Types and diagrams of rafter systems: an overview and recommendations for the installation of a roof rafter system. Frame roof: construction of the rafter system, calculation and installation of the structure For mansard roofs

A rafter system is a structure that provides the strength of the roof and serves as the basis for laying roofing material. She is shown in the photo.

The roof is a load-bearing structure that performs the following functions:

gives the structure a beautiful appearance;
takes on external loads;
protects the attic from the outside world;
transfers the load from the battens and the material on it to the walls of the building and internal supports.

The main elements of the roof include lathing, rafters and Mauerlat. Also, the supporting structure includes additional fasteners - crossbars, racks, rafter struts, spacers, etc. The reliability and strength of the roof is most influenced by the rafter system. The rafters are the main load-bearing part of the roof. The rafter system has weight not only roofing but also snow cover, wind pressure. It must withstand all these influences, therefore, the calculation is made taking into account the type of roofing material and the climatic characteristics of the region.

The structure of the rafter system

Connecting the rafters to each other stiffens the roof frame, and the result is a strong rafter structure. The load on the rafters can be quite significant, for example, during strong winds, so the frame is tightly tied to the frame of the building.

In the construction of private houses and cottages, wooden truss systems are usually used, which are easy to manufacture and install. If mistakes were made during the construction of the walls, these products can be easily processed: shorten, increase, hang, etc.

During installation, fasteners of the rafter system are used: bolts, screws, clamps, nails, staples. They are also used to reinforce the load-bearing roof structure. The interconnected elements of the roof create a truss truss, which is based on triangles, which are the most rigid geometric figure.

When choosing a material for making a system from rafters, it is necessary to take into account the structural and architectural nuances of the project. Do not forget about antiseptic and fireproof impregnation for them, as this affects the durability of the roof.

The system consists of rafter legs. Install the rafters at an angle of inclination of the roof slopes. The lower sections of the rafter legs are supported on the outer walls with the help of a Mauerlat, which contributes to an even distribution of the load. The upper ends of the rafters are supported on a bar under the ridge or on intermediate fittings. The strut system transfers the load to the load-bearing internal walls.

Types of rafters

Hanging rafters consist of the following elements: rafter leg, attic overlap, crossbar. These rafters only have two extreme anchor points. They rest directly on the walls of the house. Rafter legs respond to both compression and bending. These rafters are without braces. See also: "Supporting the rafters on the floor beams."

The design of the hanging rafters transfers a significant horizontal expansion force to the walls. In order to reduce the load, a stretch is used, with the help of which the rafter legs are connected. They do it either at the base of the rafters, or at a higher height. The stretch at the base of the rafters is at the same time the floor beam - this is important when creating mansard roofs. With an increase in the height of the brace location, it is necessary to increase its power and make sure that it is securely attached to the rafters.

Part layered rafters includes: rafter leg, mauerlat, headstock, brace, tightening. This type of rafter is installed in buildings in which there is a middle load-bearing wall or intermediate supports in the form of pillars. Elements of this design work only for bending, performing the function of the heads. The weight of the layered rafter system is less, and less materials are also required, so it is cheaper than hanging system.

Installation of a layered system is done if the supports are no more than 6.5 meters apart from each other. If there is an additional support, the rafters sometimes overlap the width of 12 meters, and if there are two supports, up to 15 meters.

Rafter legs are most often based not on the walls of the building, but on a special beam - the Mauerlat. This element can be located along the entire length of the house or be placed only under the rafter legs. If the structures are wooden, a log or timber is taken for the Mauerlat, which is the upper crown of the log house.

At brickwork Mauerlat walls are a bar installed flush with the inner surface of the walls, fenced off from the outside by a masonry ledge. A layer of waterproofing is laid between this element and the brick - for example, you can put roofing material in two layers.

If the width of the rafters is small, they can sag over time. To prevent this from happening, use a lattice consisting of a rack, a crossbar and struts. At the top of the structure, a girder is laid, which connects the rafters or trusses. This is done regardless of the type of roof. Subsequently, on this run, the ridge of the roof is made. In places where there are no load-bearing walls, the heels of the rafters rest against the side girders - longitudinal beams significant power. The dimensions of these parts depend on the expected load.

In the construction of private houses, log rafters are used - they are lighter. To create roofs on multi-storey residential buildings and industrial buildings, metal rafters are used.

Installation of roof systems

The slope angles of the slopes are selected based on the type of building and the purpose of the attic space. The amount of slope is also influenced by the material chosen to create the roofing.

If roll products are to be stacked, the angle of inclination should be 8-18 degrees. For tiles, the required angle is 30-60 degrees, for roofing steel or asbestos-cement sheets - 14-60 degrees.

The installation of the rafter system begins after the erection of the load-bearing walls of the house (for more details: "Installation of the rafter system"). Wooden rafters construction log house significantly differs from systems for houses made of aerated concrete, brick, timber frame or panel houses... The differences are significant even with the same shape, type and type of roof. As for how to treat the rafter system, it is necessary to use antiseptic and fire-fighting agents so that the roof will serve long time.

The main elements of the supporting structure are roof trusses and lathing. The roof is the outer part of the roof, which is laid on a supporting structure consisting of battens and rafters.

For the production of rafters, they take material a certain size... So, the thickness of the rafters (section) is most often 150x50 and 200x50 millimeters. For the lathing, they usually take beams and boards measuring 50x50 and 150x25 millimeters. The distance between the rafter legs is on average 90 centimeters. If the roof slope is more than 45 degrees, this step is increased to 100-130 centimeters, and if a huge amount of snow falls in the region, then it is reduced to 60-80 centimeters.

To make more accurate calculations regarding the gap between the construction legs, you need to take into account their section, the step between the supports (struts, ridge run, uprights), the type of roofing material.

The floating rafter system is attached using special brackets that allow the rafters to "sit down" together with the shrinkage of the gables and not hang over the ridge log.

Installation of the rafter system, see the video:

If the rigidity is provided by the trusses, the rafters are reinforced with diagonal ties (for more details: "How to strengthen the rafters - options for strengthening the rafter system"). For them, boards with a thickness of 3-4 centimeters can be used, nailed at the base of the extreme rafter leg and in the middle part of the adjacent one. Rafter legs are the main element of the system, so they account for most of the roof load. For this reason, the system must be correctly calculated and installed in order for the roof to be reliable (see also: "How to put rafters on a house").

The installation of the rafter system must be carried out, strictly observing all the requirements. If you do not have experience in construction, it is better to entrust the construction of the roof to specialists, since this is not an easy task, and the slightest mistakes can lead to its collapse.

Source: kryshadoma.com

The frame format stands out among a number of types of roofing. It can be performed in a rather different way, but in any case, you will need to calculate the rafters and install them according to all the rules. With proper knowledge of the problem, you can solve it on their own without resorting to specialists.

Features and shapes of the roof

Frame roof can be installed only with spans no longer than 1220 cm, while the gap from one truss to another is a maximum of 0.6 m. The dimensions of the frame fragments are determined by the spans and the calculated snow load. The rafters can be either freely installed or take the load from the attic elements. In the case of a broken roof, it is possible to provide a ceiling height sufficient for a residential attic, and it will look best on a square structure.

The multi-gable roof is considered the most difficult and hardly affordable variation for amateur builders. The balanced rafter system effectively withstands even very high loads, while having an excellent "appearance". Since the slope is steep, the risk of snow retention will be minimal. But at the same time, you will have to very carefully calculate all the structural elements, and in the process of work there will be a lot of waste. In addition, the valley will have to survive the impact of a significant amount of snow.

Purpose and types of systems

Mauerlat can be used in a wide variety of rafter systems. The mass of the roof of the house differs depending on the area occupied by the slopes and on the materials used. But in any case, the created load is very solid. When there is a ridge on the structures, a rafter frame is necessarily provided, with its feet resting on the walls. The force is applied in several directions at once, and in the cold season, the accumulation of snow only exacerbates the problem.

Mauerlat aims to eliminate this deficiency and prevent walls from collapsing. This word means a beam of significant section, which can be both wood and steel. In most cases, they take the same material that was used to form the rafters, but they necessarily achieve the continuity of the strapping or create strong and especially stable joints. They refuse to use the Mauerlat only in log houses or in buildings built using frame technology - and even there they have their own parts that perform a similar task. When it is not possible to make a continuous block, all fragments must be strictly the same length.

The T-shaped roof is characterized by the inset of two wings at a certain angle. Because of this, it is necessary to form a valley. External rafters will rest against support boards... In addition to them, there will also be basic parts directly attached to the wall. To make everything in the valley correspond to the task being solved, wooden elements with a thickness of 3.8 cm are used.The lathing is supposed to be made monolithic, the coating is attached to it with clamps every 50 cm.A typical Mauerlat is three times less thick than the load-bearing wall, and if it is made made of steel, you can slightly reduce this figure.

A reinforcing belt is often equipped under the Mauerlat. This is especially important if you plan to insulate the roof and provide reliable waterproofing. Such a belt is formed from the same mixture that is used to build the foundation. Completely the entire formwork is poured with concrete in one step, the slightest separate layers are unacceptable. In the aerated concrete wall, intermediate jumpers are cut down at top line blocks - and immediately a practical chute appears. The attachment of the Mauerlat is done either with a knitting wire, or with reinforcing bolts (but they will not help in any way without a reinforcing belt), or with construction pins.

Having dealt with the support for the rafters, you need to find out what they can be, and what is more correct to use to support the roof. Hanging rafters are used if there is no main wall inside the building, their support points are located exclusively on the outer contours.

Such props were in demand during the construction:

residential buildings with one span;
production facilities;
various pavilions;
attics.

This option should not be underestimated, thanks to engineering developments, suspended rafters are able not to bend, blocking spans of 15-17 m. But it is important to understand that they acquire all their capabilities only in close interaction with other details. We'll have to use puffs, and headstock, and crossbars. The simplest truss is made of two beams connected in the upper position, according to the configuration, such a device is close to a triangle. The horizontal connection of the frame parts is provided by tightening (a beam made of wood or metal profile).

Due to the tightening, the transfer of thrust to the walls is excluded, while the force applied in the horizontal plane is suppressed. External walls they experience the action of only those forces whose vector is oriented vertically. Builders do not always place a puff at the very bottom, often it is exhibited at the very ridge. When preparing for the construction of the attic, this element is most often placed higher than the base of the rafter legs. Then it will be possible to make a floor, against the ceiling of which you will not have to bang your head for any careless movement.

Hanging rafters for spans longer than 6 m must be reinforced with hangers and braces. In this case, the monolithic tightening is replaced by one assembled from a pair of connected beams. V classic pattern(triangular articulated) lower bases abut against horizontal parts. For the system to function properly, the ridge height must be at least 15% of the span of the trusses. The rafters act on the bend, but the tightening does not allow them to move to the sides. To make the beams bend less, the ridge nodes are cut in with the expectation of eccentricity (the occurrence of a bending force opposite in the vector).

Mansard attics are built mostly with triangular arches on three hinges, and the puffs are assigned the function of floor beams. The constituent parts of the tightening are bolted by means of an oblique or direct cut. The raised fastening can also be used in the construction of rafters under the attic. The higher it rises, the more the ceiling can be raised. But it is important to remember that at the same time the loads on all elements grow. The transfer of forces is made to the Mauerlat using a movable mount that dampens changes in dimensions from changes in humidity and temperature.

The rafters can be subject to uneven loading, since it is higher on one side. This leads to a shift in the same direction for the entire system. You can eliminate such an unpleasant effect if you take out the rafters outside the contour of the walls. With such a solution, tightening ceases to be a support, it transfers either tensile forces (if an attic is arranged), or stretched-bending (when an attic is being built). Hinged arches with the inclusion of a deadbolt differ from the previous version by replacing the sliding support with a rigid support that is identical in function. Due to the change in the type of supports, the type of stresses generated also becomes different, the rafter system turns into a spacer.

The tightening is formed in the upper lobe of the arch. Its purpose is not to endure stretching, but compressive force. An additional tightening, reinforcing the deadbolt, is needed at a significant load. Arches with hangers and struts complement the system of arches with "headstock". Such a system is needed for significant spans (from 6 to 14 m). The braces that correct the resulting bend must be resting against the headstock. Regardless of the specific type of rafter system, it is necessary to carry out all the details and their connections with each other as clearly as possible.

Not always hinged rafters can fulfill the task. Then the nipple elements come to the rescue. This type of rafters is used under hip roofs and under roofs equipped with a valley. Their length is longer than usual. In addition, they become supports for the shortened rafters of the slopes. That is why the overhead rafters have a load of about 50% more than in other structures.

Due to the increased length, it is possible to:

resist significant impacts;
form beams without cuts;
bring parts to a uniform size by pairing boards.

To build hip roof with many spans, the diagonal legs are provided with supports. Such supports are made in the form of standard struts or racks from a bar, or a pair of connected boards. The support is made through a wooden lining and a waterproofing layer directly on the reinforced concrete floor. The braces are placed at an angle of no less than 45 and no more than 53 degrees, at the bottom such a part rests on the beds. The angle of installation is less important than the ability to fix parts of the rafters at the point that is experiencing the strongest stress.

Slant rafters placed in openings up to 750 cm should be supported by struts only in the upper lobe. With a length of 750 to 900 cm, a truss truss or rack is additionally mounted below. And if the total length of the span exceeds 9 m, then for maximum reliability you need to put a stand in the middle, no other support will fit. If the selected floor is unable to withstand the load, you will have to reinforce it with a beam. The type of support in the ridge is determined by how many intermediate supports are used, what they are, how the key layered rafters are made.

In addition to the type of rafters, you need to clearly understand their material. Both wooden and metal structures can be good, but only each in its place. Even the high strength of the metal does not allow to push aside the usual wood. The tree has confidently proven its advantages over the millennia, and now it is even growing in popularity due to its excellent environmental characteristics. Boards and beams can be purchased at affordable price, and if something has not been taken into account, it is always easy to cut off the desired fragment or build up a part right on the construction site.

Sometimes there are problems associated with the operation of the created structures. Wooden rafters will have to be carefully treated with antiseptics, as well as means that block the development of mold colonies, eating by insects. The combustibility of wood is suppressed due to regular processing, and in addition, it is too difficult to find the necessary components for slopes longer than 7 m. Before installation, the walls are laid with a Mauerlat made from a log frame or based on a block of timber. The thickness of the structures is at least 180 mm, this is the only condition for a uniform distribution of loads.

Metal rafters are inevitably heavier than wood with an identical cross section. Therefore, the walls need to be strengthened, the work on their construction becomes more expensive and longer. It will not be possible to mount metal blocks manually, they will definitely be required lifting cranes... It is impossible or very difficult to adjust the dimensions and geometry of the rafters, therefore, you will immediately have to construct the walls as accurately as possible and eliminate errors in their construction. The slightest mistake can make an expensive block almost useless in practice.

Metal rafters are connected by welding, and welded joints are inevitably weakened, since corrosion develops there accelerated. The cost of work is very high, and when performing them, it is necessary to fulfill the requirements of fire and electrical safety. But there is such an indisputable advantage as the ability to support the roof slope from 700 cm and longer. If you use a special anti-corrosion paint, the durability of metal structures is fully ensured. All these advantages make it possible to quickly and comfortably build industrial buildings with significant heights and span lengths.

How to choose: what to consider?

The rafter system should be chosen as correctly and clearly as possible.

When looking for a suitable solution, you need to pay attention to the following points:

strength;
the ability to support the slopes and the roof as a whole of a certain size and geometry;
creating a positive aesthetic image of the building as a whole.

Technical parameters take priority. Even the most beautiful designs that follow the design principles will not show their positive qualities if they last too little. Experienced builders always analyze the average annual and seasonal temperatures, the financial capabilities of the developers, the maximum possible wind rate and the severity of the overlying roof. The future use of the roof space and the scale required for it are also taken into account. Wind, snow and rain cannot be underestimated, since these factors can have a very strong effect on the roof, and through it, on the rafters.

If it is reliably known that a particular area is characterized by heavy snowfall, the minimum slope angle is impractical. This point is even more relevant when using flat roofs. Under the pressure of the accumulating sediments, the frame can rapidly deform or water will pour inward. Another thing is when a certain region is often subject to the arrival of cyclones and the strong winds they bring. Here, the ramp should be made smaller, then the situation with the breakdown of single structural elements will be practically excluded.

You can avoid mistakes if you look at houses that have already been built nearby and have been in use for a long time. By accurately reproducing the structure of their roof and the rafter system interconnected with it, you can the best way take into account local specifics. But not everyone follows this path, sometimes the task is to develop an exclusively original project. Then you will have to carefully collect the initial data, carry out scrupulous calculations. In the absence of special knowledge, it is better to attract qualified performers to help.

After analyzing the total load created by wind and snow, you can sometimes find that certain parts of the rafter complex need selective reinforcement. When assessing the required angle of inclination of the roof, attention is also paid to the type of coating used. Heavy metal tile or corrugated board with a very large slope can spontaneously slide down, you have to additionally fix them, complicating your work and increasing the cost of installation. In addition, some materials have a tendency to retain water or soak in it; this can only be dealt with by making the slope steeper. Creating a good roof and rafter system that meets such conflicting requirements is not always available to the layman.

What does it consist of?

The device of the rafter system, as it is easy to see, is rather complicated and even contradictory. Each part of this structure has a strictly defined role. So, the Mauerlat is a long bar made of softwood, and strictly resinous wood is used for work. Such elements are laid out along the external load-bearing walls, attaching to the base with anchors or rods of a special design (threaded). This part transfers the load from the roof to the wall.

This is followed by such a device as a rafter leg. Under this name, a wooden beam appears, used to erect the contour of the slopes. The shape of the structure is always triangular, because it best helps the roof to withstand the destructive effects of winds, snow and other atmospheric processes. Rafter legs are placed at even distances along the entire roof, the step cannot exceed 120 cm.

The bed is of a certain importance for the support of the roof - it is wooden block replacing the Mauerlat in some cases. The beds are placed on the inner support walls. They become the base of the roofing triangle. Thanks to them, the slopes do not creep under their own weight. And it is also worth mentioning about the racks - these are vertical bars with a square section. They perceive the pressure that the ridge knot exerts downward and transmit it mechanically to the inner bearing plane. Sometimes the racks end up under the rafter legs.

The braces are designed to strengthen the entire roof structure, they tie the legs and the legs into one whole. This detail resembles a rhombus in shape. The community formed by the puff and braces is called the farm. In addition to them, you also need a crate, which is a thin board stuffed at right angles to the legs of the rafters. It helps to keep the rafter legs like unified system... Absolutely any roof covering is attached to the crate.

For soft materials, the lathing should be made inseparable, and the best remedy plywood is considered. At the very top is the ridge, which logically and physically completes the roofing triangle. The connection of a pair of opposed rafter legs is provided by a square timber that prevents the roof from collapsing altogether. And at the very bottom of the pitched roof there is an overhang, which is displayed approximately 0.5 m from the perimeter. Thanks to him, the streams of rain leaving the roof do not flood the outer bearing planes and do not harm them.

Fillets are used only in a situation where the rafter legs cannot be performed along the length that would allow organizing the overhang. The connection with low-cut planks effectively solves this problem. For fastening wooden elements rafters are most often recommended to use clamps, staples. It is undesirable to use nails, because the tree pierced by them becomes weak and fragile after a few years. Therefore, if professionals use connections made directly on the construction site, they use bolts.

But even a bolted bond weakens building structures, albeit relatively slightly. The connections are the strongest with the help of clamps or metal staples. Only their industrial production can maximize the quality of products, since only in strictly standardized and fully controlled conditions deviations from the norms, deterioration of quality are excluded. It is possible to assemble a truss structure from completely finished trusses very quickly, there is no risk in using it. Another thing is that it is required to collect information about the required characteristics as accurately as possible and transfer it to the manufacturer without distortion.

In addition to the named elements, the rafter system borders on the valley. This is the name for a special connection of a geometrically complex roof at the points where its trajectory changes. The difference from the ridge is that in such places the roofing parts form a negative angle. The technical essence of the product lies in the fact that the chute helps to drain the liquid aside. The more complex the configuration, the more should be the number of such gutters.

The cornice beam serves to abut the spacers, the other end of which abuts against the frontal board, while the drip is not deformed and its configuration is not distorted. Wind ties are those elements of the rafter system that transfer the load generated by the wind from the roof to the foundation. They not only increase the overall stability of the structure, but also help to avoid its overturning when single parts are unstable. The roof will retain its spatial rigidity even in very strong winds.

Horizontal wind ties are elements such as:

braces;
parabolic tightening;
complexes of conventional puffs;
trusses complemented by a cross-shaped lattice.

Vertical retention of characteristics under strong air pressure is ensured by wind supports and beams. Sometimes a monolithic reinforcing core is used. Engineers have come up with many other design options for wind communication. It is provided with frames and semi-frames, clamped by supports. In small buildings, rigid (resisting compression) or stretched diagonals are used, some cover two spans at once. The location of each of the elements is accurately reflected in the design documentation.

Qualitative characteristics roof systems and their composition is not so difficult to understand if you show attentiveness and diligence. But it is equally important to calculate the quantitative parameters of these systems. If you do not do this or carry out the calculations incorrectly, you can either spend too much money, or face leaks, even with destruction. individual elements.

roof bends;
average annual snow mass;
irregularities in its distribution along the slopes, depending on the steepness of the slope and the wind rose;
wind transfer of already fallen snow;

the descent of snow and ice masses, the flow of liquid water downward;
aerodynamic performance and windage of the structure;
differences in the force of impact on individual points.

Calculating everything you need, moreover, simulating realistic situations and laying a reasonable margin of safety in the project is not so easy. Moreover, it is necessary to pay attention to the addition of various loads, to their combined effect. But still, any customer is quite capable of assessing the quality of the designers' work. The loads applied to the rafter systems are divided into three key groups: main, additional and extreme.

The main category includes:

stable factors - the severity of the roof and rafter structures, additional elements installed on top of them;
long-term effects - snow, temperature;
periodically changing factors - complete calculations of snow and temperature effects, taking into account all the subtleties.

An additional group is the pressure exerted by the wind, builders and repairmen, ice and rain. The extreme category includes all natural and man-made emergencies that may occur in a particular place. Their level is forecasted with a margin to guarantee exclusion unpleasant consequences... When calculating the frame roof and the structures under it, the ultimate load is taken into account, in the case of which the entire structure crumbles. Additionally, an indicator or a group of indicators is given, upon reaching which various deformations inevitably appear.

The drift rate reflects how much more snow will be deposited on the leeward side and in front of objects (parts) that trap the flow of air. V problem areas you will have to bring the rafters as close as possible and thoroughly calculate the required thickness of the front material. The most accurate assessment of all parameters can be given only by multiplying any obtained figures by the reliability factors. As for the wind, the force it develops is aimed at dropping steep roofs and lifting from the leeward section of a flat roof. We must not forget that the air flow acts simultaneously on facades and roof slopes.

When it hits the facade, the air splits into two waves: one goes down and is no longer of interest, and the other presses tangentially on the roof overhang, trying to raise it. The action on the slope occurs at a right angle, this area is pressed into the depths. At the same time, a vortex is formed, which tangentially affects the windward sector of the slope. This vortex bypasses the ridge and begins to create lift when applied to the leeward segment. For your information: when calculating the mass of the roof, you need to take into account the severity of the rafters, insulation, waterproofing and vapor barrier.

The standard load per square meter of the roof is up to 50 kg, regardless of its size and other significant circumstances. By changing the distance from some rafter legs to others, you can set the actual distribution of loads on them. According to most experts, acceptable values will be from 60 to 120 cm. But on an insulated roof, it is worth choosing such distances that are equal to one sheet or roll of insulation materials. At the same time, it should be borne in mind that among several suitable options for placing the rafters, the one that gives the optimal effect with the minimum consumption of the materials used is preferable.

When calculating the loads carried by rafters, they always look to ensure that they do not exceed the maximum endurance of the roofing material. After all, there is no point in such an excess. If, with the planned impact, the roof still begins to sag, it is impossible to talk about a solid result. In the calculations, the payload from the structures connected to the trusses of the rafters is calculated according to the area of contact that is plotted on the drawing. Such structures include ventilation chambers, attic and first floor ceilings, water reservoirs placed on the roofs. In addition to the magnitude of the pressure on the rafter system, the sharpness of the roof slope is also calculated.

Tilt angle: value

On the forums, with the advice of specialists and in professional literature, one can find references to three units of slope measurement at once. In addition to the usual and expected degrees, there will be both percentages and ratios between the parties. Often they get along together even within the same publication or instructions from the manufacturer of roofing materials. But in fact, there is nothing mysterious about this, any consumer can understand the essence. Experts understand the angle of inclination of the roof as the angle that occurs at the intersection of the horizontal with the roof slope.

In this case, obtuse angles cannot exist in principle. Moreover, you can meet a slope steeper than 50 degrees only in decorative elements, all kinds of turrets. The only exceptions to the general rule are the ramps on the lower rows of rafters. mansard roof... In all other cases, the angles range from 0 to 45 degrees. The relative aspect ratios are calculated as the ratio between the height of the ramp and its projection onto the horizontal. This figure is equal to half the span for a uniformly constructed roof with a pair of slopes.

On a pitched roof, the proportion is equal to one, but in more complex configurations, you will still have to carry out all calculations and estimates yourself, without starting from ready-made values. The draft angle is usually expressed as a fraction, the numerator and denominator are separated by a colon. But when the resulting figures cannot be rounded to whole numbers, it is recommended to use percentages: they simply divide one by the other and increase by a hundred times. Flat roofs those that have a slope of no more than 5 degrees are considered; a slope of 6-30 degrees is recognized as small, and all other roofs are considered to be steep. Flat design radically increases useful area and is quite wind resistant, but you will need to clean it of snow by hand and reinforce the waterproofing to the limit. The slope must be consistent with the specific material, and the required values can be found in the instructions from the manufacturer. To calculate even the most complex and bizarre roof configurations, they are mentally broken into triangles and the angle in each is calculated separately.

Step, length and section of rafters

When it became clear how long the slopes were, what the angles formed by these slopes with a horizontal plane, it was time to start actually calculating the rafters. If the roof frame is made of 5x15 cm timber under the metal tile, the installation step fluctuates from 0.6 to 0.8 m. As the slope increases, the gap also increases. If the roof is tilted at 45 degrees, rafters are required every 800 mm, and for slopes of 75 degrees, another 200 mm can be added.

Next important parameter Is the length of the rafters. It is closely related to the step: if the blocks are made long, they are brought closer together as much as possible, and when a single piece is shortened, they are pushed apart. When calculating the pitch of the lathing, they proceed from the view of the tiles laid on top and from the fact that an integer number of rows should be laid out on each slope. If you get a fraction, it is better to round, decrease or increase the indicator slightly. The rafter legs under the metal tile, the section of which is 15x5 cm, range from 65 to 95 cm.It is impossible to increase the step when the sheathing section is 3x5 cm.

To make the insulation better ventilated, in the area of the upper edge of the rafters, rows of holes with a diameter of 1-1.2 cm are prepared. Ordinary rafters under the corrugated board go every 0.6-0.9 m. with a significant cross section. The lathing under the corrugated board is assembled from boards with dimensions of 3x10 cm, which are placed at intervals of 0.5 m. The interval must be calculated according to the height and thickness of the materials.

With all the revealed shortcomings of slate, it remains widely in demand. Under the slate roof, rafters with a section of 5x10-15 cm are mounted, 60-80 cm apart from each other. Most often, an average distance of 0.7 m is recommended. The pauses between the parts of the sheathing are calculated in accordance with the steepness of the material. In relatively flat areas, the support of 4 pieces of wood pays off. If the roof is steeper, put 3 bars, separated by 63–65 cm.

We must not forget that due to the responsibility of the rafter system, it is better to leave a reserve of strength than to make an unreasonably weak type of rafters. For their manufacture, a bar is used, dried to a maximum of 15%. A non-edged board of the same dryness can serve as a replacement for a timber. Under the ceramic tiles, a lathing of a 5x5 cm bar is used.In the places designated according to the calculated distance, nails for slate or simple self-tapping screws are used.

Installation: technology

Roof construction involves the use of a standard range of carpentry tools and electric drill... If metal structures are used, a grinder is required for precise cutting. Remember that it is impossible to process metal tiles or corrugated board with it, this can lead to damage to the material. A hipped roof without pillars is made using puffs that strengthen the structure.

In the hip version, it is necessary to strengthen the diagonal runs. Paired boards and a particularly strong beam are taken away to them. The connection points always have a support (stand), and the main support is placed about a quarter of the length that separates the large rafters from the ridge. Under the gables on the gable roof, shorter rafters are always made. But under the main part of the four-slope structure, extremely long parts can be placed, even more than 7 m. To keep them securely, they use either a stand that transfers the voltage to the floor, or a truss.

The first step in creating rafters under a sloping roof is the formation of a support complex in the form of the letter P. It rests on the floor beams and is held by the rafter legs. Next, they put three or more runs, two of them are brought out to the corners of the frame, and the rest are placed in the middle of the overlap. The final step in the rafter is to secure the legs. It is advisable to make rafter systems according to a pattern - connect two boards, which coincide in length with the rafters, and nail them together with a nail. The template is placed with its edges at the points of attachment of the rafter legs and fixed with a crossbar.

An additional template (this time plywood) will help you make a cutting saw. Farms are attached to the Mauerlat, starting with the extreme ones. In order not to get confused with the point of attachment of the skate, the tops of these trusses are tied with a straight rope. The massiveness of the puffs increases as you approach the ridge. If the rafters are bolted, washers or plates should be used. This will prevent the nuts from sinking into the wood.

How to install rafters with your own hands, see the video below.

When designing any residential building, architects pay special attention to the roof, since it performs not one, but several functions at once, depending on its design features. It must be said that not all future homeowners are satisfied with the usual gable roof, although it can be called the most reliable, since it has only two pitched planes and one joint between them. Many are attracted by more complex structures, which add special appeal and originality to the structure. Other, more practical homeowners prefer roof structures, which are simultaneously capable of performing the role of a roof and a second floor.

The basis of any roof is an individual rafter system, which has its own design features... It will be much easier to make the choice of the desired roof frame if you figure out in advance which types and diagrams of rafter systems used in construction practice. After receiving such information, it will become more clear how complex such structures are in installation. This is especially important to know if the roof frame is supposed to be erected independently.

The main functional tasks of roof systems

When arranging pitched roof structures, the rafter system is a frame for covering and for holding the materials of the "roofing pie". With proper installation frame structure will be created the necessary conditions for correct and non-insulated types of roofs that protect walls and the interior of the house from various atmospheric influences.

The roof structure is also always the final architectural element of the exterior design of the building, supporting its stylistic direction with its appearance. Nevertheless, the design features of the rafter systems must first of all meet the strength and reliability requirements that the roof must meet, and only then - the aesthetic criteria.

The frame of the rafter system forms the configuration and angle of inclination of the roof. These parameters largely depend on natural factors specific to a particular region, as well as on the desire and capabilities of the homeowner:

Precipitation in different periods of the year.
The direction and average speed of the wind in the area where the building will be erected.
Plans for the use of the space under the roof - the arrangement of residential or non-residential premises, or using it only as an air gap for thermal insulation of the rooms below.
A type of planned roofing material.
The financial capabilities of the homeowner.

Atmospheric precipitation and the strength of wind flows give a very sensitive load on the roof structure. For example, in regions with heavy snowfalls, you should not choose a rafter system with a small angle of inclination of the slopes, since the snow masses will linger on their surface, which can lead to deformation of the frame or roofing or to leaks.

If the area where the construction will be carried out is famous for its winds, then it is better to choose a structure with a slight slope of the slope so that the sudden gusts that occur do not tear off individual elements of the roof and roof.

Basic elements of the roof structure

Details and assemblies of rafter systems

Depending on the type of truss system chosen, the structural elements used can vary significantly, however, there are details that are present in both simple and complex roof systems.

The main elements of the rafter system of a pitched roof include:

Rafter legs that form the roof slopes.
- a wooden beam, fixed on the walls of the house and serving to fix the lower part of the rafter legs on it.

The ridge is the joint of the frames of the two slopes. It is usually the highest horizontal line of the roof and serves as the support on which the rafters are anchored. The ridge can be formed by rafters, fastened together at a certain angle, or fixed on the ridge board (run).
The lathing is slats or beams mounted on rafters with a certain pitch and serving as the basis for the flooring of the selected roofing material.
Retaining elements, which include benches, girders, racks, struts, ties and other parts, serve to increase the rigidity of the rafter legs, support the ridge, and tie individual parts into a common structure.

In addition to the aforementioned structural details, it can include other elements, the functions of which are aimed at strengthening the system and optimal distribution of the roof loads on the walls of the building.

The rafter system is divided into several categories depending on different features its design.

Attic space

Before proceeding to review different types roofs, it is worth figuring out what the attic space can be, since many owners successfully use it as utility and full-fledged living quarters.

The design of pitched roofs can be divided into attic and attic. The first option is called this way because the space under the roof has a small height and is used only as an air gap that insulates the building from above. Such systems usually include or has several slopes, but located at a very slight angle.

The attic structure, which has a sufficiently large ridge height, can be used in different ways, be insulated and not insulated. These options include the attic or gable version. If a roof with a high ridge is chosen, then in mandatory it is necessary to take into account wind loads in the region where the house is built.

The slope of the slopes

To determine optimal slope slopes of the roof of the future residential building, first of all, you need to take a closer look at the already built low-rise neighboring houses. If they have been standing for more than one year and can withstand wind loads, then their design can be safely taken as a basis. In the same case, when the owners set the goal of creating an exclusive original project, unlike the buildings standing nearby, it is necessary to familiarize yourself with the design and operational features of various rafter systems and make the appropriate calculations.

It should be borne in mind that the change in tangential and normal values \ u200b \ u200bof the wind force depends on how large the slope of the roof slopes is - the steeper the angle of inclination, the greater importance have normal forces and less tangential forces. If the roof is sloping, then the shear wind load affects the structure more, since the lifting force increases from the leeward side and decreases from the windward side.

The winter snow load must also be taken into account when designing the roof. Usually this factor is considered in conjunction with the wind load, since the snow load on the windward side will be much lower than on the leeward slope. In addition, there are places on the slopes where snow will necessarily collect, giving a large load on this area, so it should be strengthened with additional rafters.

The slope of the roof slopes can vary from 10 to 60 degrees and should be selected not only taking into account the consolidated external load, but also depending on the roofing to be used. This factor is taken into account because roofing materials differ in their weight, their fixing requires a different number of elements of the rafter system, which means that the load on the walls of the house will also differ, and how large it will also depend on the angle of the roof slope. Of no small importance are the features of each coating in terms of resistance to moisture penetration - in any case, many roofing materials need one or another slope to ensure free descent storm water or melting snow. In addition, when choosing a roof slope, you need to think in advance about how the cleaning process will be carried out and renovation works on the roof.

When planning this or that angle of the roof slopes, you need to know that the fewer joints between the covering sheets, and the more airtight they are, the less you can make the slope of the slope, of course, if it is not supposed to arrange a residential or utility room in the attic space.

If a material consisting of small elements is used to cover the roof, for example, ceramic tiles, then the slope of the slopes must be made steep enough so that water never lingers on the surface.

Given the weight of the roofing material, you need to know - the heavier the coating, the greater the angle of the slopes, since in this case the load will be correctly distributed to the rafter system and load-bearing walls.

The following materials can be used to cover the roof: either profile sheet, galvanized steel, corrugated asbestos concrete and bitumen-fibrous sheets, cement and ceramic tiles, roofing material, soft roof and other roofing materials. The illustration below shows the permissible slope angles for different types roofing.

Basic constructions of truss systems

First of all, it is worth considering the basic types of rafter systems in relation to the location of the walls of the house, which are used in all roof structures. Basic options are divided into layered, hanging, and also combined, that is, it includes elements of both the first and second types of systems in its design.

rafters

Nasal system

In buildings where internal load-bearing walls are provided, a layered rafter system is often installed. It is much easier to mount it than a hanging one, since the internal load-bearing walls provide reliable support for its elements, and in addition, this structure will require fewer materials.

For rafters in this system, the defining reference point is the ridge board, on which they are fixed. The non-thrust type of the layered system can be equipped in three options:

In the first version, the upper side of the rafters is fixed on a ridge support, called sliding, and their lower side is fixed by cutting to the Mauerlat. Additionally, the rafters in the lower part are fixed to the wall using wire or staples.

In the second case, the rafters in the upper part are cut at a certain angle and interconnected with the help of special metal plates.

The lower edge of the rafter legs is attached to the Mauerlat with movable fasteners.

In the third version, the rafters are rigidly fastened in the upper part with bars or treated boards, located horizontally, parallel to each other on both sides of the rafters connected at an angle, and the ridge girder is pinched between them.

In the lower part, sliding fasteners are used to secure the rafters, as in the previous case.

It is necessary to explain why sliding fasteners are often used to fix the rafters on the Mauerlat. The fact is that they are able to relieve the load-bearing walls from excessive stress, since the rafters are not rigidly fixed, and when the structure shrinks, they can move without deforming the overall structure of the roofing system.

This type of fastening is used only in layered systems, which also distinguishes them from the hanging version.

However, in some cases, a spacer system is used for layered rafters, in which the lower end of the rafters is rigidly fixed to the Mauerlat, and in order to remove the load from the walls, puffs and struts are built into the structure. This option is called complex, since it includes elements of a layered and hanging system.

Hip roofs are used both for low-rise residential buildings and for various outbuildings. Most often, they are built over structures with equilateral load-bearing walls, and the foundation plan is of a square type. You can find such architectural elements and over circular structures, in this case the number of slopes is more than four.

The main difference between the hipped roof is the complete absence of a ridge. This is very important detail rafter systems, it takes on the efforts of the rafter legs. The function of this element is assigned to the central support, at the top of which the rafters converge. In this regard, great attention should be paid to its strength. There are options for rafter systems without a central support, the upper heels of the rafter legs rest against each other. Such options are most often used on small buildings. All stingrays tent system are in the form of equilateral triangles.

The hipped roof has many manufacturing options, but all must have the following elements.

Types of truss systems, advantages and disadvantages of a hipped roof

Table. The main types of hipped roof truss system.

Advantages of the tent rafter system

The disadvantages include the complexity in the manufacture of the rafter system. A hipped roof has a large number of different nodes and elements, each of which can have a negative impact on stability indicators. Another drawback is the difficulty in converting attic spaces into residential ones, such options for rafter systems are extremely rarely used for attics. Sometimes the disadvantages include the increased consumption of roofing materials, but this depends not so much on the features of the rafter system, but on the professionalism of the roofers.

Timber prices

General tips for the construction of hipped roof truss systems

Specific decisions are made by the master, taking into account the size and design features of the hipped roof. But for all cases there are general recommendations, the implementation of which guarantees the stability of the structure.

Additional stops for the attachment rafters

Top of the tent system

A very complex from a technological point of view and the most important from an architectural point of view, the unit. It has two solutions: the rafter legs rest on one support, installed in the center of the building, or against each other. The first option provides maximum structural stability, but its installation requires a rigid support. The best option is that there is a load-bearing wall in the middle of the building. An acceptable option - during the construction of the ceiling, reinforced beams or beds are provided. When calculating the parameters, the maximum possible roof loads must be taken into account and a safety factor must be applied.

The use of the joint only in the upper heels of the rafters without vertical support is used only on small structures... At the same time, a complex of special construction measures is used to increase the rigidity of the structure.

Narodniks

They can be fixed using special lower stops, directly to the rafters or in a mixed way. Modern technologies allow you to fix the rafters without cutting, which not only strengthens the rafter system, but also greatly simplifies the work.

Structural solutions of a hipped roof, especially without a central support, significantly increase the spacer loads on the Mauerlat. This situation requires its strengthening, all units must be built according to accurate calculations and with unconditional compliance with building codes.

And finally general recommendation- if you do not have serious practical experience in the construction of ordinary pitched rafter systems, then it is absolutely not worth taking on a tent roof. This is one of the most complex truss systems, all of its elements are so interconnected and interdependent that the violation of the stability of one becomes the cause of the complete destruction of the roof. The hipped roof truss structure does not have completely identical elements, all must be approached individually.

Construction boards prices

Construction boards

Calculation of the tent structure

First, consider the algorithm for calculating the basic values of the system. Take into account that the roof has four slopes, each of which is an equilateral triangle. The angle of inclination of the slopes is selected depending on the coverage area and climatic zone location of the object. Taking into account the climatic zone in the tables of building codes and regulations, the minimum permissible angle of inclination of the slopes is selected.

The length of the central rafter depends on the angle of inclination and is determined by the well-known formula of a right-angled triangle. First you need to find the length of the diagonal, this is done by calculation or by ordinary measurement. The second method is easier, use a tape measure to find this value.

We have the angle of inclination of the slopes, find the tangent on the Internet, multiply it by the length of half of the leg, and the result is the height of the vertical support of the hipped roof. Next, you should calculate the length of the rafter. It is calculated using several formulas, the simplest of them is the Pythagorean theorem: c 2 = a 2 + b 2, where:

c - the length of the rafters;

a - half of the leg of the structure;

b is the height of the vertical support.

Hence - the length of the rafters is square root the sum of the squares of the indicated quantities. That's all, if it's difficult to do it yourself, then there are a large number of online calculators on the Internet. After entering the data, not only the sizes of the system elements are displayed, but their number is also counted.

Rafters, supported on two supports without any additional stops, are used for single-pitched roofs with a span of 4.5 m or gable roofs with a span of up to 9 m (Fig. 30). The rafter system can be used with the transmission of the spacer to the Mauerlat (walls) and without the transmission of the spacer.

Rice. 30. Inclined rafters without struts

Expansionless layered rafters

A bending rafter that does not transmit the thrust to the walls must have one support fixed, but freely rotating, the other freely rotating and movable.

These conditions are met by three options for fastening the rafters:

1. The bottom of the rafter leg is hemmed with a support bar or a cut (cut) is made on it with a tooth and abuts against the Mauerlat, and an enlarged horizontal cut (cut) with a bevel is made at the top of the rafter (Fig. 31). The depth of the cut (cut) in the upper part of the rafter should not exceed a = 0.25h... The length of the trim (support area) is made no more than the height of the rafter section (h)... It is recommended to cut the trim so that it does not interfere with the bending of the rafter, otherwise the cut with the side cheek will rest against the girder and we will get a spacer rafter system. The length of the beveled undercut is made at least two depths a... If it is impossible to trim the top of the rafter leg, it is hemmed with a rafter trim with double-sided fastening mounting plates or wooden sinkers. The upper ends of the rafter legs fit freely on the girder. In gable roofs, they are attached to the girder as a sliding support, but they are not fastened to each other. A gable roof, in this case, is considered as two gable roofs, adjoined to each other by a high side. Pay attention to an extremely important condition: the upper support cut or hemming of the top of the rafters are made horizontally... One has only to change the support scheme for the girder, as the rafter immediately shows the thrust. This calculation scheme for the installation of rafters due to the rigidity of the conditions for the manufacture of the upper unit (any inaccuracy in the design of the unit immediately turns the non-thrust scheme into a spacer) is practically not used for gable roofs, therefore it is more often used in pitched roofs... In addition, in gable roofs, for the lack of space on the Mauerlat when the rafters bend under the action of a load, you have to pay by opening the ridge node of the roof.

At first glance, this scheme is generally paradoxical. We clearly see an emphasis on the Mauerlat in the lower part of the rafter leg and the system seems to have to exert a horizontal force on it. However, she does not show strife. If someone wants to know why, then read the proof in the course of lectures of Professor V. G. Zalessky on pages 414–415.

rice. 31. Supporting the bottom of the rafter with a cut in the mauerlat, and the top of the rafter on the run with a horizontal cut, with a beveled cheek, does not push against the walls

2. The most common way of installing rafters is related to gable roofs. The bottom of the rafter leg is made on a slider, and the top is fixed (Fig. 32): they are tied with a nail strike or a bolt, or abut against each other and tied with wooden beads or metal toothed plates (MZP).

rice. 32. Supporting the bottom of the rafters without cutting into the Mauerlat with the top of the rafters secured, does not push against the walls

What you need to pay special attention to is the fastening of the rafter leg to the Mauerlat.It comes down only to fixing the rafters in the design position, which ensures the step of their installation. To do this, it is enough to drive one nail diagonally from both sides into lateral surface rafters or one long nail on top, or put a flexible steel plate. If fashionable fastening corners are used, then one nail will be enough to secure the bottom of the rafter, or you need to press the rafter with corners on both sides without nails at all. Do not drive as many screws into the corners or hammer in as many nails as there are holes in the corner. Otherwise, you will turn the slider into an imperfect hinge and get a spacer on the Mauerlat. Flexible wire strands are held against the wind-driven overturning of the roof; it is not necessary to transfer this function to the corners, or the rafter system should be calculated as a spacer.

3. Does not give space and rigid pinching of the ridge knot, when the bottom of the rafter leg is made on the slider, and the top is rigidly fixed (Fig. 33). However, a bending moment appears in the ridge knot, tending to destroy it. The maximum bending moment in such a structure occurs on the ridge support, and the rafter legs themselves receive less deflection. It is quite difficult to calculate such a node, and then accurately manufacture it on a construction site, therefore it is better to accept formulas for calculating the bending moment and deflection as for ordinary single-span beams.

rice. 33. Supporting the bottom of the rafters without cutting into the Mauerlat with pinching of the ridge knot, does not give space to the walls

In all three options, the rule is observed: one end of the rafter leg is made on a sliding support that allows rotation, the other on a hinge that allows only rotation. The fastening of rafters on sliders and fixed hinges is made by the most different designs... Now they are often performed on mounting plates. You can also fasten in the old fashioned way: with staples, nail strikes or using short bars and boards. You just need to choose the right type of fastener so that it allows or prevents the rafters from sliding in the support.

When calculating the rafter system, an "idealized" design scheme is adopted. It is believed that a uniformly distributed load presses on the roof, i.e. the same and even force acting evenly on all planes of the slopes. In fact, there is almost never a uniformly distributed load on roof slopes. The wind sweeping snow bags on one of the slopes and blowing snow off the other, the sun melting snow on the southern slopes, snow sliding in the spring - make the load on the slopes uneven. Under the action of an uneven load, all three of the above options for rafter systems are statically stable, but only under the condition of rigid fastening ridge run... Which ends are inserted into the gables of the walls or propped up with hip roof rafters. That is, the rafter system will be stable only if the ridge girder on which the top of the rafters rests is securely fixed against horizontal displacement.

In the manufacture of gable roofs and the support of the girder only on the racks (without resting their ends on the walls of the gables), the situation changes for the worse (Fig. 34). In the second and third variants, with a significant decrease in the load on one of the slopes, against the calculated one on the other slope, the roof will try to "leave" in the direction of a higher load. The first option, in which the bottom of the rafter leg is made with a notch with a tooth or with a filing of a support bar, and the top with a horizontal notch is laid on the girder, holds an uneven load well, but only if the racks holding the ridge girder are absolutely vertical.

rice. 34. Loss of stability of the rafter system

To give the rafter system stability, a horizontal grapple is introduced into it (Fig. 35). It increases the stability of the system, but only slightly. Therefore, in all places where the fight intersects with the racks supporting the ridge run, it is attached to the racks with a nail fight. There is a persistent misconception that the contraction always works in tension, it is not. The grapple is a multifunctional element: in non-expansion rafters, it does not work at all in the absence of snow on the roof, or it works in compression when an insignificant uniform load appears on the slopes. In tension, it works only in a pre-emergency situation when the ridge girder sags or deflects from the action of maximum loads. In fact, the scrum is an emergency element of the rafter system, which comes into operation when the roof is covered with the maximum possible amount of snow and the ridge girder bends over the entire calculated value, or when unexpected and uneven subsidence of the foundations occurs and, as a result, uneven settlement of the walls and ridge girder. The lower the contractions are set, the better. Usually they are installed at a height of at least 1.8–2 m from the floor surface, so that they do not interfere with a person when passing through the attic.

If, in the second and third versions, the lower unit for supporting the rafter leg (slider) is replaced with a slider of a slightly different design (Fig. 35, c) - with the end of the rafters out of the wall, then this will further strengthen the entire system, making it a statically stable structure for any combination of loads.

rice. 35. The scrum between the rafters increases the stability of the rafter system

Another measure to increase the stability of the entire system is rigid (which is not always possible) securing the bottom of the racks supporting the run. They are cut into the bed and attached to the ceilings in any possible way, turning the lower support unit of the rack from a hinged one (in the plane of the rafters) into a unit with rigid pinching (Fig. 36).

rice. 36. An example of fixing the strut support unit

Due to the development of low stresses in them, the section of contractions is not calculated, they are taken constructively. In order to reduce the size of the parts used in the construction of the rafter system, the cross-section of the fight is used the same size as the rafters, but thinner boards can also be used. Fasteners are installed on one or both sides of the rafters and are attached to them with a nail and / or bolts (fig. 37). When calculating the cross-section of rafters, contractions are not considered additional supports, i.e. the rafter system is calculated as if there are no fights in it at all. However, if the scrapes are bolted to the rafters, then the load-bearing capacity of the wood due to weakening by the bolt holes is reduced by using a factor of 0.8. In other words, if holes are drilled in the rafter for the installation of the bolts of the contractions, then its design resistance is applied equal to 0.8R. When fastening the fight to the rafters only with a nail fight, the weakening of the calculated resistance of the wood of the rafter leg does not occur, but it is necessary to calculate the number of nails driven. The calculation is made for the cut (bend) of the nails. For the calculated shear force, a thrust is taken, which can occur in an emergency condition of the rafter system. In general, a spacer is introduced into the calculation of the nail connection of the scrum with the rafter (H), which does not exist during normal operation of the rafters.

rice. 37. Scrum attachment unit

Let us recall once again that the static instability of a non-thrust rafter system is manifested only in roofs where it is not possible to fix the ridge girder from horizontal displacement. In houses with hip roofs and in houses with brick and stone gables, non-thrust rafter systems are quite stable and there is no need for stability measures. However, "anti-emergency" designs are fights, it is still necessary to install.

When a thrust is introduced into the calculation of the rafter system (even if it is not there), the calculation of the compressive force S changes. Now it is calculated by dividing the resultant distributed load by the sine of the rafter tilt angle S = (qL / 2) / sinα. Without going into the details of the expansion of the force vectors, let us explain this by small example... Suppose we have a rafter system with a steep slope of the slopes. When a load acts on it in an emergency state, for example, when a subsidence, a deviation from the vertical, or the destruction of a ridge run, tensile stresses will appear in the fight, neutralizing the so-called thrust. With a constant external load, the smaller the angle of inclination of the slopes, the more the spacer will grow and the more the rafter legs will be compressed. Conversely, if the rafter legs are not tied by contractions, then they work like ordinary beams laid in an inclined position. In this case, a decrease in the angle of inclination, with a constant load, reduces the compressive stresses in the rafter and increases the normal (perpendicular) force directed to the bending of the beam. Therefore, the compressive force in rafter systems without contractions is considered as S = (qL / 2) × sinα and with contractions S = (qL / 2) / sinα. Since double-slope rafter systems are almost never built without fights, and the calculation is always carried out for the worst working conditions, then on all schemes the compressive stresses will be written as S = (qL / 2) / sinα, regardless of whether there will be a thrust in these schemes or no.

When installing studs or bolts for fastening scrapes, pay special attention to the diameter of the hole for them. It should be equal to the diameter of the studs (bolts) or even less than it by 1 mm. In an emergency, the scrum will not work until it chooses a gap between the pin and the wall of the hole at this time, the bottom of the rafter legs "erodes" a few millimeters or centimeters (depending on the height of the scrum installation), which can move or unscrew the Mauerlat and destroy the cornice of the walls, and in the spacer rafter systems, where the Mauerlat is rigidly fixed, "push apart" the light walls.

Spacer rafters

A bending rafter that transfers the spacer to the walls must have two fixed supports.

We take the same options rafter schemes and replace in them the lower supports with two degrees of freedom (sliders) with supports with one degree of freedom (hinges). Simply, where they are not, we nail the support bars to the bottom of the rafter leg. Typically, a bar about a meter long and a cross-section of 50 × 50 (60) mm is used with the calculation of a nail connection. Or we support the Mauerlat in the form of a tooth. In the first version of the design scheme, in the ridge where the rafters are horizontally supported on the girder, we sew the upper ends of the rafter legs with a nail fight or fasten them with a bolt and thus obtain a hinge support.

rice. 38. Rafters rested by both ends in the Mauerlat and in each other show the strut

The design schemes of rafter systems change slightly (Fig. 38), all internal compressive and bending stresses remain the same, but an equal distance appears in the lower supports of the rafters H = (qL / 2) × ctg α, (kg). In the upper nodes, the spacer in one rafter leg is destroyed by the oppositely directed spacer from the end of the other rafter leg, so here it does not bring special trouble... However, the ends of the rafter legs resting directly against each other or through a purlin can be tested for wood crushing, although this is not required in most cases.

In fact, spacer rafters are a transitional scheme between non-spaced layered and hanging rafters. They already show the scheme of the hanging rafters, but the rudiment in the form of a ridge run still remains. When the rafters are resting on the bottom against the walls, and on horseback against each other, then the run here is like the fifth wheel in a cart. On the one hand, it doesn't seem to hurt, but on the other, you can do without it. The rafter system shows the duality of its work, which depends on the tightness of the top of the rafters to the girder and to each other. The force vector pressing on the ridge knot is distributed both to the rafter legs and to the girder. In case of subsidence, as a result of wall shrinkage or deflection from its own weight, the girder goes out of work and the force vectors are completely distributed along the rafters, and the rafters themselves turn into hanging ones.

In spacer rafter systems, the purpose of the scrum is somewhat different - it works in compression in emergency situations. Getting into work, she lowers the spacer on the walls of the bottom of the rafter legs, but does not completely remove it. She will be able to remove it completely if it is located at the very bottom, fixed between the ends of the rafter legs, but this is already another constructive scheme and the fight in it is called a tightening.

What changes with the inclusion of a fight in the scheme? We will not burden you with the layout of the force vectors, just imagine a pre-emergency situation when maximum loads are acting on the roof. Where there are no racks under the ridge girder, the girder deflects and the layered rafters, tightened by the fight, instantly turn into a hanging rafter pattern with a compressed crossbar, and the bottom of the rafter legs receives a strut according to the corresponding design scheme. Where there are racks under the ridge girder or a rigid girder, the fight also works for compression and the bottom of the rafter legs also transfers the thrust, but weaker in the same way as the top of the rafters holds the ridge girder. However, the calculation is based on the worst-case scenario.

The use of spacer rafter systems requires taking into account the effect of the spacer on the walls. You can reduce the spacer by installing rigid ridge girders. For these rafter schemes, it will be better if the calculated deflection of the ridge run turns out to be much less than the normalized SNiP. Try to increase the rigidity of the purlin by installing struts, struts or cantilever beams (changing the section height) or doing a building lift on it. This is especially true for houses made of lightweight concrete, lumber and chopped from a log. Massive brick, concrete and panel houses can easily transfer the spacer to the walls.

It should be noted that the thrust is understood as the horizontal force arising from the compressive stress S rolling down the rafter leg. In other words, thrust is a horizontal vector of forces arising from the action of a vertical load... Do not confuse it with rafter deflection. In the design scheme, the rafters are considered to be bar elements that have no height, therefore, the deflection strut is not taken into account. This is what the standardization of the deflection in building structures is aimed at. SNiP, introducing normalized deflection values, brings idealized design schemes closer to real ones. In other words, if the deflection building structure does not exceed the standard, then you should not think about the pressure from the deflection, it seems to be not there. Although in fact it is also in the spacer scheme, it manifests itself to a greater extent than in the non-spaced one. You need to pay attention to the deflection strut when building the walls of a house from aerated concrete. These blocks absolutely do not hold the bend and can be destroyed by expansion even from the deflection of the rafters. Do not use rafters on these walls. In other cases, the deflection spacing does not do much harm, for example, in brick walls it is perceived by the elasticity of the Mauerlat and steel fasteners.

The rafter system, made according to the spacer option, is a statically stable system for any combination of loads and requires rigid fixing of the Mauerlat on the wall. To hold the gap, the walls must be massive enough or equipped with an unbreakable monolithic reinforced concrete belt around the entire perimeter, like a hoop on a wooden barrel. In emergency situations, in contrast to non-thrust systems, in a thrust system, a compression scrum does not save the situation; it only partially reduces the thrust transmitted to the walls (Fig. 38.1). So that emergency situations do not occur, we collect to the maximum, the loads acting on the roof.

rice. 38.1. Compression scrum partly removes the spacer from the walls

The calculation of the rafter system with a compressed scrum introduced into it is done according to two combinations of loads. The cross-section of the rafter leg is selected according to the maximum bending moment and deflection without taking into account the work of the compressed fight. Imagine that an uneven load is acting on the roof: snow lies on one side of the slope, and on the other it has melted or slipped. The bending rafter leg will simply push the compressed grapple and will work like a regular single span beam. The selection of the section of the compressed fight and the determination of the thrust on the walls, on the contrary, will be carried out for a uniformly distributed load on both slopes. In this case, the fight will be compressed on both sides and will receive the maximum compression stress, the bottom of the rafter leg will give a reduced thrust to the wall, and the rafter leg itself will turn into a continuous beam on three supports.