Snow — roof loads
Characteristic snow loads on roofs to ČSN EN 1991-1-3 ed. 2 including the Czech National Annex. After choosing the snow region, the topography and the thermal transmittance of the roof, select a roof type or a local effect — the calculator determines the shape coefficients and the loads of every load case given by the standard and draws them in a diagram.
1 — Ground snow and coefficients
Region from the Czech snow map (NA.4). A more precise value for the site is given by the ČHMÚ digital snow map (opens in a new window) (NA.2.7). A custom value takes precedence over the region; the region still decides the National Annex conditions.
For region VIII verify the characteristic value sk against ČHMÚ data. The pre-filled value of 5.0 kN/m² is only a default.
A reduction only for roofs with high thermal transmittance, in particular some glass roofs (5.2(8)). Not below 0.8 in the Czech Republic; if the heat source is switched off, easy snow clearance from the roof must be ensured (NA.2.14).
Large flat roof — NA.2.13
For large flat roofs: Ce = 1.25 − (1.25 − Ce0)·e^(−(lc − 50)/200) for lc = 2W − W²/L > 50 m; otherwise Ce = Ce0.
Topography
Windswept: flat unobstructed area exposed on all sides without, or with little, shelter from terrain, higher buildings or trees. Sheltered: the building is considerably lower than the surrounding terrain or surrounded by high trees or higher buildings (Table 5.1). Future development around the site should be considered.
2 — Roof type
3 — Input: mono-pitch or flat roof
A flat roof is the case α = 0°. Snow guards and parapets prevent snow from sliding off — µ1 is then not reduced below 0.8 (5.3.2(2)).
Undrifted snow is case (1); drifted snow is cases (2) and (3) with half of µ2 on the windward side (Fig. 5.2, unchanged for the Czech Republic).
The valley lies between the right pitch of one span and the left pitch of the next, hence ᾱ = (α1 + α2)/2. If either pitch at the valley exceeds 60°, µ3 = 1.6 (NA.2.17).
µ4 = 0.2 + 10·h/b ≤ 2.0 applies where the tangent pitch is β ≤ 60°; steeper parts at the eaves carry µ = 0 (5.3.5). If β ≤ 60° everywhere, ls = b (NA.2.19).
h is the height from the edge of the upper roof to the surface of the lower roof at the wall; for a sloping lower roof this is h1 (NA.2.20, Fig. NA.1a). Snow weight density γ = 2 kN/m³ (5.3.6).
For roughly horizontal roofs with a projection or obstruction (6.2): µ2 = γ·h/sk with γ = 2 kN/m³, drifts on both sides.
s is the most onerous undrifted load case of the roof under consideration; layer depth d = s/γ with γ = 2 kN/m³ (NA.2.24). The load acts at the edge of the roof.
s is the most onerous undrifted load case on the roof area from which snow could slide. Snow guards prevent sliding, so µ ≥ 0.8 and s = 0.8 · Ce · Ct · sk (6.4, 5.3.2(2)). Friction between snow and roof is taken as zero.
Load arrangement
- The characteristic value sk follows the snow map of the National Annex; a more precise value for a specific site can be taken from the ČHMÚ digital snow map. A value below 0.7 kN/m² is raised to 0.7 kN/m² to NA.2.7.
- Amendment Z1 of 2025 only adds that ČSN EN 1991-1-3 ed. 3 applies in parallel and replaces ed. 2 on 31 March 2028; it does not change any relation used by the calculator.
- The calculator determines characteristic snow loads for persistent and transient design situations. It does not cover load combinations, exceptional snow loads or exceptional snow drifts (Annex B is not used in the Czech Republic), tilted panels on flat roofs to NA.2.11b, loads from snow clearance or unusual roof shapes. This is not a substitute for a complete structural design.