Snow drift calculator
A step in a roof collects a drift that can be several times the balanced snow load. This evaluates the ASCE 7-22 leeward drift equation in its published form, applies the step cap and gives the width and surcharge the roof beam has to carry.
| Quantity | Value | Unit |
|---|---|---|
| Ground snow load | 1.20 | kPa |
| Upper roof length | 15.0 | m |
| Step height | 2.00 | m |
| Drift height hd | 0.71 | m |
| Drift width w | 2.85 | m |
| Snow density | 1121 | kg/m³ |
| Peak surcharge | 7.83 | kPa |
ASCE 7-22 §7.7.1 leeward drift; the equation is empirical and is evaluated in its published imperial form.
hd = 0.43·lu^(1/3)·(pg + 10)^(1/4) − 1.5 · Imperial form: lu in ft, pg in psf, hd in ft.
w = 4·hd · Widened when the step limits the height.
p = γ · hd · Added over the drift width.
A clear path from inputs to results.
A step in a roof collects a drift that can be several times the balanced snow load, and it is the load that breaks roofs. ASCE 7 gives an empirical equation for the drift height; this evaluates it in its published units rather than re-fitting it in SI.
How to use
- Enter the ground snow load, the upper roof length and the step height.
- Read the drift height, width and the peak surcharge.
- Check whether the step capped the height — the width widens to conserve the drift area.
The formula
Reference: ASCE 7-22 §7.7.1, evaluated in feet and psf and converted.
Worked example
15 m upper roof, 2 m step, 1.2 kPa ground snow
- pg = 25.1 psf, lu = 49.2 ft.
- 1
hd = 0.43 × 3.664 × 2.434 − 1.5 = 2.34 ft = 0.71 m. Width 2.85 m. Density 70 pcf → surcharge 7.8 kPa.
0.71 m drift, 2.85 m wide, 7.8 kPa peak.
Interpreting the result
The cube root on length is why a long roof does not produce a proportionally bigger drift; the height difference matters far more, and the cap at hc is what usually governs.
Assumptions & limitations
- Leeward drift only; windward and sliding drifts are separate.
- Ground snow load from the ASCE map or the authority having jurisdiction.
- Surface roughness and exposure are already in pg.
- Drift load adds to the balanced snow load.
Frequently asked questions
Why is the equation in imperial units?
It was fitted in imperial and the coefficients do not convert cleanly, so the inputs are converted instead.
Does this cover sliding snow?
No — sliding snow from a steep upper roof is §7.9 and can be larger.
What if the drift is taller than the step?
It is capped at hc and the width widens so the drift triangle keeps its area.
Where does the density come from?
γ = 0.13·pg + 14 pcf, with a floor of 70 pcf, both from §7.7.1.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.