Is a backwater valve required in Ontario?
Where a building drain or branch could be subject to backflow from the public sewer — generally where fixtures are below the level of the adjoining street — the OBC requires protection by a backwater valve.
A backwater valve is a one-way flap in the drainage piping that closes when the municipal sewer surcharges during heavy rain, preventing sewage from backing up into basement fixtures. The trigger condition is elevation: fixtures below the adjoining street level (nearly every finished basement with a bathroom or floor drain) are at risk. The valve must be a certified device, installed accessibly for maintenance — typically in a pit with a removable lid near where the building drain exits. In renovations that add basement plumbing, inspectors routinely require one, and many Ontario municipalities offer subsidy programs for retrofitting them because basement flooding claims are the largest category of urban water damage.
If your basement has a drain or toilet lower than the street, a storm-surcharged sewer can flow backwards into your house. The backwater valve is a one-way door that slams shut when that happens. Cheap to install during construction, miserable to need and not have.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.