Calculation of snow and wind load. Correct calculation of the roof truss system Calculation of the snow load on the roof

Snow load areas

The first thing to decide is which area by the weight of the snow cover the area under consideration belongs to. This information can be found on special maps in regulatory documents. The main regulatory document regulating the snow load - SP 20.13330 *

Fig.1 Map of the Russian Federation by snow cover weight (click to enlarge)

*Please note that SP20.13330 is 2011 and 2016, and the maps in these documents are different. At the time of publication of the article, the joint venture of 2011 is mandatory. but in the near future JV 2016. will officially become valid and the calculation will need to be carried out according to the cards of the new document. The calculation of the snow load can also be found at SNiP 2.01.07-85*, but this calculation will not be valid because norms are outdated.

Snow load calculation

Snow loads are calculated according to SP 20.13330 *

The normative value of the snow load on the horizontal projection of the pavement should be determined by the formula:

S 0 =C e C t µS g

where C e is the coefficient that takes into account the removal of snow from the coverings of buildings under the influence of wind or other factors, taken in accordance with 10.5-10.9 of SP 20.13330; C t - thermal coefficient, taken in accordance with 10.10 SP 20.13330; µ is the coefficient of transition from the weight of the snow cover of the earth to the snow load on the cover, taken in accordance with 10.4 of SP 20.13330; S g is the normative value of the weight of snow cover per 1 m2 of the horizontal surface of the earth, taken in accordance with 10.2 (see Table 1 below).

The design value of the snow load is determined by multiplying the standard value by the reliability factor for the snow load:

S=S 0 *γ f

Reliability coefficient for snow load γf = 1.4.

Snow load table

S g - the standard value of the weight of the snow cover per 1 m 2, depending on the area of ​​\u200b\u200bthe snow load, is determined from table 1.

Table 1: Table of snow loads depending on the area

For example:

Snow load in the Moscow region and St. Petersburg (III snow region on the map) - S 0 =C e C t µS g\u003d 1 * 1 * 1 * 1.5 \u003d 1.5kPa \u003d 1.5kN / m2 \u003d 150kg / m 2 S=S 0 *γ f = 150*1.4=210kg/m2.Snow load in the Moscow region (IV snow region on the map) - S 0 =C e C t µS g\u003d 1 * 1 * 1 * 2 \u003d 2kPa \u003d 2kN / m2 \u003d 200kg / m 2 S=S 0 *γ f = 200*1.4=280kg/m2

Snow load calculation online calculator

For a faster calculation on our website, you can use the online snow load calculator. If you have any difficulties, you can order a calculation by writing to us by mail in the contacts section.

Fig.2 Online calculator for calculating the snow load.

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Various forces act on the roof structure. The calculation of the load on the roof includes such influences as: the weight of the roofing material, rafters and battens, insulation, lining carpet, snow and wind load. Let's consider each of these loads separately.

Rafter calculation

If you are building a house on your own, and you do not have sufficient knowledge in the field of engineering and architecture, then the calculation of the roof load can be ordered from a specialized organization or from a private designer. If the building is not so demanding on technical calculations, then everything can be done on its own.

The impact of the force of the wind

Snow load can destroy the roof, but the wind load can also tear off the coating. The greater the angle of the roof slopes, the greater the wind load on the structure. The smaller the angle, the stronger the lifting force will be, tending to rip off the roof. That is why the calculation of the area of ​​\u200b\u200ba gable roof is so important. First, determine the length of the rafter leg. Knowledge of the school geometry course is useful here, since the rafter makes up a right triangle with adjacent walls, so by calculating the length of the hypotenuse, you can determine the required indicator.

It is a little more difficult to calculate the cross section of the rafters and the distance between them. To do this, we will calculate the wind load on the roof according to the formula: Wp = W * k * C. W - wind pressure, which is taken from the SNiP tables. k is a coefficient depending on the height of the building, it is also indicated in the normative document mentioned above. C is the aerodynamic coefficient used to calculate lift from leeward and windward.

Coefficient C can be either positive or negative. The first case occurs if the wind presses on the surface of the slopes, this is true for large angles. The second case occurs on sloping roofs, when the wind "flows" along the slopes. To counteract these forces, depending on the pitch of the rafters, so-called "ruffs" are installed in the walls of the house. These are metal pins to which the rafter legs are tied with wire. In windy regions, each rafter is tied, under normal conditions this is done through one beam, having previously been completed according to the available data.

Calculation of the floor beam, look at the video:

Roof weight load

The weight of the roofing material itself has a serious impact on the characteristics of the truss system. At the same time, different materials can vary significantly in their weight. The more the roof weighs, the greater should be the angle of inclination of the slopes. You also need to know how to calculate the square meters of the roof, since the larger its area, the more it will depend on the influence of external loads.

The pressure force of the roof on the rafters can be calculated, knowing the characteristics of the material. They are often specified in the technical data or instructions from the manufacturer. Depending on the type of roofing material, a certain version of the crate is selected. So, to create it, OSB plate, plywood or edged board is used. The average weight of these materials can be obtained from the normative tables or technical data from the manufacturer. For example, bars with a section of 4 * 6 or 6 * 6 cm are used under a slate roof, while OSB boards or plywood are used under bituminous shingles.

The calculation of the square of the roof depends on its type. very simple for pitched roofs. In more complex structures, the roof should be divided into elementary shapes - rectangles and triangles, the area of ​​\u200b\u200bwhich is easily determined (more details: ""). It is also important to take into account the overhangs of the roof on the cornices. The distance between the rafters is determined based on the thickness of the roofing material.

No less important is the thermal calculation of the roof, on the basis of which the insulation and its thickness are selected. These two indicators greatly affect the overall weight of the roof structure. In addition, this includes the weight of vapor and waterproofing, as well as the inner lining of the attic. The thickness of the insulation is calculated by the formula: T \u003d R * L. Where R is the thermal resistance of the structure to be insulated, L is the thermal conductivity of the selected insulation (selected according to SNiP II-3-79 standards).

Suppose that the roof is insulated with glass wool URSA M-20, the house is located in the central region. Then the thickness of the insulation will be: T \u003d 4.7 * 0.038 \u003d 0.18 m \u003d 18 cm. In this case, 4.7 is the thermal resistance taken from the SNiP standards, and 0.038 is the thermal conductivity coefficient, which was indicated by the material manufacturer. Knowing the density of the insulation (indicated in the technical data) equal to 18-21 kg / sq.m., you can calculate the weight of the material.

Similarly, the weight of hydro and vapor barrier, as well as finishing material, is calculated. The calculation of roof heating is also important, as it affects the thickness of the insulation. Also, the heating system that will be installed in the attic will add to the weight of the roof structure.

In order to take into account the weight of the rafter structure itself, you should draw its plan. Average values ​​​​for layered rafters and runs are taken into account - 5-10 kg / sq.m., for hanging rafters - 10-15 kg / sq.m. To obtain a certain margin of safety of the structure, the resulting loads are multiplied by a factor of 1.1.

In order to more accurately determine the weight loads on the roof, it is necessary to carry out a thermal calculation of the roof, an example of which can be found on the pages of our portal.

The key feature of our climate is seasonality. As a result, the factors affecting the roofs of houses change: the amount of precipitation, strength, wind direction, and others. Snow load on the roof is one of the main components of the future construction project, taking into account which the type of truss system, material parameters, battens and roofing options are determined.

What should be known about such impacts, and their consideration at the construction design stage?

How snow affects the roof

It is clear that the snow that has fallen on the roof surface has a mass, which creates pressure on the entire system. However, the generated load is uneven and constantly changing.

• During the cold season, the snow cover increases. But the main danger is the alternation of thaws and frosts, as a result of which the mass of even one layer increases.

On a note

It is worth knowing that thawing and freezing of snow compacts it, and as a result, the mass grows.

• The snow cover is not static, it is in constant motion: it slides off the slopes, it is blown away by the wind. As a result, pressure is distributed unevenly in different parts of the roof. In particular, this factor is manifested on roofs with non-standard configurations (the so-called broken types).
• Since the snow slides down the slope, its large mass accumulates on the overhangs, which also does not have a beneficial effect on the roof structure.
• Snow cover creates impacts not only on the roofing itself and the truss system, but also on gutters, which often results in the collapse of the latter.

To eliminate or reduce the adverse effect of snow load on roofs, a whole concept has been developed to solve the problem. It includes cleaning the surface on existing roofs, changing designs, or calculating, and laying down certain properties even at the design stage of a house being built.

Accounting for snow loads on existing roofs

Naturally, it is best to take into account all the factors of snow loads at the construction stage and include them in the draft project. But, what should be checked or considered in the option when the house is already built?

• In the finished building should be measured. Optimally, if this value is from 45 to 60 degrees, then the snow cover simply will not accumulate on the surface, shifting from the roofing.

However, in this case, one more factor must be taken into account - the wind. The greater the angle of inclination of the slopes, the higher the structure will be, which means that the influence of the wind will increase.

Evenly distribute snow flows over the surface will help fixtures mounted on the flooring - snow retainers and snow cutters. Such elements will “break” the entire mass into several parts, distributing them approximately evenly over the entire area. Also, depending on the crate, it is selected, on solid options, types of devices are possible, in other options it is better to install snow cutters that break the snow stream into separate parts.

However, care must be taken with the installation of such devices. on roof slopes with an inclination angle of more than 5 degrees, otherwise this may lead to the accumulation of significant masses of snow on the surface of the flooring.

• In order to avoid the accumulation of large amounts of snow on the roof eaves, a heating system should be considered. Installation along the edge of the roof deck will help eliminate the freezing of blocks of snow and ice. The system can be controlled in automatic and manual modes.

It is important to know that in addition to direct methods of reducing and eliminating snow pressure on roofing, it is worth taking care of waterproofing. The formation of even a slight frost on the surface creates an obstacle to the flow of water, as a result of which moisture can get under the roof material.

The roofs of already built buildings, as a rule, are already designed for a certain snow load in a given region, however, additional measures and adaptations will help eliminate the negative consequences of both the overload itself and the accompanying processes (leaks, destruction of the flooring, etc.).

Calculation of snow loads in accordance with building codes

Without taking into account the climatic features of winters in this region, the roof may simply not withstand the amount of snow that has fallen, the truss structures are deformed with further destruction.

On a note

The weight of fresh fallen snow is about 100 kilograms per 1 cubic meter of volume, wet is heavier - 300 kg / m³.

Knowing the mass of precipitation, it is already possible to calculate the effect of snow on the surface from the thickness of the fallen cover. Why in SNiP (building codes and regulations 2.01.07-85 "Loads and impacts" paragraph 10) formulas are included that can be used to make calculations. But, it is necessary to know exactly the average thickness of the snow cover for a particular region and, accordingly, the impacts created.

In order to be able to make an accurate calculation, a map of the country has been compiled, where the territory is divided into 8 regions with approximately the same conditions.

1. For example, for Moscow and the Moscow region, the load is approximately 180/126 kg / m³,
2. Nizhny Novgorod region - 240/168 kg / m³,
3. and in mountainous areas this figure can vary 560/392 kg/m³.

Taking into account such data, the total snow load on the roof is calculated using the following formula:

S – is the desired total snow load;

Scalc - estimated snow load (look at the map, specify specifically for your region);

µ is a coefficient that takes into account the angle of inclination of the roof.

The value of the roof slope is taken depending on the following indicators:

• When the slopes are less than 25 degrees - one;
• Tilt from 25 to 60 degrees - coefficient 0.7;
• With slopes of more than 60 degrees, this indicator is not taken into account at all.

That is, having such data it is quite simple to make calculations. For example, for the Nizhny Novgorod region, the estimated snow load is 240 kg, the house is designed with slopes at an angle of 30 degrees, which means that the calculation is as follows - 240 × 0.7 = 168 kg / m³. After that, you can select the appropriate details of the roof truss structure.

Flat roof types

These types of roof structures are unacceptable for regions with high rainfall during the cold season, as large volumes of snow will accumulate on such a surface. The result will be excessive snow pressure on the structure. In areas with a warm climate, roofs of this type should have a margin of safety, as well as a continuous crate. A prerequisite is the installation of heating cornices, to remove precipitation from overhangs through drainage systems.

The slope of the slope planes towards the drain funnels in such situations should exceed 2 degrees, which will ensure a full runoff of precipitation.

When designing construction, outbuildings or with a flat roof, they are guided by the same rules and calculations of snow loads as for conventional gable (or more) types of roofs. However, for flat roof structures on such buildings, it is better to choose rafters from thicker materials, and mount the crate solid.

Self weight of the roof structure

In addition to snow loads, it is worth considering the mass of the roof structure itself. This is done to reduce pressure on the walls of the building, and also so that the roof does not collapse under its own weight, loaded with precipitation.

The optimal value for residential buildings is approximately 50 kilograms per 1 meter of area.

The calculation is carried out by summing the mass of 1 m² of each layer of the roofing cake, and multiplying by a factor of 1.1. For example, the weight of 1 square of lathing with boards with a cross section of 25 mm is about 15 kg / 1 m², a heat insulator of 100 mm is 10 kg / 1 m², metal tile flooring is 4-5 kg ​​/ m² (depending on the thickness of the sheet). In total, we have 15 + 10 + 4 = 29 × 1.1 = 31.9 kg / 1 m². Also, do not forget about the mass of rafters.

Taking into account these indicators, the optimal material options are selected, as well as the types of battens and rafters. Subsequently, this approach will allow you to change the roofing without fear of destruction of the existing structure.

The calculation of snow impacts on the ceiling is one of the components of the project of the future home, which should not be ignored. Neglect of simple calculations, and careless selection of the appropriate variant of the design of coverings can lead to serious consequences up to destruction.

In particular, snow load calculations are important for complex roof options, since uneven distribution of precipitation on the surface will create overloaded areas. In this case, more durable materials should be selected to create a greater margin of safety on such parts of the roofs.

If everything is done correctly, then such a roof will last its operational life without problems, and even when changing the material of the roofing deck.

During the construction of the roof, special attention should be paid to the calculation of its bearing capacity, since a huge amount of forces are constantly acting on the structure. One of the forces that acts on the roof is the snow load, according to which the roof is built. It is she who determines how thick the supporting elements will be and how to build a rafter system. Its value is calculated according to a special formula, according to SNiP.

Snow load and its negative impact

Usually, up to 5% of snow cover is removed from a pitched roof during the day. It is blown away by the wind, slips or becomes covered with infusion. But the remaining amount negatively affects not only the structure, but also the person:

1. Snow weight can increase during a hard frost after warming. In this case, deformations of the truss system, waterproofing and thermal insulation are possible.
2. Snow load on roofs that have a complex structure, as a rule, is distributed unevenly.
3. Snow sliding towards the eaves can be dangerous for people nearby, so the installation of snow retainers is mandatory.
4. Sliding snow, in addition to being dangerous to humans, can harm the drainage system. That is why it is necessary to clean it off in time or install snow retainers.

Roof cleaning from snow mass

The most effective way to remove snow from a roof is by hand shoveling. But it is very dangerous for independent conduct without prior preparation. That is why, a properly calculated snow load can help not to constantly remove snow.

The angle of inclination of the roof slope has a positive effect on snowmelt. The most optimal roofing option for regions where there is a high probability of a large amount of snow is from 45 to 60 degrees.

In order to reduce frost and prevent the formation of icicles, cable heating can be installed around the perimeter of the roof. It can be automated or manual.

Calculation of the load of snow on the roof

Even at the design stage of the roof, in order to avoid damage to its structure during heavy rainfall, design measures are carried out. The average weight of snow is 100 kg per cubic meter. meter, and wet precipitation weighs even more, which is 300 kg per 1 cu. meter. Knowing these approximate values, it is quite easy to calculate the allowable snow load.

But this will also require knowledge of the thickness of the falling snow layer. You can measure this indicator on a flat area, and multiply the resulting number by a coefficient that assumes a margin and equals 1.5. In order to take into account the regional indicator, you can use a special map. It became the basis for obtaining the rules of SNiP and other regulations. In general, the indicator is determined by the following formula:

S=S calc. *μ

In accordance with this formula, its components are deciphered as follows:

• S calc - the value of the weight per square meter of a horizontal platform.
• μ - coefficient of slope of the roof.

Usually, as mentioned earlier, calculations are made according to the snow load map, which is presented below:

In accordance with SNiP, there are such indicators of the roof slope coefficient:

• If the roof slope is less than 25 degrees, then the coefficient is 1.
• If the slope of the roof is in the range from 25 to 60 degrees, then the coefficient will be equal to 0.7.
• With a slope of more than 60 degrees, the coefficient can be completely ignored.

This also takes into account the side from which the wind blows. This is necessary, since in any case there will be less snow on the windward side than on the leeward side.

In order to better understand how the snow load is calculated, let's present an illustrative example for the Moscow region. The calculated roof has a slope of 30 degrees. So, according to the requirements of SNiP, we calculate:

1. In the map we find the location of the Moscow region and reveal that it belongs to the third climatic region. Here, the value of the load on the roof is 180 kg per 1 sq. meter.
2. According to the formula, we calculate the total weight of snow. To do this, we multiply 180 by a factor equal to 0.7. We get the number 126 kg per sq. meter.
3. Already according to this indicator, a rafter system is created, which is calculated by the maximum numbers.

In addition to this option, there is a complete calculation, which is also presented in SNiP and has a corresponding table there. The calculation is carried out according to the following formula:

Q1 = m*Q

Here, m, which is calculated by the interpolation method, acts as an indicator of the coefficient. With a roof slope of 30 degrees, it is equal to 1, and at 60 degrees - 0.

A standard indicator can be calculated. To do this, you need to use an atlas in which changes in SNiP are recorded or calculate the indicator using the formula: Q2 \u003d 0.7 * Q * m. If the calculation is made for the structure that is mounted in areas with constant winds blowing snow from the roof, then it is necessary to add the coefficient C to the formula. It is equal to 0.85. But there are a number of conditions for adding this indicator. This is a wind speed of at least 4 m / s, the average monthly temperature in the winter months is not higher than -5 degrees, and the slope should be in the range from 12 to 20 degrees.

Important! If it is not clear how to calculate the load yourself, then it is better to contact specialists.

Features of installing snow retainers

If the roof structure is correctly made, taking into account the calculations, then the snow from the roof can not be removed. And in order to avoid strong slipping, snow retainers are installed without fail. Such designs are very convenient and help not to remove snow from the roof during heavy rainfall.

Typically, tubular-type snow retainers are installed, which can be used with a snow load of not more than 180 kg per 1 sq. meter. If the weight of the snow cover is greater, then the structures are installed in several rows. SNiP regulates cases and rules when the installation of snow retainers is necessary:

1. The slope is more than 5%, and there is also an external drain.
2. There must be a minimum of 0.6 m from the edge of the roof to the installed snow retainer.
3. If tubular structures are installed, then only a continuous crate is provided under them.

Features of calculating the snow load for flat roofs

A sufficiently large amount of snow accumulates on a flat type roof, so all the requirements for calculating the snow load must be met so that the roof can withstand such a weight for a long time.

In most of Russia, flat roofs are not created, since a layer of snow can create an excessive load on the rafter structure. But, if, nevertheless, the project of the house provides for just such a reinforced concrete or other roof and it cannot be replaced, then during installation it is necessary to provide a heating system to ensure high-quality water runoff from it.

Important! A flat roof should have a minimum slope of 2 degrees so that water from the entire surface can drain without problems.

Conclusion

The calculation of the snow load on the roof will help create the optimal design of the truss system, as well as keep the roofing in good condition. The correctness of the calculation depends on theoretical knowledge in this area, which can be obtained by reading this article.

Snow is a pleasant joy for many, and sometimes a great disaster for them, especially when there is a lot of it. In determining the weight, it is important to understand by its calculations, first of all, for builders, so that the roofs would not collapse.

In some countries, snow is an excellent building material, for example, in the construction of the Igloo among the Eskimos, and on holidays for the construction of original sculptures.

Snow formation as a natural phenomenon

Snow is a natural phenomenon formed by the crystallization of small water droplets in the atmosphere and falling to the ground as precipitation. Snow formation takes place in the atmosphere when microscopic water particles begin to cluster around similarly sized dust particles and crystallize. Initially, the size of the formed ice crystals does not exceed 0.1 mm. But in the process of falling to the earth's surface, depending on the temperature of the external environment, they begin to "overgrow" with other frozen water crystals and increase proportionally.

The patterned shape of snowflakes is formed due to the specific structure of water molecules. Usually these are six-pointed patterned figures, with a possible angle between the faces of either 60 or 120 degrees. In this case, the main "central" crystal forms the shape of a hexagon with regular faces. And the crystalline rays that have joined in the process of falling can give the snowflake a wide variety of shapes. Given that in the process of falling snowflakes are exposed to wind, temperature changes, they can re-increase the number of crystals, in the end they gain not only a flat, but also a three-dimensional shape. On the surface, this may seem like a heap of frozen water droplets, but if you look closely, then in the original structure all such attachments will have the right angles.

As a rule, the color of snow is white. This is due to the presence of air in its internal structure. In fact, snow is 95% air. This is what determines the “lightness” of snowflakes, as well as a smooth landing on hard surfaces. Later, when the light passes through the crystallized water, taking into account the air layers and begins to scatter, the snowflake acquires a visible white color. But this is a classic. If there are other elements in the atmosphere, including tiny particles of dust, burning, polluted by industrial emissions of air mixtures, the snow may acquire other shades.

Usually snowflakes have dimensions not exceeding 5 mm in diameter. But in history, there are cases of the formation of “giant” snowflakes, when the size of each “instance reached a diameter of up to 30 cm. At the same time, given the many factors that affect the formation of these natural creations, it is believed that it is simply impossible to find two identical snowflakes. And even if visually it seems to you that they are completely similar, looking at them under a microscope, you will understand that this is far from being the case. Variations of their possible forms today are unlimited.

How much does 1 cube of snow weigh - dependencies on dependencies

• From ambient temperature
• From time since rainfall
• From additional precipitation in the form of rain
• From the density of caking

Have great weather at home!