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
Details, formula, and sources
total = 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 concentrates the same total into 32 psf. Adds to the lower roof's own balanced load. A design aid, not a substitute for the engineer of record.
total = 0.4 x pf_upper x W (lb/ft); distributed over min(15 ft, lower_width); surcharge = total / distribution width (psf).
The ASCE 7 §7.9 sliding snow load on a lower roof, by name.
ASCE 7 is available through the ASCE Library at ascelibrary.org; the §7.9 sliding-load relation is public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: pf_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 the lesser of 15 ft or the lower-roof widthASCE 7 §7.9
- Superimposed added to the lower roof's own balanced snow loadASCE 7 Ch. 7