Having a flat roof cleared of snow

What winter does to a flat roof

A digest from the National Research Council of Canada describes what happens. A flat roof has slight low spots where meltwater collects. When the weather turns colder, this water freezes in a thin layer, then thaws, then freezes again. The digest dates from 1968, and the Council keeps it as a historical reference.

According to this text, a thin layer of ice is unlikely to damage the roofing. According to the same digest, if a crack or a hole lets water under the covering, freezing can break or tear it. The digest adds that ice on a poorly drained flat roof is rarely visible.

Clearing snow from a flat roof is judged on two things: the membrane is still intact, and the water drains away.

The membrane comes first

The Régie du bâtiment du Québec (RBQ, Quebec's building authority) recommends leaving a thin layer of snow on the roof to keep the waterproofing membrane in good condition. In its March 2023 news release, the Régie states that specialized companies have the right equipment and know the work methods that avoid damaging the membrane.

Before hiring a company, ask it these questions:

• Which tools touch the roof covering?
• How much snow is left in place at the end?
• What does the crew do with ice that is stuck to the roof?

The drains, the water's way out

Tell us where the drains are, if you know, and ask that they be cleared.

From inside, watch the top-floor ceilings during a thaw. Photograph a stain that appears or grows, and report it to us with the time.

The roof edge is not cleared

The CNESST (Quebec's workplace health and safety board) sets the rules for snow removal workers on a flat roof. It provides for a warning line that leaves a 2-metre buffer zone around the edge, and a guardrail where the snow is thrown down. Expect a strip of snow to remain all around once the work is done.

The CNESST also asks snow removal workers to list, before winter, what will become an obstacle under the snow, such as a skylight, a damaged section or an opening. If your roof has any, mention it in your request.

How the crew gets up, and where the snow falls

Ask us how the crew will reach the roof. If it goes through the inside, by a hatch or a shared stairway, occupants must know and the way must be clear.

On the ground, the CNESST asks that the area where snow and ice fall be marked off and closed to access. Work out with us which side the snow will be thrown on: the yard, the entrance, the parking area. Warn the tenants and neighbours concerned, and have the cars moved. The Régie calls on the public to respect the safety perimeters that are set up.

After the work, ask that doors, stairs and balconies be cleared. The Régie stresses the ones that serve as emergency exits.

Frequently asked questions