Snow load is often the governing load for light steel roofs and for buildings at high altitude. This guide explains how roof snow load is calculated to EN 1991-1-3 (Eurocode 1, Part 1-3). The numbers in the example only illustrate the method; in a real project the ground snow load comes from the national annex or the relevant regulation for the site’s region and altitude.
Snow load formula
s = μi · Ce · Ct · sk
- sk: Characteristic ground snow load (kN/m²), set by region and altitude.
- μi: Roof shape coefficient, depending on the roof’s shape and pitch.
- Ce: Exposure coefficient: 0.8 for windswept sites, 1.0 for normal sites, 1.2 for sheltered sites.
- Ct: Thermal coefficient, usually 1.0.
Snow load acts per square metre of the roof’s horizontal projection, not of the sloping surface.
Roof shape coefficient μ1
For monopitch and duopitch roofs, by roof pitch α:
- 0° ≤ α ≤ 30°: μ1 = 0.8
- 30° < α < 60°: μ1 = 0.8 · (60 − α) / 30
- α ≥ 60°: μ1 = 0 (snow is assumed not to stay)
Unbalanced snow on duopitch roofs
On duopitch roofs, not only the balanced case with both sides equally loaded is checked, but also the unbalanced cases where wind moves snow to one side: μ1 on one side and 0.5 μ1 on the other. Whichever is more onerous for the structure is used.
Snow drift
Snow accumulates behind parapets, on lower roofs next to higher ones and in the valleys of multi-span roofs. In these areas the shape coefficient, and so the snow load, is considerably higher than on a plain roof and must be calculated locally.
Worked example
To illustrate the method with example values: sk = 0.75 kN/m², Ce = 1.0, Ct = 1.0.
- Roof pitch 20°: μ1 = 0.8 → s = 0.8 × 1.0 × 1.0 × 0.75 = 0.60 kN/m²
- Roof pitch 45°: μ1 = 0.8 × (60 − 45) / 30 = 0.4 → s = 0.30 kN/m²
- Unbalanced case on a 20° duopitch roof: one side 0.60 kN/m², the other 0.30 kN/m².
Common mistakes
- Applying snow load to the sloping roof area instead of the horizontal projection
- Missing snow drift at parapets and changes in roof level
- Not checking the unbalanced case on duopitch roofs
- Using the wrong altitude or an out-of-date regional value
This guide is for information only; design calculations should be carried out by a competent engineer to the national annex and project specification. For calculation and reporting of snow, wind and seismic loads, see our structural analysis and load calculations service.
Frequently asked questions
Which standard is used to calculate snow load?
In Europe EN 1991-1-3 (Eurocode 1); in Türkiye it is published as TS EN 1991-1-3, and some older Turkish specifications refer to TS 498. The calculation follows the project specification.
Where do I find the characteristic ground snow load s_k?
From the snow load map and tables in the national annex or the relevant regulation, based on the site’s region and altitude.
Does snow load act on the sloping surface or horizontally?
In EN 1991-1-3 snow load is given per square metre of the roof’s horizontal projection.
Is snow load calculated differently for steel buildings?
The snow load itself does not depend on the building material; the difference is in checking the steel members under that load. On light steel roofs, snow drift and wind suction should be considered together.
Related pages
- Structural analysis and load calculations
- How to calculate wind load
- Machine design
- Engineering services
Get your load calculation done
Send us the dimensions, location and any drawings of the structure, and we will calculate and report the snow, wind and seismic loads. WhatsApp: +90 547 909 99 90 · info@dokuti.com