Why snow slides
Three things decide whether snow stays put.
- The slope. The steeper the roof, the less snow holds on, in general.
- The roofing. Metal is smooth and snow slides off it easily. Asphalt shingles are rougher and generally hold snow better.
- A thaw. Sun, rain or heat rising from the house melts the underside of the layer. A thin film of water forms between the snow and the roof, and the slab can let go all at once.
The Régie du bâtiment du Québec (RBQ) points out that alternating rain and snow makes snow denser and heavier.
Where it lands
Keeping a safe distance from the walls, look at what lies under each edge of a sloped roof.
- The entrance. The door, porch and outdoor staircase of a plex often sit right under the roof edge.
- The sidewalk. When the facade is close to the street, roof snow lands where people walk.
- The parking spot. A car parked along the wall or in the lane can take the whole slab.
- The roof below. A balcony, an awning, a shed or a temporary carport takes the impact and the weight.
The RBQ states that ice and icicles at the edge of a building are a danger to pedestrians and cars.
What holds snow back
Snow guards. These are bars or small pieces fastened along the bottom of the slope. They hold the layer so it melts in place or comes down in small amounts. We install snow guards mostly above an entrance, a sidewalk or a parking spot. Their number and layout are decided with the slope, the roofing and the roof structure.
Clearing the roof edge. When the layer threatens to slide, we clear the roof edge above the entrance and walkways. The work is done with tools that do not cut the roofing, and the area on the ground is closed off while the snow comes down.
A marked-off area. As long as the snow is up there, close the walkway below with tape, cones and a sign on the door. The RBQ asks the public to respect the safety perimeters set up by owners.
What you can do from the ground
You do not go up on the roof or on a ladder. For anyone who wants to clear snow themselves, the RBQ names a single tool. A roof rake with a telescopic handle lets you work from the ground. It is not for use under a slab of snow that is about to slide, near power lines or against ice.
Otherwise, park the car elsewhere, use another door if you have one and take photos of the roof. Those photos help us prepare the visit.
Frequently asked questions
How do you stop snow from falling off a roof?
You stop snow from falling off a roof by installing snow guards along the bottom of the roof, or by having the roof edge cleared before the snow slides. In both cases, the work at height is our job, not yours.
Why does snow fall off all at once?
Snow falls off all at once when the underside of the layer melts during a thaw. The water that forms between the snow and the roofing takes away the slab's grip, and the whole slab slides off in one go.
When should you call a company?
Write to us as soon as snow or ice hangs over a place where people walk. Describe the roof and what lies under it in the form.
Official sources, consulted October 8, 2026: