Snow Drift Surcharge at a Roof Step or Parapet (ASCE 7 Ch. 7)
The triangular surcharge that collapses lower roofs, which the flat-roof snow-load never adds.
Example
You enter
- Upwind fetch lu (ft) 100
- Ground snow load pg (psf) 30
You get
- Snow density gamma 17.9 pcf
- Drift height hd 3.52 ft
- Drift width w 14.1 ft
- Peak drift surcharge pd 63.0 psf (at the step, tapering to 0)
Details, formula, and sources
ASCE 7 Chapter 7 leeward drift height hd = 0.43 (lu)^(1/3) (pg + 10)^(1/4) - 1.5, density gamma = 0.13 pg + 14 (capped 30 pcf), peak pd = hd gamma over a 4 hd width. A 100 ft upwind roof at pg 30 psf piles 63 psf at the wall on top of the balanced load; a heavier 50 psf snow zone drives it to 83 psf. Leeward form, on top of snow-load's balanced number. A design aid, not the engineer of record's stamped design.
gamma = min(0.13 pg + 14, 30) pcf; hd = max(0.43 (lu)^(1/3) (pg + 10)^(1/4) - 1.5, 0) ft; w = 4 hd (hd <= hc) else min(4 hd^2/hc, 8 hc); pd = hd gamma psf.
The ASCE 7-22 Chapter 7 leeward snow-drift provisions -- the drift height 7.7.1, the snow density 7.7.1, the surcharge pd = hd gamma, and the drift width -- by name.
ASCE 7 is available through the ASCE Library at ascelibrary.org; the Chapter 7 drift relations are public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: lu_ft, pg_psf, gamma_pcf, hd_ft, w_ft, pd_psf
- Drift height hd = 0.43 (lu)^(1/3) (pg + 10)^(1/4) - 1.5, floored at zero for a short fetchASCE 7-22 7.7.1
- Density gamma = 0.13 pg + 14, capped at 30 pcfASCE 7-22 7.7.1
- Surcharge and width pd = hd gamma at the step over a 4 hd width (full-height triangle, hd <= hc)ASCE 7-22 7.7.1