Snow Drift Loads on Roofs (ASCE 7 §7.7) Made Simple

Some of the worst roof snow failures are not from uniform snow. They come from drifts that pile up where wind dumps snow against a taller wall, parapet or rooftop unit. ASCE 7 §7.7 covers these drift surcharge loads, and they often govern the design of a lower roof.

Where drifts form

A drift builds on a lower roof next to a taller upper roof (a roof step), against a parapet, or beside a long rooftop screen. Wind carries snow off the upper surface and deposits it in a triangular drift on the lee side.

What sets the drift size

The leeward drift height depends on the ground snow load and the length of the upper roof feeding it: hd = 0.43 × (Lu)^(1/3) × (Pg + 10)^(1/4) − 1.5 ft. The snow density γ = 0.13 Pg + 14 (capped at 30 pcf) turns that height into a surcharge load pd = hd × γ, which sits on top of the balanced load.

When you can skip it

If the clear height above the balanced snow is small relative to the balanced snow depth (hc/hb < 0.2), drift need not be considered. Otherwise, run it: our snow drift calculator does the full §7.7 check.