Rain-on-Snow Surcharge (ASCE 7-22 7.10)
The rain-on-snow surcharge added to a balanced flat-roof snow load.
Example
You enter
- Balanced flat-roof snow Pf (psf) 15
- Ground snow load Pg (psf) 18
- Roof slope (deg) 1
- Eave-to-ridge distance W (ft) 100
- Surcharge value (psf, ASCE 7-22: 8) 8
- Risk category II
You get
- Total with surcharge 23.0 psf
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe rain-on-snow surcharge added to the balanced flat-roof snow load where the ground snow Pg is no more than pm,max (25, 30, 35, or 40 psf for risk category I to IV) and the roof slope (deg) is less than W/50 (W the eave-to-ridge distance). ASCE 7-22 sets it at 8 psf, up from 5 psf under a flat 20 psf trigger, 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 35, risk category II) takes no surcharge, staying 15 psf. Balanced case only, both triggers required. A design aid, not a substitute for the engineer of record.
Formula or tableapplies = (0 < pg <= pm,max) AND (slope_deg < W/50), pm,max = 25 / 30 / 35 / 40 psf for risk category I / II / III / IV; total = pf + (applies ? 8 psf : 0).
Source and editionThe ASCE 7-22 §7.10 rain-on-snow surcharge load, by name.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the §7.10 trigger and surcharge are public.
What governsEstimate. AHJ and licensed professional govern.
API field namespf_psf, pg_psf, slope_deg, eave_to_ridge_ft, surcharge_psf, risk_category, total_psf
- Trigger 0 < Pg <= pm,max (25 / 30 / 35 / 40 psf by risk category) and slope (deg) < W/50ASCE 7-22 §7.10 and Table 7.3-4
- Surcharge 8 psf added to the balanced Pf onlyASCE 7-22 §7.10
- Balanced only not added to drift, sliding, or partial-load casesASCE 7-22 Ch. 7