Foundation cracks in Montreal: types, diagnosis, repair

Guide by Pietro Di Benedetto, President of Sous-Sols Experts Inc. RBQ-licensed contractor #5668-8765-01, SOPREMA-certified applicator. 500+ projects completed since 2013.

TL;DR. A thin vertical crack on a Montreal foundation wall is almost always cosmetic shrinkage from the concrete cure. A horizontal crack, a stair-step crack in a block foundation, or any crack that widens with the seasons is structural until proven otherwise. Width, direction, and behavior over time determine urgency. Polyurethane injection runs $400 to $700 for a typical vertical crack. Structural repair runs $2,500 to $8,000 depending on extent. The on-site inspection is free.

Are foundation cracks normal in a Montreal home?

Yes, most foundation cracks in Montreal are normal. The Greater Montreal area sits on heavy clay across most of the plain and the South Shore, with winters that put basements through dozens of freeze-thaw cycles per season. Almost any poured concrete foundation built before 1990 has at least one hairline crack by year five. The presence of a crack is not the question. The width, direction, and behavior over time are. A thin vertical crack that has not changed in two years is cosmetic. A horizontal crack that appeared this spring is a different conversation. This guide is the triage rubric we run through on first inspections, written for the homeowner who is staring at a crack in May and trying to decide whether to call.

How to tell a cosmetic crack from a structural crack

Three signals separate a cosmetic crack from a structural one: width, direction, and presence of other movement signs in the house. A cosmetic crack is less than 1 mm wide, runs vertically (or close to it), has not changed in at least one full season, does not leak, and is not paired with sticking doors, sloping floors, new wall cracks upstairs, or new drywall cracks. If all those criteria are met, you are looking at a shrinkage crack: the concrete dried and contracted during the first years, and the crack is how it relieved the tension. This happens on almost every residential foundation in Quebec. It becomes a problem only when water starts coming through, at which point the fix is injection, not structural reinforcement. A structural crack, by contrast, is more than 2 mm wide, widening, horizontal, or paired with other movement signs. The Régie du bâtiment du Québec requires an RBQ licence for this type of repair because the underlying diagnosis is complex: a structural crack is not something to fix with a hardware-store product.

What does a vertical foundation crack mean?

A vertical crack generally means concrete shrinkage, minor settlement, or seasonal cycling. It is rarely structurally urgent on its own. Vertical cracks run top to bottom (or close to it) on a poured concrete wall, often originating at a form-tie point, a window corner, or a basement door corner. It is the most common crack type in Greater Montreal homes built between 1960 and 1990. The action depends on behavior: if dry and stable, monitor; if leaking, polyurethane injection; if widening visibly over months, get a structural engineer involved before any product is specified. A vertical crack that widens at the top while staying narrow at the bottom is a settlement signal, not a hairline shrinkage signal, and deserves a closer look than a uniform-width crack. The injection repair is documented on our crack injection services page.

What does a horizontal foundation crack mean?

A horizontal crack in a foundation wall is structural until proven otherwise. It usually indicates that the wall is under lateral pressure (water-saturated soil, mature tree roots, or frost in poorly drained soil) exceeding its capacity. On a poured concrete foundation, a horizontal crack typically appears in the upper third of the wall, where lateral pressure peaks. On a block wall, it often follows a mortar joint across several consecutive blocks. The action is not injection: the product will not solve anything because the cause is not in the crack, it is in the soil and the drainage design. The repair runs through an evaluation of the existing French drain, exterior waterproofing, and in advanced cases reinforcement of the wall itself (steel anchors, interior buttresses, or section rebuild). A horizontal crack demands a full diagnosis before any quote. See our foundation repair and waterproofing page for the complete process.

What is a stair-step crack in a block foundation?

A stair-step crack follows the mortar between concrete blocks and traces a staircase shape. That is the signature of differential movement: one part of the wall is sinking faster than another. On block foundations of postwar Montreal homes built between 1945 and 1975 (the postwar boom in Rosemont, Ville-Émard, Saint-Léonard, and the rest of the island), it is the most common crack pattern. The cause is almost always settlement of the soil under the footing, often tied to a failing French drain that let water soften the base under one section of the wall. Simple repair is possible only for stair-step cracks that are thin, stable, and dry: inject or repoint. When the crack is active (it has widened since last year, or mortar is falling out), treatment requires stabilization of the underlying soil (expanding resin injection or steel piers) before the wall itself is repaired. This is one of the rare contexts where a structural engineer should evaluate the home before the contractor proposes a fixed-price quote.

Why do Montreal foundations crack?

Four causes dominate in the Greater Montreal area: concrete shrinkage during cure, clay soil that swells with moisture and shrinks when dry, freeze-thaw cycles that push and pull the wall in both directions, and a failing perimeter drainage system that lets groundwater rise against the foundation. Shrinkage is the most innocent cause. The other three are active-movement causes. The clay of the Montreal plain and the South Shore (Longueuil, Saint-Lambert, Saint-Hubert, Brossard) is particularly reactive: a dry summer followed by a wet spring can induce several centimetres of vertical movement under the footings. That is why a French drain in good condition is not a luxury in Quebec, it is the element that protects the foundation from the soil’s moisture swings. CAA-Quebec notes in its homeowner guide (French) that foundation problems in Quebec are almost always tied to water, directly or indirectly, and seasonal visual inspection is the best preventive defence.

How much does foundation crack repair cost in Montreal?

Foundation crack repair costs in Montreal vary widely by type. For a standard leaking vertical crack, polyurethane injection runs $400 to $700, warranty included. For a structural crack (horizontal or stair-step) requiring wider repair with steel anchors or buttresses, the price runs $2,500 to $8,000 depending on extent. For complete foundation wall rehabilitation with exterior excavation, SOPREMA membrane, and a new French drain, the project runs $12,000 to $30,000 depending on the home and access. These ranges are 2026 averages in the Greater Montreal area. Several factors push prices upward: restricted access, excessive depth, party wall with neighbour, landscaping to protect, attached garage. Factors that push prices down: single crack on an otherwise sound foundation, direct garden access, soil that does not require piering. The free on-site inspection delivers an exact price for your situation before any commitment.

Can you repair a foundation crack yourself?

For a thin vertical crack that does not leak, yes: monitoring and a bit of patience is enough. For a leaking crack, polyurethane injection kits exist in Quebec hardware stores. The risk is threefold. First, the consumer product is not the same as the professional product: kits use low-viscosity polyurethanes that penetrate cracks wider than 1 mm poorly. Second, pressure injection requires equipment that doses the product at 1,500 to 3,000 psi; the manual pumps in kits do not reach that level, and the product stays at the surface. Third, a crack that looks vertical to the eye may actually be the beginning of a stair-step crack or structural movement; homeowner visual diagnosis is rarely complete. For a repair you want to last 20 years with a written warranty, an RBQ-licensed contractor gives a better value-per-dollar than an $80 kit. For a temporary fix before a move or while waiting for a professional evaluation, the kit can do the job.

Repair from inside or outside: how to choose?

For a single leaking crack, injection is always done from inside. No excavation, no landscaping to redo, no window to clear. For a horizontal crack, an active stair-step crack, or multiple cracks distributed across the wall, exterior becomes the better option: we excavate, replace the waterproofing membrane (SOPREMA is the industry standard in Quebec), and replace the French drain at the wall footing if implicated. Exterior intervention costs more but treats the cause, not just the symptom. The decision also depends on access: if the cracked wall is shared with a neighbour or bordered by an attached garage, exterior is not possible and we work from inside with an interior drain and membrane applied on the dry side. Our team lays out both options and the cost of each at the inspection; the choice is yours.

When to call an expert vs a structural engineer?

For 90% of residential cracks in Montreal, an RBQ-licensed contractor with documented foundation experience (written quotes, warranty, verifiable past projects) is sufficient. A structural engineer becomes necessary when: the crack is horizontal and has visibly evolved since last year; the home shows multiple concurrent movement signs (sticking doors, sloping floors, drywall cracks upstairs); the wall is out of plumb (leaning inward or outward); or a financial institution, insurer, or potential buyer requires a sealed report. In those cases, the engineer diagnoses and the contractor executes per the signed plan. Sous-Sols Experts works with a network of Quebec engineers for files that require it; we never sign a structural plan ourselves, that would be outside our RBQ licence scope.

Request a free inspection

If you are looking at a crack and you cannot decide between monitoring and repairing, the on-site visit and diagnosis are free with Sous-Sols Experts in the Greater Montreal area (Island of Montreal, Laval, South Shore, North Shore, West Island). Pietro Di Benedetto and his team typically arrive within 48 hours, examine the crack and the rest of the basement, and provide a written quote within 24 hours. No obligation, no sales pressure. Call (514) 979-5425 or use the contact form. RBQ 5668-8765-01, SOPREMA-certified applicator, 500+ projects completed since 2013.