How a backwater valve works
The valve sits on the main drain, inside the foundation or in a chamber cut into the slab. A ball or flap stays open under normal conditions, letting the house's water flow freely out to the sewer.
When the level in the municipal sewer rises above the drain's level — typically during a heavy rain that overwhelms a combined system — the pressure closes the mechanism and blocks the return flow. Once the level drops back down, the valve reopens on its own.
Signs it's time to install one
A backup that has already happened once is the clearest signal: whatever weak point let water in the first time will do it again at the next heavy rain unless it's fixed.
A house sitting lower than its neighbours on the same street, a finished basement that had never been wet until recently, or an area still on a combined sewer network are also worth taking seriously, even without a backup having happened yet.
Maintenance: what to check every year
The mechanism needs to stay free of debris to close properly; a valve that's never opened for inspection risks jamming at the worst possible moment. An annual check, usually in fall before the cold rainy season, takes a few minutes.
Installing a valve doesn't replace drain maintenance elsewhere: if there are roots or a sag upstream, the underlying problem is still there — only the basement's front door has been shut.
Financial assistance: check before the work starts
Some cities and boroughs in the Montreal area offer financial assistance for installing a backwater valve. Eligibility criteria, amounts and deadlines vary from one municipality to another and change over time.
The safe approach is to check directly with your city or borough before work begins, rather than relying on a figure seen elsewhere — some programs require the application to be filed before billing.