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Ron Rodrigue

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"Danger" Snow Weight of snow on Roofs

02-04-09
Ron Rodrigue

The Snow Weight Of Snow This Winter

Most roof collapses from heavy snow loads are caused by failures, not failure of framing members. Snow loads are based on figures published by the American Society of Civil Engineers [ ASCE ]. Estimates for the actual weight of snow range from 1 to 1.5 psf per inch of depth. The density of snow increases as the depth increases. Fot example, in 48-inch-deep snow, the design density is more than 2.4 psf per inch while the actual density is probably 1.5 psf per inch or more.

Snow Depth Design Weight Est. Actual Weight

6 inches 7.5 psf 6-9 psf

12 inches 16.1 psf 12-18 psf

18 inches 26.1 psf 18-24 psf

24 inches 37.8 psf 24-36 psf

30 inches 51.8 psf 30-45 psf

36 inches 69.0 psf 36-54 psf

42 inches 90.0 psf 42-63 psf

48 inches 117 psf 48-72 psf

Generally, loads get higher where more snow falls, when roofs are shallower rather than steeper, and where drift loads can accumulate. Note that while a 30-inch-deep snowfall corresponds to a design load of about 52 psf, the actual weight of that snow is probably somewhat less, in the neighorhood of 40 psf. For obvious reasons, engineers err on the high side when calaulating design loads. JUst the same that is a lot of weight, so it's no surprise that an improperly designed or built roof { or deck, for that matter} can fail in a big snowstorm or heavy snow depth saturated with rain. So shovel your snow off the rooof or deck before it rains in your area.

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