Repair a Foundation Crack

Foundation cracks appear in nearly every house built on a concrete slab or poured wall. Most are harmless settling cracks, thin as a credit card edge, that show up in the first few years as the house finds its permanent footprint. But some cracks leak, widen seasonally, or signal movement that needs attention. The difference between cosmetic and structural isn't always obvious from the surface, but width, direction, and water intrusion tell you what you're dealing with. Repairing a crack properly means matching the fix to the problem. Hairline cracks under an eighth inch get epoxy or flexible sealant. Wider cracks that weep water need polyurethane injection or hydraulic cement. Horizontal cracks, stair-step patterns in block walls, or anything wider than a quarter inch deserves a structural engineer's eyes before you touch it. This guide covers the repairs you can handle yourself—the common vertical settling cracks and minor water intrusion that don't require underpinning or wall replacement.

1. Clean and assess the crack. Use a wire brush to remove loose concrete, paint, and debris from the crack and surrounding area. Vacuum out all dust with a shop vac. Examine the crack width with a ruler—anything under a quarter inch is repairable with standard methods. Check if the crack is actively weeping water by taping a piece of plastic over it for 24 hours and checking for moisture behind it.
2. Widen hairline cracks for epoxy. For cracks under an eighth inch, use a concrete chisel and hammer to widen the crack to a quarter inch in a shallow V-shape. This creates a mechanical key for the epoxy to grip. Brush out all dust again and wipe with a damp rag, then let dry completely for at least an hour.
3. Mix and inject epoxy compound. Follow the epoxy kit instructions for mixing ratio and working time. Load the mixed epoxy into the injection gun and insert the nozzle into the lowest point of the crack. Inject slowly, moving upward as the epoxy fills and begins to ooze from the crack face. Fill until epoxy appears at the next injection point, typically every eight to twelve inches up the wall.
4. Apply hydraulic cement to larger cracks. For cracks wider than a quarter inch, mix hydraulic cement to a putty consistency. Dampen the crack with water, then pack the cement firmly into the crack with a trowel, working from bottom to top. Overfill slightly and smooth flush with the wall. Hold pressure for three to five minutes as the cement begins its initial set.
5. Install injection ports for active leaks. If water is actively seeping, drill holes at 45-degree angles into the crack every six inches using a hammer drill with a half-inch masonry bit. Insert plastic injection ports and seal around them with fast-set epoxy paste. Wait for the paste to cure per manufacturer instructions before injecting polyurethane.
6. Inject polyurethane foam. Starting at the lowest port, inject polyurethane slowly until foam appears at the next port up or begins to seep from the crack. Move to the next port and repeat. The foam will expand and harden within 30 minutes, forming a waterproof seal that flexes with minor wall movement.
7. Smooth and finish the repair. Once the epoxy or cement has fully cured, use a grinder with a masonry wheel to smooth any protruding material flush with the wall surface. Wipe clean and apply concrete bonding primer if you plan to paint or apply waterproofing coating over the repair.
8. Monitor the repair over one season. Mark the ends of the repaired crack with pencil lines or small pieces of tape. Check monthly for three months, then seasonally. If the crack reopens or new cracks appear parallel to the repair, consult a structural engineer before attempting further repairs.