Snow load under DBN V.1.2-2: snow region, coefficients, distribution schemes

How the roof load is built up

The starting value is S0, the characteristic snow load. It is the weight of the snow cover on 1 m² of ground that may on average be exceeded once in 50 years. It is measured in pascals, and in the tables of the standard it is given in Pa.

The roof load is obtained from S0 by multiplying by the coefficient C and by the reliability factor. C itself has three factors: μ, the coefficient for the transition from snow on the ground to snow on the roof; Ce, the roof use factor; and Calt, the altitude factor. The load is referred to the horizontal projection of the roof, not to the length of the slope.

The standard gives three design values of snow load: limit, operational and quasi-permanent. The limit value is used in checks of the first group of limit states; the operational and quasi-permanent values are used where the second group is concerned, in particular deflections and long-term action.

Snow region and the S0 value

S0 is found from the map of Ukraine by the weight of the snow cover or from the table of towns in the reference annex. The table is convenient for preliminary estimates, but the map remains the basis, and for a site outside a town or in the mountains the town value may be inaccurate.

The table below gives S0 for regional centres as they appear in the base text of DBN V.1.2-2:2006. Amendments to the standard may have refined them, so for a real project the values are taken from the current edition, from the map or the table, and checked against the site.

Values from the reference annex to the base text of DBN V.1.2-2:2006, for regional centres. They show the spread across the country (from about 0.8 to over 1.7 kPa) but do not replace the value from the current edition for your site.

Coefficient μ and distribution schemes

The standard gives μ in an annex of schemes. For every roof shape there is a scheme with one or several loading variants, and intermediate coefficient values are found by linear interpolation. The coefficients themselves are in drawings, so they are not copied here: take the value from the scheme for your roof.

The list of schemes shows which roof shapes the standard distinguishes. It is worth looking through before the calculation, so as not to take the scheme of a monopitch roof for a building with a lantern or a height difference.

Partial loading and uneven snow

Uniform snow is not always the worst case. If the members work worse under partial loading, the standard requires schemes with snow on half or a quarter of the span, and for roofs with lanterns, on zones of width b. For a truss this means a separate check of diagonals and verticals: under one-sided snow the forces in them can change sign. Where needed, the planned extension of the building is allowed for too.

The standard requires variants with increased local load to be taken into account for slabs, decking and purlins, and for those elements of the load-bearing structures (trusses, beams, columns) whose sizes are set by exactly that variant. For structures, the schemes of the annex may be replaced with simplified ones of equivalent effect. In frames and columns of industrial buildings only uniform snow may be taken, except at height differences between roofs, where the increased load is mandatory.

At a height difference the load depends on the height of the difference (in the formulas it is taken as not more than 8 m), on the share of snow carried by the wind to the difference, and on the lengths of the roof portions the snow is carried from. The length of the zone of increased snow is limited, in particular to not more than 16 m. For μ in this zone the standard gives upper limits: 4 if the lower roof belongs to a building and 6 if it is a canopy. A canopy next to the wall of a tall building is therefore in the hardest regime of all height-difference cases. All these numbers are checked against the text of the standard before use.

Lanterns: limits on μ by element type

For roofs with longitudinal lanterns the standard separately limits μ in the zones of increased snow, and the limit depends on what is being calculated. For steel profiled decking it is lower than for trusses, beams and purlins. This is an example of why the load on decking and on a truss from the same snow differs.

The numbers are read from the text of the standard, but the schemes next to them are in drawings: check against the current edition before use.

Ce, Calt and the roof regime

Ce shows how much snow actually stays on the roof, allowing for clearing and melting. It is set by the design brief. For unheated roofs of shops with increased heat release, at a slope of more than 3 % and with proper drainage of meltwater, the standard says to take Ce = 0.8. If there is no data on the roof regime, Ce = 1 is allowed.

Calt takes into account the height H of the site above sea level in kilometres: Calt = 1.4H + 0.3 for H of 0.5 km and above, and Calt = 1 below. The formula is meant for mountain terrain and gives an approximate value on the safe side. For example, at 1 km Calt is 1.7. If snow survey results exist for the site, S0 is found by their statistical processing, and then Calt = 1.

Both coefficients are easy to underestimate in a design: Ce = 1 with no note in the brief sets a level the customer may later fail to maintain, and a mountain site without Calt ends up loaded too lightly. So the roof clearing regime and the site altitude are worth writing into the input data.

Combinations and the erection period

Snow enters the design combinations as a variable load. With the limit and operational values it is short-term, and with the quasi-permanent value it belongs to the long-term loads. In main combinations that include permanent loads and at least two variable ones, the variable loads are multiplied by a combination factor of 0.95 for long-term and 0.90 for short-term loads. If a combination has three or more short-term loads, 1.0 may be taken for the first by influence, 0.8 for the second and 0.6 for the rest.

For the erection period, the snow, wind, ice and climatic temperature effects included in the design combinations are to be reduced by 20 % under the standard. This applies to an unfinished structure while it has no roofing on it, but it does not cancel the stability checks of individual members during erection.

In the deflection check, snow does not enter every group of requirements. How this looks in a calculation is shown in the articles on the beam and the truss: both say that short-term loads are not taken for the aesthetic-psychological requirement. Frame columns are covered in Steel column calculation. Wind load on poles and masts is in the article on wind regions.

What data a roof calculation needs

To find the snow load on a canopy, truss or frame you need the town and the site altitude above sea level, the roof shape with the slope angle, whether there are lanterns, height differences and parapets, and also whether the building is heated and whether the snow is cleared. Without this the coefficients cannot be chosen.

We prepare KM and KMD drawings, fabricate steel structures to the customer’s drawings and design frames: KM and KMD design, steel fabrication. The snow load for the frame calculation is set by the project design or the customer’s input data. If there are none, we can help draw up the list of input data. Erection on site is done by our partner construction company.

Standards and validity

• DBN V.1.2-2:2006 «Loads and actions. Design standards» with Amendments No. 1 and No. 2: the section on snow loads, the reference annex with values for towns, the annex with snow load schemes. As of October 2026 the document is listed as in force; its validity and current edition need to be checked before the calculation.
• DBN V.1.2-14:2018 «General principles of reliability and structural safety of buildings and structures» and DBN V.2.6-198:2014 «Steel structures. Design standards»: consequence class, member checks. They are used in the calculation articles; their validity and current edition need to be checked.

The numbers from maps, tables and schemes in this article are for reference. A real project uses the values from the current edition of the standard, not from this text.