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: 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.
Formula and source
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 ยง7.9 sliding snow load on a lower roof, by name.
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