Clients are sometimes surprised by how heavy the design snow load on an Ontario roof is — often more than the weight of the roof structure itself. The short explanation: we don't design for an average winter. We design for the worst plausible one, plus the ways snow behaves badly.
The one-in-fifty winter
Design ground snow loads in the building code are based on a 1-in-50-year statistical event for your specific location. Two towns an hour apart can have meaningfully different design values, which is why "my neighbour's engineer used less" is never a useful comparison — the code value is tied to the map, not to opinion.
Snow doesn't fall evenly
The bigger surprises come from how snow accumulates. Wind sweeps snow off one side of a roof and piles it on the other. A lower roof beside a taller wall collects a drift that can be several times the ground load. Sliding snow from an upper metal roof can land on a lower one as a concentrated dump. The code has specific provisions for each of these, and roof geometry — steps, parapets, valleys, additions — often governs the design far more than the base snow value.
Why this matters for additions
Here's the practical takeaway: adding to a building changes the snow on the part you didn't touch. A new second storey creates a drift zone on the existing lower roof, which may have been perfectly adequate before and undersized after. It's one of the most common things we check in renovation design — and one of the least obvious to owners.
Winter is a load case. A good design means you never have to think about it — which is, frankly, the goal of most good structural engineering.