Sliding Snow Load on a Lower Roof (ASCE 7 7.9)
The surcharge from snow sliding off a slippery upper roof onto a lower roof.
Example
You enter
- Upper-roof flat snow load Pf (psf) 20
- Upper-roof eave-to-ridge length W (ft) 40
- Lower-roof width available (ft) 15
You get
- Total (lb/ft) 320
- Surcharge on lower roof 21.3 psf
Checked against published worked examples from: AWC / M. O'Rourke, R. Mazikins, J. Showalter.
Details, formula, and sources
Scopetotal = 0.4 x the upper roof's flat snow load Pf x its eave-to-ridge length W (lb/ft), distributed over the lesser of 15 ft or the lower-roof width. A 20 psf / 40 ft upper roof drops 320 lb/ft, which over 15 ft is a 21.3 psf surcharge; a narrow 10 ft catch roof takes only its 10/15 share, 213 lb/ft at the same 21.3 psf (the rest falls off). Adds to the lower roof's own balanced load. A design aid, not a substitute for the engineer of record.
Formula or tabletotal = 0.4 x pf_upper x W (lb/ft) over 15 ft, reduced in proportion (x lower_width / 15) on a lower roof narrower than 15 ft; surcharge = total / distribution width (psf).
Source and editionThe ASCE 7 §7.9 sliding snow load on a lower roof, by name.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the §7.9 sliding-load relation is public.
What governsEstimate. AHJ and licensed professional govern.
API field namespf_upper_psf, eave_ridge_ft, lower_width_ft, total_lb_ft, surcharge_psf
- Total sliding load 0.4 x upper Pf x upper eave-to-ridge W, per foot of eaveASCE 7 §7.9
- Distribution uniform over 15 ft; a lower roof narrower than 15 ft takes total x width/15 at the same psfASCE 7 §7.9
- Superimposed added to the lower roof's own balanced snow loadASCE 7 Ch. 7