Minimum Roof Snow Load (ASCE 7-22 7.3.3)
The floor a low-slope roof's design snow load cannot fall below.
Example
You enter
- Ground snow load Pg (psf) 15
- Risk category II
You get
- Minimum roof snow Pm 15.0 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
ScopePm = Pg up to pm,max, and pm,max above it, where pm,max is 25, 30, 35, or 40 psf for risk category I, II, III, or IV. The design flat-roof snow is the greater of this minimum and the computed Pf, so the exposure/thermal/slope reductions cannot drop a low-slope roof below a single heavy snowfall. Pg 15 gives 15 psf; Pg 35 caps at 30 psf for risk category II but stays 35 psf for an essential facility (IV, capped at 40). Balanced case only. A design aid, not a substitute for the engineer of record.
Formula or tablepm = min(pg, pm,max), pm,max = 25 / 30 / 35 / 40 psf for risk category I / II / III / IV; governing = max(pm, pf_computed).
Source and editionThe ASCE 7-22 §7.3.3 minimum snow load for low-slope roofs and Table 7.3-4 (pm,max), by name.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the §7.3.3 minimum relation is public.
What governsEstimate. AHJ and licensed professional govern.
API field namespg_psf, risk_category, pm_psf
- Minimum Pm = min(Pg, pm,max); pm,max 25 / 30 / 35 / 40 psf for risk category I / II / III / IVASCE 7-22 §7.3.3 and Table 7.3-4
- Governing design flat-roof snow = greater of Pm and computed PfASCE 7-22 §7.3.3
- Low-slope, balanced applies to low-slope roofs, balanced case onlyASCE 7-22 §7.3.3