Ice dam on the roof (barrage de glace): what it is, how serious, what it costs

An ice dam, « barrage de glace » in a French report, is a ridge of ice that forms along the edge of the roof, often with large icicles. Snow melts on the upper part of the roof, warmed by heat escaping from the house. The water runs down to the edge, which sticks out past the wall and stays cold, and freezes there. The ice builds up into a dam.

Meltwater that arrives afterward is trapped behind it. It can back up under the shingles and get into the attic or the walls.

Outside of winter, the inspector cannot see it, and infers it from the condition of the attic and the roof. What matters is the cause: heat leaking into the attic, because of thin insulation, blocked ventilation or air leaks around the hatch.

It becomes serious when the report also notes water stains on the top-floor ceiling, damp wood in the attic or a roof near end of life: the water then has a way in. Otherwise, it is mostly an invitation to fix the attic insulation and ventilation.

The lasting fix is in the attic, not on the roof. Adding insulation: $900 to $2,000 for one section, $2,300 to $5,000 for the whole attic. Clearing the soffit air intakes: $400 to $1,000 for one side of the house. Installing baffles that keep the air path open above the insulation: $400 to $900 for one roof slope.

If water has already come inside, add finding the leak: $200 to $450.

These are ballpark figures for Quebec, to be confirmed with quotes.

1. What makes you say the roof is vulnerable: the insulation, the ventilation, the condition of the shingles?
2. Were there water stains on the top-floor ceiling or on the wood in the attic?
3. Are the soffit air intakes clear?

Ask the seller whether there were large icicles along the roof edge, or water inside, during a winter.

Does your report mention ice dams? Upload it: in five minutes, I tell you what comes first, what to have checked and what to ask the seller.

Not by you, on the roof: it is dangerous, and a pick or shovel damages the shingles. A roofer can remove the ice with steam. The real fix is still in the attic.

They can open a path for the water in a difficult spot, but they do not fix the cause. Proper attic insulation and ventilation solve the problem at the source.