Unbalanced Snow Load on Gable Roof (ASCE 7 7.6.1)
The ASCE 7-22 7.6.1 unbalanced gable case.
Example
You enter
- Ground snow load pg (psf) 30
- Balanced sloped-roof snow ps (psf) 25
- Roof slope (rise on 12) 4
- Eave-to-ridge length W (ft) 30
- Winter wind parameter W2 (Figure 7.6-1) 0.5
You get
- Windward slope load 7.5 psf
- Leeward peak (psf) 48.5
- Surcharge extent from ridge 10.5 ft
Checked against published worked examples from: ASCE 7-22 Section 7.6.1 (unbalanced snow load, gable roofs).
Details, formula, and sources
ScopeWindward slope drops to 0.3 ps while the leeward carries ps plus a wind-blown ridge drift surcharge (hd gamma/sqrt(S)), with hd from the 7-22 winter-wind relation. Applies in the ~1/2:12 to 7:12 slope band; with W <= 20 ft (simply supported prismatic members) the windward slope is 0 and the whole leeward slope carries pg; sizes the leeward rafter and ridge the balanced case misses.
Formula or tablegamma = min(0.13 pg + 14, 30); hd = 1.5 sqrt(pg^0.74 W^0.70 W2^1.7 / gamma) (Eq. 7.6-1, lu = W with no 20 ft floor); windward = 0.3 ps; surcharge = hd gamma/sqrt(S) (S = 12/rise); leeward_peak = ps + surcharge; extent = 8 hd sqrt(S)/3.
Source and editionThe ASCE 7-22 Section 7.6.1 unbalanced snow load on gable/hip roofs, by name.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the Chapter 7 snow provisions are public.
What governsEstimate. AHJ and licensed professional govern.
API field namesground_snow_pg_psf, flat_roof_ps_psf, roof_rise_on_12, eave_to_ridge_ft, w2, windward_psf, leeward_peak_psf, extent_ft
- Applicability band slope ~2.38-30.2 deg (1/2:12 to 7:12); W <= 20 ft takes windward 0 / leeward pg; outside the band only the balanced load governsASCE 7-22 7.6.1
- Leeward surcharge ps + hd gamma/sqrt(S), tapering over 8 hd sqrt(S)/3 from the ridgeASCE 7-22 7.6.1