What this calculator does
A roof does not carry the snow that falls on the ground. It carries the snow ON the roof, which depends on the shape, how steep it is, how warm it is and how much wind can clear it. The roof coefficients can easily double or halve the answer, and the drift at an upstand can add more than the uniform load.
How to use it
- Enter the ground snow load for your site from your local map or building official — it is not looked up here.
- Enter the roof shape, exposure and thermal coefficients for this roof from your code work.
- Enter the roof plan area, then the drifting zone area and drift coefficient for any upstand or obstruction.
- Read the drift percentage: it is often the number that governs near an obstruction rather than the flat roof.
Governing formula
p_k = μ · C_e · C_t · s_k Where:
-
μ= Roof shape coefficient -
C_e= Exposure coefficient -
C_t= Thermal coefficient -
s_k= Ground snow load, kN/m²
Derived Equations:
F = A · p_k Worked example — 1.5 kN/m² on the ground, shallow roof
The figures below were produced by running this page's own engine with the example inputs (Ground snow load (kN/m²), Roof shape coefficient (μ) (–), Exposure coefficient (C_e) (–), Thermal coefficient (C_t) (–), Roof plan area (m²), Drift height (m), Drift coefficient (–), Area within the drifting zone (m²)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Roof snow load | 0.6 kN/m² |
|---|---|
| Total snow force on the roof | 120 kN |
| Drift adds this much extra | 20 % |
| Extra force from the drift | 24 kN |
| Combined roof coefficient | 0.4 – |
| Equivalent snow depth | 0.09 m |
| What this means | The roof carries 0.600 kN/m² and the drift adds only 20 % — the uniform load governs at this geometry. |
- The combined roof coefficient is 0.4 × 1 × 1 = 0.4.
- That gives 0.4 × 1.5 = 0.6 kN/m² on the roof, 120 kN over 200 m².
- The drift loads 1.2 kN/m² over 20 m², adding 24 kN — 20 % on top.
Assumptions & limitations
Engineering calculation. Roof snow load from the ground value and the shape, exposure and thermal coefficients, plus the extra force from drifting at an upstand.
- Every coefficient is YOUR input. This tool holds no code table, names no code edition, and selects nothing for you.
- The ground snow load s_k is entered from your own site data, not looked up. It must match your local authority and code edition.
- The drift calculation is a simple upstand relationship from factors you supply. It does not model drift geometry around an actual roof obstruction, a parapet profile or a neighbouring building.
- Static snow only. Roof snow melting, sliding, ponding instability, ice dams, rain-on-snow surcharge and drifting against the wind are not modelled.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 1991-1-3 and ASCE/SEI 7 — snow actions, as the framework behind the coefficients.