Analysis & loading · Wind, snow & cyclone
Snow loads with drifting at a roof step calculator
Roof snow load for alpine and sub-alpine sites. Balanced and unbalanced shape coefficients for mono- and duo-pitch roofs, and the drift against a higher roof or parapet (μw = (b1 + b2)/2h limited by γh/sg, drift length 2h), with the design line load on purlins. Enter the ground snow load sg for the site from AS/NZS 1170.3 Section 5 (depends on region and altitude).
Standard: AS/NZS 1170.3:2003 roof snow load s = μ·Ce·Ct·sg, drift at steps (shape coefficients as EN 1991-1-3)
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Ground snow load sg (site, 1/R) | 3 | kPa |
| Exposure coefficient Ce | 1 | |
| Thermal coefficient Ct | 1 | |
| Roof | Duo-pitch (gable) | |
| Roof pitch | 15 | ° |
| Step / parapet height above the roof (0 = none) | 2 | m |
| Upper roof width (feeding the drift) | 15 | m |
| Lower roof width | 10 | m |
| Snow density γ | 2 | kN/m³ |
| Purlin spacing | 1.2 | m |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Balanced shape coefficient μ1 | 0.80 | |
| Balanced roof snow s | 2.40 | kPa |
| Unbalanced (duo-pitch, α ≥ 15°) | 1.20 / 2.40 kPa | |
| Drift coefficient μw | 1.33 | |
| Drift length ls | 5.0 | m |
| Peak drift load at the step | 4.00 | kPa |
| Purlin line load at the step (ULS = Su) | 4.80 | kN/m |
| Purlin line load, balanced | 2.88 | kN/m |
- ✓ Drift limited by the step height (μw ≤ γh/sg)
- ✓ Roof pitch < 60° (snow retained)
Method, formulas and assumptions
- s = μ·Ce·Ct·sg; μ1 = 0.8 for 0–30°, 0.8(60 − α)/30 for 30–60°, 0 above 60°. Duo-pitch unbalanced case: 0.5μ1 on one side and μ1 on the other.
- Drift at a step: μw = (b1 + b2)/2h, 0.8 ≤ μw ≤ γh/sg and ≤ 4, over ls = 2h (5 ≤ ls ≤ 15 m), reducing linearly to the balanced value.
- AS/NZS 1170.0 ULS combination: 1.2G + Su + ψcQ (Su = s at the annual probability for the importance level).
- sg for Australian alpine sites (Section 5) increases with elevation above about 1200 m in the Alpine region; confirm from the standard.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.