Reference

Methodology

RoofHelm implements the snow load procedure of ASCE/SEI 7-22, Chapter 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures). Every factor below is exactly the value the calculator uses. This page is the single source of truth.

The equations

Flat-roof load	Pf = 0.7 · Ce · Ct · Is · Pg	Eq. 7.3-1
Sloped load	Ps = Cs · Pf	Eq. 7.4-1
Minimum load	Pm = Is · min(Pg, 20)	§7.3.4, slope < 15°
Rain-on-snow	+5 psf when 0 < Pg ≤ 20 and slope < W/50	§7.10
Snow density	γ = 0.13 · Pg + 14 ≤ 30 pcf	Eq. 7.7-1
Drift height	hd = 0.43 · L^⅓ · (Pg+10)^¼ − 1.5	§7.7
Unbalanced	gable/hip: leeward Ps + hd·γ/√S surcharge	§7.6.1

Ground snow load, Pg

Pg is a site-specific input, set by the ASCE 7 Hazard Tool or your local building department. RoofHelm never assumes it. The per-state pages provide a planning range read from the ASCE 7 ground snow load map; the value adopted by your jurisdiction governs a permit.

Exposure factor Ce, Table 7.3-1

Terrain	Fully exposed	Partially exposed	Sheltered
B	0.9	1	1.2
C	0.9	1	1.1
D	0.8	0.9	1
Above treeline	0.7	0.8	0.8

Thermal factor Ct, Table 7.3-2

Heated building	1
Just above freezing or cold ventilated	1.1
Unheated or open	1.2
Freezer	1.3
Heated greenhouse	0.85

Importance factor Is, Table 1.5-2

Risk Category I	0.8
Risk Category II	1
Risk Category III	1.1
Risk Category IV	1.2

Slope factor Cs, §7.4 and Fig. 7.4-1

Cs is 1.0 up to a breakpoint slope, then falls linearly to 0 at 70°. The breakpoint depends on whether the roof is warm (Ct ≤ 1.0), cold ventilated (Ct = 1.1) or cold (Ct ≥ 1.2), and on whether the surface is slippery: warm slippery 5°, warm other 30°; cold-ventilated slippery 10°, other 37.5°; cold slippery 15°, other 45°.

What this tool covers, and what it does not

RoofHelm computes the balanced roof snow load, the §7.3.4 minimum and the §7.10 rain-on-snow case; the §7.6.1 unbalanced case for hip and gable roofs; and, on the drift page, the §7.7 drift surcharge at roof steps. It does not yet resolve sliding snow (§7.9), partial loading (§7.5), the monoslope and sawtooth unbalanced cases (§7.6.2 and §7.6.3), or curved roofs. Those can govern on specific geometries and must be checked by a qualified engineer for a permit submittal. Always confirm the governing ground snow load and have the design reviewed by a licensed professional.