Flat roof membrane (membrane élastomère): what it is, how serious, what it costs

On a flat roof, water does not run off the way it does on shingles: it can sit there. The covering has to be fully watertight. Today in Quebec, it is often an elastomeric membrane, « membrane élastomère » in a French report: bitumen modified with polymers, in two layers, often torch-welded. The top layer is covered with granules that protect it from the sun.

You find it on many plexes, townhouses and additions. Older flat roofs are sometimes tar and gravel, called « multicouche » in French.

On a flat roof, a small weakness can be costly: water goes straight into the top-floor ceiling. The signs an inspector notes: blisters, lifting seams, missing granules, water pooling after rain, flashing coming loose at the parapet (the low wall around the roof) or around the drains.

Sometimes the flat roof was not inspected up close, for lack of access. In that case, the age of the membrane and the absence of water stains on the ceiling are your best clues, and an evaluation by a roofer before the deadline is reasonable.

Having the membrane evaluated by a roofer: $200 to $500. The roofer will tell you whether a repair is enough or whether the membrane is near end of life.

Redoing a flat roof depends on the area, the layers to remove, the drains and the parapets: only a roofer’s quote can price it.

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

1. Could you get onto the flat roof? If not, where did you look at it from?
2. Did you see blisters, lifting seams, pooling water or loose flashing?
3. Were there water stains on the top-floor ceiling?

Ask the seller for the year of the membrane, the roofer’s name and the warranty: on a plex, the upper units depend on it.

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

Often 20 to 30 years when it is well installed and maintained, less if water pools or the drains clog. The roofer’s invoice is what gives you its real age.

Yes. Keeping the drains clear of leaves and debris, and having a roofer look at the roof from time to time, for example after a harsh winter, extend its life.