Concrete Crack Repair: Foundations, Slabs, and Driveways

Reading the Crack

Hairline cracks (less than 1/16 inch wide): normal shrinkage cracking. Appears within the first year as concrete cures. Not structural. Seal to prevent water infiltration.

Vertical cracks in foundation walls: usually settlement cracking or shrinkage. If both sides of the crack are flush (not offset), the wall has not moved differentially. Seal and monitor.

Horizontal cracks in foundation walls: this is the one that worries engineers. Horizontal cracking indicates lateral soil pressure pushing the wall inward. If the wall is bowed inward above or below the crack, this is a structural issue requiring professional assessment.

Stair-step cracks in block foundation walls: follow the mortar joints in a step pattern. Indicates differential settlement — one corner of the foundation is sinking more than the other. Monitor width over time; if it is widening, consult an engineer.

Wide cracks (over 1/4 inch) or cracks with one side offset from the other: structural movement has occurred. Do not just fill these — determine the cause before repairing.

Monitoring Cracks

Before repairing, determine if the crack is active (still moving) or dormant (stabilized). Place a pencil mark at each end of the crack and a mark across the crack at its widest point. Date the marks. Check monthly.

If the marks move apart, the crack is active. Repair with a flexible material that can accommodate movement. An epoxy rigid repair on an active crack will re-crack.

Cracks that only appear or widen seasonally (wider in winter, tighter in summer) are responding to frost heave or thermal expansion. This is common and not necessarily structural — but the movement should be small (under 1/8 inch total).

Filling Hairline and Small Cracks (Under 1/4 Inch)

For slabs and driveways: clean the crack with a wire brush and compressed air. Fill with concrete caulk (polyurethane or silicone-based). Tool the surface smooth. The caulk remains flexible and moves with seasonal changes.

For foundation walls: apply a coat of hydraulic cement or concrete patching compound. Press it into the crack with a putty knife. Feather the edges smooth. For cosmetic repairs, this is sufficient.

For actively leaking cracks in foundation walls: use hydraulic cement, which sets in 3-5 minutes even with water flowing through the crack. Mix a small batch, roll it into a rope, and press it firmly into the crack starting at the top. Hold each section until it grabs.

Epoxy Injection for Structural Cracks

Epoxy injection fills the full depth of a crack in poured concrete walls, bonding the two sides together at near-original strength. This is the professional-grade repair for structural cracks in poured foundations.

The process: seal the surface of the crack with paste epoxy. Install injection ports every 6-8 inches along the crack. Inject liquid epoxy starting from the lowest port, filling until it flows out the next port up. Plug the finished port and move to the next.

Kits are available for DIY (Simpson, Emecole) but the technique takes practice. Over-pressuring blows out the surface seal. Under-pressuring leaves voids. For a first-time repair on a critical foundation crack, professional installation is worth the cost.

Epoxy is rigid when cured. Only use it on dormant cracks. For active cracks, use polyurethane foam injection instead — it remains flexible.

Polyurethane Foam Injection

Polyurethane foam expands as it cures, filling voids and conforming to irregular crack shapes. It remains flexible and accommodates minor movement. It also reacts with water, making it the preferred method for actively leaking cracks.

The injection process is similar to epoxy but the material behaves differently — it expands 10-20x its injected volume. Use less material than you think. Over-injection can create hydraulic pressure that widens the crack.

Polyurethane does not bond the concrete together like epoxy — it fills and seals but does not restore structural strength. For cracks where both sealing and structural repair are needed, some professionals inject polyurethane first to stop water, then epoxy to restore strength.

Slab and Driveway Crack Repair

Clean the crack with a pressure washer or wire brush and remove any loose material. For cracks deeper than 1/2 inch, fill the lower portion with backer rod (foam rope) to within 1/4 inch of the surface.

Apply self-leveling concrete crack filler (the kind in a caulk tube works for narrow cracks). For wider cracks, use a concrete patching compound mixed to a pourable consistency and troweled smooth.

For a cosmetic match, resurface the entire slab with a thin concrete overlay after repairing the cracks. This hides the repairs and gives a uniform appearance. Overlays are 1/8 to 1/4 inch thick and bond to the existing slab with a polymer adhesive.

When to Call a Professional

Horizontal cracks in foundation walls with visible inward bowing. Stair-step cracks in block walls that are wider than 1/4 inch or widening over time. Any crack where one side is offset vertically from the other (indicating shear movement). Cracks accompanied by doors or windows that no longer close properly, or floors that slope noticeably.

A structural engineer (not a foundation repair company) can assess whether the crack is cosmetic or structural and recommend the appropriate repair. Foundation repair companies have a financial incentive to recommend expensive solutions — an independent engineer assessment first is money well spent.

Frequently Asked Questions