Why depth tells you nothing
Fresh snow is mostly air. As it packs down, gets wet and freezes again, it holds less air and more water. The snow loses height without losing any weight, and it gains weight when rain is added.
A roof can look less loaded than the day after the storm and actually carry more. That appearance is the first trap to avoid when deciding when to clear a roof.
What makes snow heavier
The Régie du bâtiment du Québec (RBQ) puts it in one sentence. Alternating rain and snow makes snow denser and heavier. In another press release, the RBQ adds that freezing rain combined with snow buildup can weaken roofs.
In Montreal, these conditions often come back several times in one winter.
- Rain. Snow soaks it up like a sponge.
- A thaw. The surface melts, the water seeps into the layer and freezes again when the cold returns.
- Freezing rain. It adds its own weight on top of the snow.
- Snowfalls that add up. Each new fall adds to the earlier ones and packs them down.
- Trapped water. On a flat roof with a frozen or buried drain, meltwater adds to the snow.
Wind concentrates snow. A drift against a parapet or a taller wall loads one part of the roof more than the rest.
Why two roofs do not carry the same load
What a roof can carry depends on its design, the era it was built in, the condition of its framing and what has been added since.
Plexes often carry such additions. A deck on a flat roof, a heat pump, several layers of roofing stacked on top of each other. None of these additions can be seen from the sidewalk.
The role of the structural engineer
Only a structural engineer can tell what a given roof can carry. That calculation is not a roofer's job. We clear the roof and tell you what we saw up there.
See an engineer if:
- a sign of overload appeared this winter or a previous one, even if it has since gone away
- you have added weight to the roof or plan to
- the framing has already suffered from a leak or other damage
What to do in the meantime
The RBQ asks owners to watch for signs of overload inside the building, like a door that starts to stick or a ceiling that sags. If one of them appears, move away from the area, and call 911 if the danger is immediate. The RBQ then recommends clearing the roof, whatever the amount of snow.
Frequently asked questions
How much snow can a roof hold?
There is no snow depth that applies to every roof. The weight of the same depth of snow changes depending on whether it is fresh, packed, wet or frozen, and every roof has its own capacity. The RBQ gives no number and asks owners to watch for signs of overload instead. Only a structural engineer can tell what your roof can carry.
Is wet snow heavier than fresh snow?
Yes. The RBQ states that alternating rain and snow makes snow denser and heavier.
Can I weigh or measure the snow on my roof myself?
No. You would have to go up on the roof, which is dangerous, and the result would say nothing about what the structure can carry. Watch the inside of the house instead, and look at how the snow has built up from the ground.
Official sources, consulted October 8, 2026: