Sewer backup: protecting your basement on the South Shore
A basement backing up during a storm is not inevitable. Here is what causes it, and what your municipality requires — or subsidises.
Updated
Why a basement backs up
During intense rain, the municipal sewer receives far more water at once than it can carry away. Pressure builds in the street main, and it looks for the easiest path back down. That path is often a house’s service connection — and the lowest outlet in that house is the basement floor drain.
On the South Shore this is not a theoretical scenario. The City of Longueuil explains that part of its network is still combined: storm and sanitary pipes “eventually meet in a combined network downstream”. On 13 September 2022, nearly 70 mm of rain fell on Longueuil in under an hour, and water entered homes, businesses and schools.
What a backwater valve does
A backwater valve is a device fitted to the drain line. It lets water out normally, but closes when water tries to come back toward the house. It is a simple mechanical part, and that is exactly why it must stay accessible and be checked periodically: a valve jammed with debris protects nothing.
It is not an optional accessory from a regulatory standpoint. The Québec Construction Code requires plumbing fixtures to be protected against potential sewer backup, and the RBQ publishes a backwater-valve best-practice guide jointly with the CMMTQ.
What your municipality requires or subsidises
The rules vary enormously from one South Shore city to another, and that is the first thing to check. Here is what the municipalities publish themselves:
- La Prairie — mandatory. Every building connected to the sanitary sewer, whatever its year of construction, must have a backwater valve and a submersible pump. An owner who is not compliant forfeits any damages claim against the City.
- Candiac — mandatory for buildings served by the sanitary sewer. Compression-type ("squeeze in") valves are expressly prohibited there.
- Châteauguay — an active subsidy program for rehabilitating plumbing systems to reduce backups. The building must have been built before 2008, and the application requires the contractor’s RBQ licence and CMMTQ membership.
- Contrecœur — a subsidy of up to $500 for purchasing and installing a valve, offered for homes built before 1 January 2000.
- Saint-Lambert — the stormwater retention bylaw covers drainage of exterior paved surfaces below street level, which must be connected to a pumping system and fitted with a check valve.
- Boucherville and Brossard — both cities publicly recommend checking or installing a backwater valve, but no subsidy program or general requirement appears on their pages.
Before you call: three checks
Conditions, amounts and deadlines change. Always confirm directly with your city before committing to work — several programs require the application to be filed in a specific way, and some are retroactive only from a given date.
- Check whether your municipality requires a valve, reimburses one, or neither.
- Ask whether a permit is required before the work: several cities require one.
- Confirm the contractor holds a valid RBQ licence — several subsidy programs formally require it in the application.