Shed and Garage Snow Load: What to Know

Sheds and detached garages are often built with minimal roof framing: lightweight trusses, spaced 24 inches on centre, with thin roof decking. In heavy-snow regions, these outbuildings can be at more risk than the main house.

Risk Category I lowers the load, but does not remove it

ASCE 7 classifies sheds and minor storage buildings as Risk Category I, which gives an importance factor Is = 0.8. That reduces the design load by 20% compared to a house. But in a high-snow area, even 80% of the full design load can exceed the capacity of light framing.

The unheated factor adds load

A shed has no heat, so the thermal factor Ct = 1.2. This adds 20% load, which largely offsets the Risk Category I reduction. For an unheated shed in a heavy-snow region: Pf = 0.7 × Ce × 1.2 × 0.8 × Pg. The 1.2 and 0.8 nearly cancel, leaving you close to a standard residential design load.

Common failure modes

Sheds fail in snow because: (1) the framing was undersized for the actual ground snow load in that location; (2) snow accumulates against a taller adjacent structure and creates a drift over the shed; (3) the owner treats it as unimportant and never rakes it. All three are avoidable.

What to do before the season

Calculate the design snow load for your ground Pg and your shed's thermal and exposure conditions. Verify your framing span against a span table for the tributary area and roof load. If the numbers don't work, add a mid-span interior post or a collar tie to stiffen the framing. Rake sheds after every major snow event.