Making Sense of Deck Loads
The local building official often has the last word, so it’s always good to check how he or she addresses snow and live loads.
I’m designing a deck in northern Wisconsin where the ground-snow load is 60 psf (lb. per sq. ft.). When sizing the footings, beams, and joists, do I need to design them for the snow load (60 psf) plus the live load (40 psf) for a total of 100 psf, or just for the snow load since it’s the larger of the two? In other words, is the ground-snow load additive to the live load or not?
—Will P., via email
Mike Guertin: There’s been confusion about how to address snow load and live load on decks for a while. Some code officials, engineers, and contractors have added the two loads together when designing decks and others have just done as you suggest, basing the calculations on the larger of the two loads, either snow load or live load. There wasn’t a clear answer in the International Residential Code until the 2015 version, where a footnote to the ledger fastener table [R507.2] reads, “Snow load shall not be assumed to act concurrently with live load,” which means you only have to design for the greater of the two loads—in your case, the 60 psf snow load.
Though most of the country is still using older versions of the IRC, you can point to the 2015 (or 2018) IRC footnote as a qualified reference should a question arise from your local building official. Still, the local building official often has the last word, so it’s always good to check how he or she addresses snow and live loads.
Drawing by Christopher Mills.