Minimum Roof Snow Load (ASCE 7 7.3.4)
The floor a low-slope roof's design snow load cannot fall below: 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.
Formula and source
pm = (pg <= 20 psf) ? Is x pg : 20 x Is; governing = max(pm, pf_computed).
The ASCE 7 ยง7.3.4 minimum roof snow load Pm, by name.
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