Snow Drift Load at a Roof Step
Snow Drift Load at a Roof Step
Wherever a roof steps down — at a parapet, a lower adjoining roof, or next to a taller building — wind-blown snow accumulates disproportionately in the resulting pocket, piling up far deeper than the roof's general "balanced" snow load. This drift load is a localized surcharge that can be several times the uniform design snow load, concentrated exactly where the structure below (often a lower roof's framing) may be least expecting extra load.Roof collapses in heavy snow years are disproportionately traced to under-designed drift zones rather than the uniform snow load, because designers sometimes size the general roof for the base snow load and only later account for drift at specific step locations — every roof step, parapet, and rooftop unit needs its own explicit drift check, not just the overall roof.
The additional pressure from a snow drift is p_d = gamma_s·h_d, the snow unit weight times the drift height, applied as a triangular or rectangular surcharge at the step location. where gamma_s is the unit weight of the drifted snow and h_d is the height of the snow drift above the general roof snow depth, giving the resulting drift surcharge pressure p_d.
Inputs
gamma_s := 3 kN/m^3=3000 kg/m^2/s^2
h_d := 0.6 m=600 mm
The drift surcharge pressure is simply the weight of the extra snow column height above the general roof snow depth.
p_d := gamma_s·h_d=1800 Pa
Results
A drift surcharge of about 1.8 kPa on top of the general roof snow load is a meaningful addition — often comparable to or larger than the uniform design snow load itself — and it is added over a limited drift width, not the whole roof. Because drift height depends strongly on the height and length of the adjacent roof step (per code drift formulas), even a modest step next to a long roof run can produce a tall, heavy drift; this is why drift geometry is checked at every step, not assumed proportional to roof area. The framing directly under the drift zone (often the lower roof's joists or purlins near the wall) needs to be checked against this concentrated surcharge, in addition to the general uniform snow load elsewhere on the roof.
Assumptions & limitations
- Snow unit weight is a representative average; actual drifted snow density varies with age and compaction.
- Drift height is taken as a given input; real designs compute it from roof step geometry per code drift equations.
- Drift load is applied as a simplified uniform surcharge rather than the code's triangular distribution.
- Only the drift surcharge is shown — it is additive to (not a replacement for) the general roof snow load elsewhere.