Minimum Roof Snow Load (ASCE 7 7.3.4)
The floor a low-slope roof's design snow load cannot fall below.
Example
You enter
- Ground snow load Pg (psf) 15
- Snow importance factor Is (Table 1.5-2) 1
You get
- Minimum roof snow Pm 15.0 psf
Details, formula, and sources
Pm = Is x Pg where the ground snow Pg is 20 psf or less, or 20 x Is where Pg is over 20 (Is the snow importance factor). 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 / Is 1.0 gives 15 psf; Pg 30 caps at 20 psf; Pg 25 with an essential-facility Is 1.1 gives 22 psf. Balanced case only. A design aid, not a substitute for the engineer of record.
pm = (pg <= 20 psf) ? Is x pg : 20 x Is; governing = max(pm, pf_computed).
The ASCE 7 §7.3.4 minimum roof snow load Pm, by name.
ASCE 7 is available through the ASCE Library at ascelibrary.org; the §7.3.4 minimum relation is public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: pg_psf, importance, pm_psf
- Minimum Pm = Is x Pg for Pg <= 20 psf, else 20 x IsASCE 7 §7.3.4
- Governing design flat-roof snow = greater of Pm and computed PfASCE 7 §7.3.4
- Low-slope, balanced applies to low-slope roofs, balanced case onlyASCE 7 §7.3.4