Roof Snow Load Calculator — BS EN 1991-1-3

The UK is divided into ground snow load zones (zone number Z) on the map in Figure NA.1 of the BS EN 1991-1-3 UK National Annex, with sk,0 = 0.1Z + 0.2 kN/m². As an approximate regional guide, southern England is around Zone 1 (0.30 kN/m²), the Midlands Zone 2, northern England Zone 3, and Scotland Zones 4–5 — but always read your exact Z off Figure NA.1, as the boundaries are geographic, not by county. The base value is then adjusted for altitude.

Snow load increases with altitude per BS EN 1991-1-3 UK National Annex (Equation NA.1, clause NA.2.8). Below 100 m AOD no correction is applied. Above 100 m the altitude term (A − 100)/525, in kN/m², is added to the base zone value: sk = sk,0 + (A − 100)/525, where A is altitude in metres (valid up to 1500 m). At 300 m in Zone 2: sk = 0.40 + (300 − 100)/525 ≈ 0.78 kN/m². The correction is linear with altitude (added on, not a multiplier).

The shape coefficient (μ1) reduces snow load on steeper roofs because snow slides off. For pitches up to 30°, μ1 = 0.8. Between 30° and 60°, it reduces linearly. Above 60°, it’s zero (no snow retention).

Drift loading occurs where snow accumulates against obstructions like parapet walls, changes in roof level, or adjacent taller buildings. Drift loads can be significantly higher than the uniform snow load and must be checked separately.

This calculator provides indicative snow loads based on BS EN 1991-1-3 for preliminary assessment. For structural design, always consult a chartered structural engineer who will consider all load combinations and safety factors. Also check wind loading with our Roof Wind Load Calculator.