How to Repair Cracked Concrete in a Foundation

Assess the Crack Before You Repair It

Small, hairline cracks in a foundation are common and often cosmetic, but wider or growing cracks can signal structural movement that needs immediate attention. Before you buy any product, clean the crack thoroughly with a wire brush and compressed air, then determine whether it is active or stable by marking the ends with a pencil and checking over several days.

Choose the Right Repair Method

The best approach depends on crack width and whether water is passing through.

• Epoxy injection: Best for structural cracks up to about 6 mm (¼ inch). Inject low-pressure epoxy resin to restore bonding and strength.
• Polyurethane foam injection: Works well for cracks where water leakage is present, as the expanding foam seals and flexes with minor movement.
• Hydraulic cement: Suitable for larger cracks or minor spalling; it sets quickly and tolerates slight water seepage during application.
• Concrete patching compound: Use for surface crazing or shallow chips after cleaning and dampening the area.

Step-by-Step Repair Process

For epoxy or polyurethane injection, drill 6 mm (¼ inch) holes at a 45-degree angle along the crack, install port sleeves, and inject material from the lowest port upward. For hydraulic cement, wet the area, trowel the mix into the crack in layers, and keep it damp for at least 24 hours. Always follow manufacturer cure times before applying any load or backfill.

Prevent Future Cracks

Improve exterior drainage by extending downspouts, grading soil away from the foundation, and installing a French drain if water pools near the house. Control indoor humidity and avoid rapid temperature swings in crawl spaces, as freeze-thaw cycles and soil pressure are the most common triggers for new foundation cracks.

When to Call a Professional

Contact a structural engineer or foundation specialist if cracks are wider than 6 mm, show stair-step patterns in block foundations, or are accompanied by bowing walls, sloping floors, or doors that no longer close. These signs can indicate ongoing settlement or lateral pressure that a DIY patch will not resolve.