1 m 30 cm.
When calculating the step, you need to take into account the length of the beam. Taking the maximum length, the distance between the rafters is minimal. And, on the contrary, with a minimum length of rafters, the step is maximum.
When laying ceramic tiles, it is important to consider that you will need to move along the roof. The maximum safe step of the rafter legs for such a movement is 80 cm.
When laying ceramic tiles, it is important to calculate the span of the crate. This distance is directly related to the size of the roofing material. A standard tile sheet is 40 cm long. Laying occurs with an overlap of 50 to 90 mm. When calculating the step of the crate, the size of the overlap is subtracted from the length of the tiles. The result is a step of 305-345 mm.
For a shed roof for the construction of a house from a profiled beam, it is not difficult to make a calculation. If the roof is of a complex multi-pitched shape, then each distance between the rafters is calculated separately for the step of the crate. By fixing the cord on the opposite side of the roof slope, you can easily mark the rows.
Metal tiles are used more often than ceramic or corrugated board. Externally, the roof resembles ceramic tiles, but unlike it, it is easier to install and lighter.
The metal tile weighs from 35 kg / m 2 of the roof. This makes it possible to lighten the design of the rafters and use beams of a smaller section. In this case, the step between the rafter legs of the roof frame increases and is equal to 60 to 90 cm. A beam is used with a section of 50x150 mm.
To create a ventilated space in the beams under the roof, holes are drilled with a diameter of 12-13 mm.
The design of the roof frame for metal tiles is not much different from corrugated board or ceramic tiles. But there is a small peculiarity: the support from above is attached to the ridge run from above, and not from the side, as in other cases. So a ventilated gap will appear under the metal tiles, which will eliminate the accumulation of condensate.
Ondulin is a soft roof used for the construction of a house made of glued laminated timber or other material. Ondulin is produced in the form of sheets, it looks like painted slate, but light. This material is great for wooden houses, both low-rise and large.
Rafter beams under ondulin are placed with a distance of at least 60 cm, maximum 90 cm. For the rafter system, a beam of coniferous wood with a section of 50x150 or 50x200 mm is used. A smaller section will not provide sufficient strength to the rafters.
The crate on the rafters is placed from a material with a section of 40x50 mm in increments of 60 cm. This is quite enough for attaching sheets of ondulin with an overlap of 30 cm. Ondulin is fastened with special nails that are sold in the kit.
Slate is rarely used to cover the roofs of modern houses. But in summer cottage construction and households. buildings, this material is indispensable. It is low cost and easy to install.
Rafter under slate
Rafter legs for slate are used with a section of 50x100 or 50x150 mm. The fastening step between the rafters is not less than 60 cm and not more than 80 cm.
The crate for slate is made of bars 50x50 mm or a wide board 30x100 mm. The crate is laid from the steps, depending on the slope of the roof. For a steep shed roof, it is 45 cm. The consumption is 4 bars per 1 sheet of slate. For a flat shed or gable roof 63-65 cm, so the consumption is reduced to 4 bars per sheet.
The step of the rafter system under the slate roof structure differs. For the host buildings are often mounted shed.
Whether a safety margin is needed during installation depends on what shape the roof is. And the distance between the beams of the rafters directly depends on this.
Shed roofing is more durable and easier to assemble. The thickness of the rafters is selected depending on the type of wood, its strength and the specifics that this or that design has. The step between them can be 60-140 cm. The distance also takes into account whether the structure will be insulated. If yes, then the step should correspond to the width of the insulation.
The thickness of the rafters must be calculated depending on the slope of the roof. For a slight slope of 15-20º, you can use a material with a section of 50x100 mm. For a steep slope of 45º, stronger ones with a section of 50x150 mm are needed.
If the calculation of the foot step of the frame of a shed or other type of roof is not correct, then the roof can be taken away, and the beams will sag and bend due to the severity of the structure. In this case, simple repairs are not enough, you will have to redo the entire structure. Therefore, it is so important to correctly calculate the distance between the legs of the rafters, depending on the roofing material used: corrugated board, ceramic or metal tiles, ondulin or slate, etc.
The distance between the rafters is a fundamental parameter on which the strength and reliability of the roof structure, its service life, and the possibility of using certain roofing materials depend.
The strength and service life of the roof depend on many factors: the quality of building materials, climatic conditions, the reliability of the crate.
But the supporting structure of the roof is the foundation on which the entire structure rests.
The rafter system must be accurately calculated, correctly mounted and reliably protected from destructive external influences.
The truss system is the supporting structure of the entire roof. It consists of rafter legs, vertical struts and inclined struts.
Each rafter is located at a certain distance from the next - this distance is called the "rafter step".
The strength of the roof structure, the maximum allowable load per square meter and the materials that can be used for roofing depend on it.
According to GOSTs, the minimum allowable value of the rafter pitch is 60 cm, the average is more than 1 m.
To determine the approximate pitch, you can use the following formula: D / (D / m + 1), where D is the length of the roof from ridge to ridge, m is the approximate rafter pitch.
All results must be rounded up to the nearest higher integer. It is obvious that such a formula serves only for approximate calculations.
To determine the exact step size, the following factors must be taken into account:
In addition to the loads listed above, the roof must be capable of supporting the weight of at least one adult person, so that in the event of a repair or installation of an antenna, the installer can safely climb onto the roof.
If you plan to install a chimney, then its location must be included in the calculations initially so that in the future it will not be necessary to remove part of the roof and install additional support points.
For a shed roof, building a truss system is quite simple. Most often, the rafters are laid directly on the crown, without the use of additional supports and supporting structures.
That is why the maximum angle of inclination of a shed roof is limited to 30 degrees: the absence of additional load-bearing structures and supporting beams means that the entire load falls on the walls of the building and the foundation.
The optimal angle of inclination is 15 - 25 degrees. The maximum allowable span of rafters does not reach 6 m.
When building a shed roof, be sure to take into account the direction of the wind and the possible additional load from the weight of snow accumulated on the roof.
For houses located in regions with strong winds and low rainfall, you can guess the slope angle at which the roof is cleared of snow due to gusts of wind.
A gable roof is a system of two inclined slopes connected by a ridge. One of the main advantages of this design is the ability to more evenly distribute the load between the truss system and the load-bearing walls of the building.
In addition, the gable frame allows you to lean the rafters on each other, which gives it additional strength.
The overall strength of the roof structure increases as the slope angle approaches 45 degrees. It is this slope that is considered optimal for regions with heavy rainfall.
With an increase in the angle of inclination, on the one hand, stability increases significantly, so you can take a larger rafter step.
On the other hand, the windage of the roof increases, so for windy regions the optimal angle of inclination does not exceed 20 degrees.
Despite the emergence of a large selection of modern roofing materials, the classic version - slate - is still very popular, mainly because of the cheapness and ease of installation.
The distance between the roof rafters under the slate is calculated taking into account the characteristics of the material: the slate is quite fragile, but at the same time it is able to withstand heavy weight loads.
The permissible range of the rafter step for slate is from 80 cm to 1.5 m. The average step length, 120 cm, is most often used.
Since the slate itself weighs quite a lot, the material for the supporting structure must be chosen durable, for example, bars with a cross section of at least 75 mm by 150 mm.
The length of the rafter pitch and the thickness of the crate are interrelated: the more durable the crate you plan to install, the smaller the pitch, and vice versa.
The slate sheet has a standard length of 175 mm, the lathing pitch is selected so that each slate sheet has at least three support points (one in the center of the sheet and two closer to the edges).
The step of the lathing depends on the degree of the slope of the roof: 63 - 67 cm is enough for a flat one- or two-slope roof. The minimum gap for a steep roof is 45 cm.
You can set the exact length of the rafter pitch for slate only by making accurate measurements and calculating the total weight of all roofing materials.
Do not forget to take into account the weather conditions (possibility of snow accumulation, strong wind gusts) and the load of additional equipment (antenna or chimney). If an attic is equipped in the attic, then consider the weight of the heaters.
Metal roofing is one of the most popular roofing materials on the market. It is unpretentious in handling, durable, looks beautiful.
In addition, metal tile is one of the lightest roofing materials (only 35 kg per square meter), it can be laid on a fairly light support, thereby reducing the load on the walls of the building and the foundation.
The average distance between the rafters under the metal tile is 60 - 95 cm for a gable roof with a slope of 20 - 45 degrees.
The size of the bars is chosen taking into account insulating and waterproofing materials. For a simple roof made of metal, a section of 50 - 150 mm is sufficient.
But in most cases, to create a mansard roof, it will be necessary to lay a heater with a thickness of 150 - 200 mm under the metal tile.
Taking into account the weight of the insulation, the truss system should be more durable, the recommended size of the bars increases to 200 mm by 50 mm.
When calculating the distance between the rafters, consider not only the length of the sheets of metal, but the insulation.
With the correct location of the rafters, you can significantly save on timber.
Do not forget about the peculiarities of mounting a roof made of metal: this material does not pass air well, as a result of which condensation often accumulates under the roof.
Fasten the top support of the truss system to the ridge run instead of the side part. This will create a small air gap, increase ventilation and help protect the roof from destructive moisture.
A distinctive feature of corrugated board is lightness and rigidity, therefore, as in the case of metal tiles, the requirements for the rafter system are not so high.
The distance between the rafters under the corrugated board should be in the range from 60 cm to 120 cm. The optimal section of the rafter leg depends on the span between the supports.
So, for a span of 3 m, a beam with a size of 40 mm by 150 mm is chosen, for a span of 5 m - a size of 50 mm by 180 mm.
What distance is permissible between the rafter legs depends directly on the cross-section of the bars: the larger the rafter step, the more durable the material must be used for the rafters. When choosing a step of more than 80 cm, increase the thickness of the bars by 20 - 25%.
Do not forget to take into account the weight of the crate under the corrugated board. The distance between the rafters of a shed roof of 60 cm will require a crate with a minimum cross section of beams of 25 mm by 100 mm.
With a rafter pitch of 80 cm, it is 30 mm by 100 mm, etc. The angle of inclination of the roof also plays an important role: with a slope of less than 15 degrees, it is recommended to lay a continuous crate under the corrugated board, which is much heavier than a sparse one.
Because both are relatively light weight, the supporting structure can be light enough to reduce stress on the load-bearing walls and foundation of the building.
The higher the degree of inclination, the greater the distance between the rafters is allowed.
On a gable roof with a slope of less than 10 degrees, it is recommended to install a continuous crate, which increases the load on the roof structure.
In this case, it is better to use a thicker beam measuring 40 mm by 50 mm, and minimize the rafter step (60 cm).
An attic is an attic space that can be used as a living space. The mansard roof should ensure the normal functioning of such a room. In the construction of the roof, a truss structure is usually used, and the pitch between the rafters of the mansard roof is an important indicator of its reliability.
Huge loads caused by the weight of the roof, wind, climatic factors are perceived by the rafter system. The distance between the rafters of a mansard type roof determines how much of the load falls on each element. Only the right choice of distance will ensure the stability of the entire roof.
Rafters are called load-bearing beams, on which protective, additional and external coverings of the mansard roof are attached.
The elements are usually made in the form of a durable wooden beam or board with a thickness of at least 50 mm. Sometimes a log can be used. For particularly durable buildings, metal and reinforced concrete beams are used.
For the roof of the attic, one of two options for truss systems (layered or hanging) is used, as well as a combination of both options in one design. The layered type is characterized by the support of each of the rafters on the wall of the structure. The hanging type implies linking the elements into a common frame, which is fixed only on the extreme supports.
According to the type of truss system used, mansard roofs are conditionally divided into the following types: single-pitched, gable, broken, hip, hipped and vaulted. In private construction, single-pitched, double-pitched or sloping roofs are most widely used. In a shed roof, the rafters rest on walls of different heights, which ensures that the roof slopes (slope) in one direction. The gable roof has two inclined planes, with each of the rafters resting at one end on the wall, and at the other end connected to another beam. This design forms a triangle, and the angle between the elements determines the slope of the slope. The sloping roof also has two slopes, but each of them has a break line in which the laying angle changes.
When installing the attic roof truss system, fastening and installation of rafters in the form of simple geometric shapes are used. The greatest rigidity (strength) has a linkage into a triangle, which is used in the construction of a mansard roof. So, the most common gable roof includes a series of triangles of rafters, connected by longitudinal lags (girders). The linkage of the rafters into a triangle is provided by the lower transverse beam (Mauerlat). To facilitate the fastening of the outer roof covering and the redistribution of its weight on the rafters, a lattice is made in the form of transverse bars or boards.
A sloping roof combines two types of rafter connections. The lower rafters with the help of a Mauerlat and a rack are connected into right-angled triangles, which, in turn, are fastened with a longitudinal run between themselves at the top. At the bottom, the attic rafters rest on the wall of the house. The upper ones are connected into a triangle by analogy with a gable design.
Figure 1. Table for selecting the section of a beam for rafters.
The lower end of the rafter is fixed on the transverse log, and the upper ends are connected together through the longitudinal upper run. The lower corners of the bonded triangle are interconnected using a longitudinal lower run. The system formed is fixed on the lower truss system. To strengthen the upper triangles, additional vertical racks are used. Thus, the mansard roof is a surface with a break on each side. From the wall, a slope with greater steepness begins, and then it takes on a more gentle appearance.
Mansards with longitudinal bars (including floor beams) are made by cutting rafters into a bar by a third of its height. Fastening to the cross beams is advisable to produce a screw connection. With such fastenings, the functions of two different truss systems are separated, and they are calculated as separate systems.
When choosing the design of the rafter system, the size of the beam and the number of elements, it is important to take into account all the loads acting on the rafters. These loads can be divided into permanent and temporary, periodic or short-term nature. Under a permanent load, the weight of all elements of the attic roof should be taken: the truss structure itself with a grate, external roofing, additional protective and insulating layers, elements of the attic interior. The weight of the external roof can vary greatly depending on the type and material of the coating.
Natural factors should be taken into account as temporary or periodic loads. This is, first of all, the weight of snow in winter. The wind has a significant impact, and the direction of such a load can be different. For some areas, this factor may be decisive. The possibility of storm water flows cannot be ignored either. In addition, it is necessary to take into account the weight of people and materials when carrying out repairs on the roof.
The geometry of the roof and rafter system has a significant impact on the distribution of loads. The main parameters include the length and width of the roof, as well as the steepness of the slope. The length of the roof greatly affects the distribution of the load, so for long lengths it is necessary to use reinforcing vertical posts. An increase in the width of the roof leads to an increase in the load on all attic rafters, as their length and the total weight of all elements increase. For wide roofs, a broken type is more suitable due to the presence of intermediate vertical racks in them and the redistribution of loads between different truss systems.
Changing the slope slope affects the parameters ambiguously. An increase in steepness, on the one hand, reduces the accumulation of snow cover and redistributes the load on the load-bearing walls of the house, on the other hand, the length of the rafters and the windage of the roof increase, which is dangerous in windy areas. The concentration of loads on the walls can also adversely affect the reliability of the house, because with a decrease in loads on the rafters, constant loads on the masonry walls increase.
The calculation of the number of rafters and installation parameters is based on the fact that high-quality material was used during construction. In this regard, the material for the truss system should be selected based on the following conditions.
Only high-quality timber with a section of at least 50x100 mm should be used as the main rafters.
All wooden elements during installation must be well dried (permissible moisture content - no more than 15%). The number of even small defects on the beam cannot exceed 3 pieces per 1 m. The tree is treated with an antiseptic before installation. Coniferous wood performed the best. Vertical racks are made of a bar with a size of at least 100x100 mm with a check of their vertical location using a plumb line.
After choosing the design of the mansard roof (based on the recommendations of experts and in accordance with the reference data), the main design parameters are the distance between the rafters () and their number. Usually the distance between the rafters is from 0.6 to 1.5 m. The calculations are based on the fact that the optimal load should be 40-60 kg per 1 m of the rafter length, and the maximum allowable beam deflection is 1/250 of its length.
The number of rafters per slope is calculated after measuring the length of the slope and selecting. The length of the slope is divided by the step value, 1 (one) is added to the result. The result is rounded up to the nearest whole number.
A specialist can calculate the distance between the rafters, taking into account all factors, but in practice they use reference recommendations. So, for example, for rafters from a board measuring 50x180 mm and a slope length of 3 m, the average step is 1.5 m; with a length of 3.5 m - 1.2 m; and with a length of 4 m - 0.9 m.
The distance between the rafters varies significantly for roofs with different coatings. Ceramic tiles are one of the heaviest roofing materials. For rafters from a bar measuring 50x150-60x180 mm, the recommended distance between them is 80-130 mm (depending) on the steepness of the slope. With a slope of 15 °, the pitch is chosen to be 80 cm. With an increase in the length of the rafter, the pitch is increased within the recommended range.
The distance between the rafters for roofs with metal roofing is set smaller than for natural tiles. The optimal step is 60-95 cm for a bar measuring 50x150 mm. When using a corrugated board coating, the step is in the range of 60-90 cm with a sufficient beam cross section from 50x100 mm to 50x150 mm.
The lightest coating is obtained when using ondulin. The optimal distance between rafters measuring 50x50 mm is 60-80 cm and decreases when a larger beam is installed. When covering the roof of the attic with slate, a beam measuring from 50x100 mm to 50x150 mm is used. The step is set in the range of 60-80 cm.
When installing rafters on a mansard roof, the following tools are used:
When installing a truss system on a mansard roof, it is important to determine the optimal distance between the rafters. The correct choice of this parameter will allow you to calculate the optimal amount of material and ensure the reliability of the entire roof.
