Rain-on-Snow Surcharge (ASCE 7-22 7.10)
The rain-on-snow surcharge added to the balanced flat-roof snow load where the ground snow Pg is 20 psf or less and the roof slope (deg) is less than W/50 (W the eave-to-ridge distance). ASCE 7-22 raised this to 5-8 psf (commonly 8) from the older flat 5, because a low-slope roof in wet-snow climates holds rain a steeper roof would shed. A Pf of 15 psf at Pg 18 low-slope becomes 23 psf; a deep-snow region (Pg 25) takes no surcharge, staying 15 psf. Balanced case only, both triggers required. A design aid, not a substitute for the engineer of record.
Formula and source
applies = (pg <= 20 psf) AND (slope_deg < W/50); total = pf + (applies ? surcharge : 0); ASCE 7-22 surcharge 5-8 psf (default 8).
The ASCE 7-22 ยง7.10 rain-on-snow surcharge load, by name.
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