Foundation crack repair: when to worry, who to call

Almost every concrete foundation cracks a little as it cures, and most of those cracks are harmless. Others point to the structure moving or water getting in, and call for a professional opinion before the problem grows. Here is how to tell them apart.

1. Cracks that are usually harmless

• A thin vertical crack, hairline wide, that does not grow.
• A dry crack, with no trace of water or efflorescence (white deposit).

Write down the date, measure the width, draw a pencil mark at each end and take a photo: next season, you will know whether it has moved.

2. Signs to have checked

• A horizontal crack, or a wall bowing inward.
• A stair-step crack in a block wall.
• A crack wider at one end than the other, or one that grows over time.
• Water coming into the basement through the crack, especially during the thaw or after heavy rain.
• Doors that stick, sloping floors, new cracks in the walls upstairs or around windows.
• A basement floor that heaves or cracks in a star pattern: in some areas, including Greater Montréal, the fill under the slab can contain pyrite. Only a lab test can confirm it.

A crack that lets water in also lets soil gases in: before finishing a cracked basement, test for radon (see our radon guide).

3. Repairing a foundation in Greater Montréal: permits and help

Foundation work nearly always needs municipal approval. In Montréal, the City (in French) requires a permit as soon as the work affects the foundation or changes the structure; when the structure is affected, it asks for plans signed and sealed by an engineer. In Boisbriand, foundation work (in French) needs a permit or a certificate of authorization. In Repentigny, where many renovations are simply declared online, repairing a foundation with piles still needs a permit (in French). In Sainte-Thérèse, the City (in French) requires a permit to change the foundation or the structure, even for low-cost work.

As for financial help, do not count on a grant in advance. In Belœil, for example, the cracked-houses component (Maisons lézardées) of the Rénovation Québec program helped stabilize foundations with piles, but the City (in French) says, on the page read on November 1, 2026, that the program is suspended until further notice, with no new government funding since April 1, 2025. Ask your municipality what is available before signing a contract.

4. Engineer or contractor: who to call

To find out whether the structure is affected, first have the foundation assessed by an independent professional, such as a structural engineer, who has nothing to sell you. For the work itself (injection, exterior sealing, French drain, waterproofing, piles, underpinning), choose a company whose RBQ licence covers it: check it, then compare several offers with our guide to quotes. A serious quote says which method will be used, over what length, whether the yard will be dug up and what warranty covers the repair.

To find companies, see renovation contractors in Montréal, in Boisbriand or in Belœil, or describe the crack and get quotes. If water comes in at the bottom of the walls, also read our guide on the French drain.