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: 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.
Formula and source
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.
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