House problems · Sewer backup · Valve and insurance
The backwater valve is mandatory, subsidized in Montreal, and the insurer's first question after a claim. Here is the rule, the grant, the insurance and what the seller of a house that already backed up must say.
A sewer backup is municipal wastewater coming back up through the basement floor drain, toilet or shower when heavy rain saturates the mains. Since 2008 to 2015, the plumbing by-laws of Montreal, Laval, Longueuil and most cities in the region require a backwater valve on basement fixtures, with annual maintenance; the City of Montreal makes it mandatory on every fixture for any building already flooded, and subsidizes it through RénoPlex's flood stream ($80 per normally-closed valve, $560 per valve under an existing slab, $1,500 for a sump and pump, minimum request $500). Home insurance only covers backup through an endorsement, with a limit (often $10,000 to $50,000) and a deductible, which the insurer can refuse or cap after a claim or without a valve. At the sale, a past backup is disclosed in the Declarations by the seller, with repairs and claims; the buyer asks for the claims history and checks the valve and its maintenance date.
In combined-sewer neighbourhoods (wastewater and stormwater in the same pipe), typical of areas built before 1970, a 30 mm rain in one hour fills the municipal main and pressure pushes water to the lowest point connected to it: the floor drain of the lowest basement on the street. Basements finished below street level, houses at the bottom of a slope and streets at the end of the network back up first. The backwater valve closes when water rises and protects the fixture; one valve per fixture (floor drain, toilet, shower, basement sink) or one valve on the main connection, depending on the layout, and it must be cleaned every year or it stays open.
The Quebec Construction Code (plumbing chapter) requires a backwater valve on fixtures that can back up, and the Régie du bâtiment publishes directives on eligible types (normally open on the connection, normally closed on fixtures). Montreal, Laval, Longueuil and most cities in the region added to their by-laws the obligation to install them in existing buildings, with annual maintenance, and Montreal requires a valve on every basement plumbing fixture for any building that has already suffered a backup or flood. An owner without a valve who backs up may see their claim contested and be held liable for damage to the adjoining neighbour.
In Montreal, stream 3 of RénoPlex (flood protection) subsidizes, for 1 to 5 unit buildings under $3,860 per m² of property value, $80 per normally-closed valve, $560 per valve installed under an existing slab, $1,500 for a sump with pump, with a $500 minimum request and approval before the work. Laval and Longueuil have had similar programs at times; check your city's page. Rénoclimat's new adaptation stream (October 1, 2026) adds $700 to $1,000 for this simple work everywhere in Quebec. See RénoPlex and Rénoclimat.
Sewer backup is not covered by a basic policy: you need the "water damage, sewer backup and infiltration" endorsement, with a per-claim limit (often $10,000 to $50,000 depending on the insurer and the house) and a deductible. After a claim, the insurer generally requires the valve and may reduce the limit or refuse renewal; without a valve in a city that requires it, it may reduce the payout. Keep proof of valve installation and maintenance, repair invoices and correspondence: they serve the insurer and the buyer.
The Declarations by the seller form asks whether the building has suffered water infiltration, backups or claims: a past backup is disclosed, with the date, the damage, the repairs and the payout received. Hiding it is a classic latent defect, all the more since the property's claims history remains available to the buyer's insurer. A file reading "backup in 2019, valves on all fixtures, sump and pump, no claim since, annual maintenance" sells without a discount; a redone basement with no explanation and no valve raises doubts.
Yes, under the plumbing code for fixtures that can back up, and under the municipal by-laws of Montreal, Laval, Longueuil and most cities, with annual maintenance; Montreal requires it on every fixture after a claim.
A few hundred dollars for a valve on an accessible fixture, more under an existing slab to break; Montreal subsidizes $80 to $560 per valve and $1,500 for a sump and pump.
Yes, with the repairs and the claims. A buyer who discovers it after the sale through the insurance history has recourse.
Only with the sewer backup endorsement, within the set limit and subject to the valve required by the city.
These problems are easier to settle before listing than after the buyer's inspection: see the pre-purchase inspection in Quebec, latent defects in Quebec and renovating before selling.
The Loaa & Manseur team, led by Lotfi Manseur and Loaa Al Daow, is the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 of large residential teams according to the official RE/MAX ranking, computed on actual transactions. Its Google profile counts 221 reviews with a 4.9 rating. Its offices are at 55 avenue de l'Équinoxe in Brossard, one bridge from the island, and every broker's licence can be checked in the OACIQ public register. Three things to verify yourself before calling us, and to demand of any other broker.
Also available in French: version française de ce guide.
Published September 7, 2026 by the Loaa & Manseur team, the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 according to the official RE/MAX ranking. Tell us about your project.
Quebec runs on civil law, so several French terms have no direct English equivalent: hypothèque is a hypothec and not a mortgage, vices cachés are latent defects, and the taxe de bienvenue is legally transfer duties. The full list is in our Quebec real estate glossary.
Équipe no 1 de RE/MAX Platine en 2024 et en 2025, 6e au Québec, 15e au Canada et dans le top 50 mondial au classement officiel RE/MAX. Réponse en moins de 24 heures, sept jours sur sept.