Terrebonne, QC repair guide
Basement Flooding in Terrebonne: The Mille Îles Floods of 2017 and 2019, and the Fixes That Still Make Sense
Terrebonne sits on the north bank of the Rivière des Mille Îles, and in the springs of 2017 and 2019 the river came up over the old town’s riverside streets, into Île-des-Moulins’ neighbours and along the low shores of Lachenaie and La Plaine’s creeks, twice in three years. The provincial flood-plain maps and rules that followed changed what can be built, rebuilt and insured along the river. Most Terrebonne basement flooding is more ordinary, sewer backup in a summer storm or groundwater in the melt under the subdivisions that have doubled the city since 2000, but the river is the reason the rules are what they are. This guide sorts the three apart and says what Quebec requires of the plumber.
By Michael, founder, fixmystuff.ca · Reviewed and updated 2026-09-11 · About 7–10 minutes
The river, the sewers and the melt
River flooding announces itself: the City and the province issue warnings as the Ottawa River system fills in April, the Mille Îles rises over the riverside streets of the old town and Lachenaie, and the water table under the low streets rises with it. Sewer backup arrives at the floor drain, a basement toilet or the laundry standpipe during or just after heavy rain and smells like sewage; the old town on the oldest sewers is most exposed, though the newer subdivisions have flooded too when a storm outran their storm sewers. Groundwater arrives clear at the wall-floor joint in the melt, and the post-2000 houses depend on a sump pump and a French drain.
Photograph where the water first appeared and when. A river warning, a storm and a smell, or a melt and clear water each point to a different fix.
| What you see | Likely cause | Who fixes it |
|---|---|---|
| River rising, City warning | Mille Îles flooding | City measures, insurer |
| Sewage from the floor drain in a storm | Sewer surcharge | Plumber: backwater valve |
| Clear water at the wall in the melt | Groundwater, failed drain or sump | Plumber or waterproofer: sump, drainage |
| Slow drains in dry weather | Collapsed or rooted lateral | Drain contractor: camera, repair or lining |
What the fixes do, and what Quebec requires
A backwater valve is a one-way flap on the drain that closes when the sewer surcharges so sewage cannot flow back into the house; Quebec’s plumbing code has required them on the below-grade fixtures of most homes for years, and the City of Terrebonne’s by-laws require them and gutter disconnection. A sump pump with a pit collects groundwater from the French drain and pumps it outside; a battery backup keeps it running through the outages that come with the storms. Quebec insurers have held that a missing or unmaintained backwater valve can reduce a claim.
The City requires a permit for the work and has offered protective plumbing programs at times; check the City’s current page before signing a quote.
- City of Terrebonne permit and any current program checked
- RBQ-licensed plumbing contractor
- Camera inspection recording and locate
- Backwater valves present and tested on every below-grade fixture
- Sump pump tested in March, backup and alarm
The flood plain after 2019
After the 2017 and 2019 floods the province redrew the flood-plain maps along the Mille Îles and the Ottawa River system and adopted rules that restrict rebuilding, enlarging and in some cases repairing homes in the highest-risk zone, with a compensation program for owners who chose to leave. If your house is in the zone, the City’s permit office will tell you what basement work is allowed and what is not, and your insurer will use the same maps; overland flood coverage along the river became hard to get. A waterproofer or plumber quoting work near the river should know which zone the house is in before quoting.
Who owns which pipe
The City of Terrebonne owns the sanitary and storm sewers beyond your property line and the water supply; the lateral on your property is yours, subject to the City’s by-laws. A camera inspection with a locate tells you which side a break or blockage is on. Keep the recording; an insurer will want it, and sewer backup and overland water are separate endorsements on a Quebec home policy. Excavation and drainage work must be done by an RBQ-licensed contractor, and the RBQ’s register shows the licence.
Questions to ask before booking
- Is my address in the post-2019 flood zone, and does that limit the work?
- What is your RBQ licence number, and does it cover plumbing and excavation?
- Will you camera the lateral and give me the recording and locate?
- Does every below-grade fixture have a working backwater valve, and will you pull the City permit?
- Does the sump quote include a battery backup?
Frequently asked questions
Does Quebec require a backwater valve?
Yes, on the below-grade fixtures of most homes under the plumbing code, and the City of Terrebonne requires them and gutter disconnection under its by-laws. Insurers may reduce a claim where one is missing. Test it every spring.
Who owns the sewers in Terrebonne?
The City of Terrebonne owns the sanitary and storm sewers beyond your property line and the water supply; the lateral on your property is yours, subject to the City’s by-laws.
My house is near the Mille Îles. Will a sump pump help in a flood?
Not against the river itself. River flooding is a matter of the City’s warnings, the provincial flood-plain rules adopted after 2019 and your insurer. A sump pump handles groundwater.
Can I renovate my basement in the flood zone?
It depends on the zone. The post-2019 rules restrict rebuilding, enlarging and some repairs in the highest-risk zone. The City’s permit office will tell you what is allowed at your address before you plan the work.
Nearby communities
North-shore plumbers cover Terrebonne, Mascouche, Repentigny and Laval across the river, and the Mille Îles flood rules apply along both banks. The RBQ rules and the plumbing code are the same across Quebec; permits and programs are municipal.
- Plumbers in Laval
- Plumbers in Montreal
- Plumbers in Repentigny
- Plumbers in Mascouche
- Plumbers in Longueuil
Browsing more broadly? Start from the Terrebonne repair hub for every local guide in one place.
Sources and further reading
- Ville de Terrebonne: sewers and floodingSewer responsibility, permits, flood zones and any current program.
- Québec: flood-plain maps and rulesProvincial flood zone mapping and the post-2019 rules.
- Régie du bâtiment du Québec: licence registerVerify a contractor’s licence and subcategories.
- Insurance Bureau of Canada: water damageSewer backup and overland water coverage.
Prices, programs, and acceptance rules can change. Check each source and obtain a written local quote before deciding.