Rainwater and flood zones in Laval
Updated September 28, 2026
According to the city, most buildings hit by Debby are on sewer networks built between 1960 and 1982, combined or pseudo-separate. The city estimates that these networks protect against a rain with a return period of 2 to 5 years. On Île Jésus, the development of former farmland left some sectors drained by ditches and others by underground pipes.
The city defines a cuvette as a hollow in the ground whose slope sends water to a low point, and notes that a house in a cuvette can be more exposed to sewer backup and water infiltration. In heavy rain, water can pool there on streets and lots when the sewer is full. After Debby, the city found that the hardest-hit sectors were mostly in cuvettes, where a garage driveway sloping down toward the house adds to the risk.
Heavy rain events
What public sources report about heavy rain in Laval, at sector level.
Thunderstorms left up to 60 mm of rain in some sectors of Laval. Residents reported a few minor water infiltrations and water pooling on the roadway, but no major event was reported to the city. The article does not say whether any sewer backup occurred.
Sources: La Presse, 14 juillet 2025
The remnants of Debby, combined with a low from the Great Lakes, left 125 to 200 mm of rain between 9 a.m. and 9:30 p.m., according to radar estimates. The city reports flooding from surface runoff, storm-sewer backup and overflowing inner streams. The heaviest rain fell on the former towns of Fabreville, Sainte-Rose and Auteuil. According to La Presse, in Laval as in Montréal, the vast majority of the damage happened in areas away from watercourses, where sewer backups flooded basements.
Sources: Ville de Laval, rapport sur la tempête Debby · Ville de Laval, rapport technique Debby, mai 2025 (PDF) · La Presse, 22 août 2024
Good to know in Laval
- By-law L-11870 requires backwater valves on every drain branch lower than the crown of the street. Only normally closed valves are allowed, and no valve may be installed on the building's main collector. (Ville de Laval, règlement L-11870 (PDF))
- Under L-11870, a driveway sloping down toward the house may slope at most 10% and needs a hump at the top at least as high as the curb or sidewalk, where there is one. It is prohibited if the building has neither a storm sewer allowing a gravity connection nor a combined sewer. (Ville de Laval, règlement L-11870, article 4.01.03 (PDF))
- The city's report on its handling of Debby notes that reverse-slope driveways may no longer be built for single-family homes. It also notes that some owners have not built the required hump at the top of driveways that slope down toward the house. (Ville de Laval, rapport de gestion de l'événement Debby, avril 2025 (PDF))
- Under L-11870, water from a pitched roof must drain onto a permeable surface at least 1 m from the building. Sending it to a driveway, or connecting a downspout to the foundation drain, is prohibited. (Ville de Laval, règlement L-11870 (PDF))
- For Debby, the city compared the readings of its rain gauges with intensity-duration-frequency (IDF) curves: for durations over two hours, the rain reached a return period of 100 years or more across almost the whole territory. (Ville de Laval, rapport de gestion de l'événement Debby, avril 2025 (PDF))
- According to the city's engineering report, an overloaded sewer is not the only cause of a flooded basement: a blocked or missing French drain, window wells with no drain, missing or poorly kept backwater valves, a sump pump with no backup battery, a lot sloping toward the foundation, a driveway hump that is too low. (Ville de Laval, rapport technique Debby, section 3.1 (PDF))
Rivers and lakes: what the check does not cover
Waterways in Laval: Rivière des Prairies and Rivière des Mille Îles.
In spring 2017, the city reported flooded streets along both rivers. On the Rivière des Prairies: Laval-sur-le-Lac, Îles-Laval, Sainte-Dorothée, Laval-des-Rapides, Chomedey and Duvernay. On the Rivière des Mille Îles: Laval-Ouest, Sainte-Rose, Fabreville, Auteuil and Saint-François. In spring 2019, the mayor declared a state of emergency, and the situation was particularly precarious in the Îles-Laval archipelago. In April 2026, high water on both rivers made some streets impassable through minor flooding, notably in Laval-Ouest and Fabreville. The city tracks water levels with telemetry stations around Île Jésus.
Sources: Ville de Laval, bilan du 11 mai 2017 (Newswire) · Radio-Canada, 28 avril 2019 · Ville de Laval, communiqué d'avril 2026 · Ville de Laval, inondations
The check is about the rain that falls on the area and runs off over the ground. River and lake flooding is not part of it: in the simulation, a nearby river or lake holds no water. For a waterfront property, the official map comes first.
What we know about the ground in Laval
These values come from the public sources the check uses. They describe the area, not an address.
| 1 m terrain model (NRCan HRDEM) | Available across the whole area |
|---|---|
| Nearest rainfall gauge (ECCC) | Sainte-Dorothée (Laval), 11 km awayLocal rainfall statistics. |
| Municipal street drains | Not modelled: with no open municipal inventory, the check's depths are an upper bound wherever a working network would carry water away |
| Mapped culverted road crossings | 131Culvert lines mapped in OpenStreetMap (ODbL). |
Data Source: Environment and Climate Change Canada, Short Duration Rainfall Intensity-Duration-Frequency Data, used under the ECCC Data Servers End-use Licence.
Area analysed: adapted from Statistics Canada, Census Subdivision Boundary File (cartographic), 2021 Census. This does not constitute an endorsement by Statistics Canada of this product.
Probed on September 28, 2026.
The official map and what it does not show
Québec's flood-zone maps cover rivers and lakes. They do not show rain water collecting on a lot, in a low spot or at the bottom of a sloping driveway. Check the official map for the address first: it is what says whether a property is in a flood zone.
- Flood-zone maps (Ville de Laval, in French)
- Check a property on the Government of Québec site (in French)
FloodVerity does not reproduce these maps and does not state any regulatory zone.
Check an address in Laval for $39
We drop a 100-year, one-hour design storm on the ground around the property, route the runoff, and email you the peak depth of rain water at the address, where it collects, a map and a 3D view.
What the check does not do
- This check models rain falling on the area itself and running off over the terrain. Water arriving from a river or lake, sewer backup, ice jams and storm surge are not modelled.
- The pipe network itself, roof and lot drainage, and sewer backup are not modelled.
- Where the structures inventory records a culverted road crossing, it is lowered to a partly blocked opening rather than solved hydraulically.
- It does not replay the storms described on this page: it applies a 100-year, one-hour design storm.
- It is a screening: a simulated storm on public terrain data, not a hydraulic study, not an official flood-zone determination, not an insurance opinion.
Notaries and brokers in Laval
We run the same check over a list of addresses and deliver a spreadsheet ranking them by the modelled rain depth. Priced per address, on quote.
Email [email protected]Other towns and areas
The FloodVerity check is a screening: it applies a design rainstorm to public terrain data and simulates where the rain runs and collects. It is not a hydraulic study or an official flood-zone determination, and it covers neither river and lake flooding nor sewer backup.