Free Tool · EN 1991-1-3:2003
Derive EN 1991-1-3 snow actions for roofs. Computes characteristic ground snow load sk with altitude correction, roof shape coefficients μ1/μ2 per Table 5.2, exposure coefficient Ce (Table 5.1), thermal coefficient Ct, and design snow load s = μi·Ce·Ct·sk for undrifted and drifted cases in kN/m².
EN 1991-1-3 §5.3 — roof shape coefficient distribution
National Annex divergence
NL NEN-EN 1991-1-3/NA:2011: single zone s_k = 0.56 kN/m² (flat country; linear altitude correction +0.10 kN/m² per 100m).
DE DIN EN 1991-1-3/NA:2012: four zones Z1/1a/2/3 (0.65–1.10 kN/m² at sea level); altitude formula s_k = max(s_k0, 0.31 + (A/256)²).
1. Ground snow load s_k (§4.1 + Annex C)
2. Exposure coefficient C_e (Table 5.1) / 3. Thermal coefficient C_t (§5.2(8))
4. Shape coefficient μ₁ (Table 5.2)
| Case | Slope 1 (μ) | Slope 2 (μ) |
|---|---|---|
| Undrifted | 0.8 | 0.8 |
| Drifted Case I | 0.4 | 0.8 |
| Drifted Case II | 0.8 | 0.4 |
5. Design snow load s = μ·Ce·Ct·sk (eq. 5.1)
| Case | Slope 1 s (kN/m²) | Slope 2 s (kN/m²) |
|---|---|---|
| Undrifted | 0.68 | 0.68 |
| Drifted Case I | 0.34 | 0.68 |
| Drifted Case II gov | 0.68 | 0.34 |
ψ factors (EN 1990 load combinations)
Pair with these tools
For full ULS load combinations per EN 1990 §6.4.3.2, pair snow with wind load.
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Snow loads integrated into full frame analysis
Pro automatically applies s_k, μ, and C_e/C_t to every rafter and portal frame in your drawing set — combined with wind (EN 1990 §6.4.3.2) and output to the fabrication drawing.
See Pro plans →How is the characteristic ground snow load s_k determined?
EN 1991-1-3 uses national zone maps. s_k,0 is the reference value at sea level; altitude correction per Annex C eq. C.1: s_k = s_k,0 · [1 + (A/728)²]. For the DE NA: s_k = max(s_k,0, 0.31 + (A/256)²). For NL: single zone s_k = 0.56 + 0.10·(A/100) kN/m².
What is the shape coefficient μ₁?
μ₁ governs roof snow load: μ₁ = 0.8 for α ≤ 30°; linear decay to 0 at α = 60°; 0 above. This reflects that steep roofs shed snow more effectively. The drifted case uses 0.5·μ₁ on one slope (asymmetric).
When does the drifted case govern?
For duopitch roofs the drifted arrangement (one slope at 0.5·μ₁, other at μ₁) is checked alongside the undrifted symmetric case. The drifted case often governs the lighter (windward) slope at low pitches because of redistribution.
What does the valley accumulation coefficient μ₂ mean?
For multi-span (saw-tooth) roofs, wind and sliding snow accumulate in the valley. μ₂ = μ₁ + μ_w + μ_s, capped at 2.0. μ_w depends on bay widths and ridge-to-valley height; μ_s on sliding from steep slopes (α > 30°).