Snow drift load calculator
The roof step
Drift surcharge of +73.1 psf (leeward) over a 15.23 ft width, on top of the 28 psf balanced load.
| Snow density, γ | 19.2 pcf |
| Balanced snow height, hb | 1.46 ft |
| Clear height, hc | 4.54 ft |
| Leeward drift height | 3.81 ft |
| Windward drift height | 2.03 ft |
| Governing drift height, hd | 3.81 ft |
§7.7: hd = 0.43·(L)^(1/3)·(Pg+10)^(1/4) − 1.5; surcharge pd = hd·γ. Windward case uses 0.75× of the lower-roof fetch.
Notes & questions
01When do I need to check snow drift?+
Whenever a lower roof sits next to a taller roof, a parapet or a long rooftop unit. ASCE 7 §7.7 says drift can be ignored only when the clear height above the balanced snow is small (hc/hb < 0.2); otherwise the drift surcharge often governs the lower roof.
02How is the drift height calculated?+
The leeward drift height is hd = 0.43 × (Lu)^(1/3) × (Pg + 10)^(1/4) − 1.5 ft, where Lu is the upper roof length and Pg the ground snow load. The windward case uses 0.75× of the same formula with the lower-roof length. The larger governs, capped at the clear height.
03What load does the drift add?+
The peak surcharge is pd = hd × γ, where γ = 0.13·Pg + 14 pcf (capped at 30). It is a triangular load that sits on top of the balanced load against the step, tapering to zero over the drift width.