How to Repair a Cracked Garage Slab Foundation

Identify the Problem

Start by walking the slab and marking any cracks, low spots, or moisture stains. Measure crack width with a ruler; hairline cracks (<1 mm) often need only sealing, while wider gaps (≥3 mm) require filler and reinforcement. Check for uneven settlement by placing a long level or straightedge across the floor; gaps indicate sections that have shifted.

Prepare the Work Area

Clear the garage of vehicles, storage, and utilities. Sweep the slab clean and use a shop‑vac to remove dust. For cracks, expose the edges by chiseling away loose concrete to at least 1 cm depth, creating a V‑shaped groove that will hold repair material securely.

Choose the Right Repair Method

Common methods vary by crack size and slab condition:

• Epoxy injection – best for structural cracks up to 6 mm; provides a strong, water‑tight bond.
• Polyurethane foam – expands to fill gaps, ideal for active movement and water infiltration.
• Concrete resurfacing – applies a thin overlay (¼–½ in) to smooth minor unevenness and seal the surface.

Step‑by‑Step Repair Process

1. Epoxy or Polyurethane Injection

Mix the two‑part epoxy according to the manufacturer's instructions. Attach injection ports at 6‑inch intervals along the crack, then pump the epoxy until it seeps out of the opposite side. For polyurethane, use a caulking gun and inject slowly, allowing the foam to expand and fill voids. Clean excess material with a scraper before it hardens.

2. Patching Larger Gaps

For gaps wider than 6 mm, place a clean, non‑shrink concrete mix (1 part cement, 2 parts sand, 0.5 part water‑reducing admixture) into the prepared groove. Embed steel reinforcement mesh if the crack spans more than 1 ft. Trowel smooth and feather the edges into the surrounding slab.

3. Leveling Sunken Areas

Apply a self‑leveling underlayment over low spots. Prime the area with a bonding agent, then pour the underlayment, allowing it to flow and settle. Follow the product's cure time before driving onto the slab.

Waterproofing and Prevention

After repairs cure, seal the entire slab with a penetrating concrete sealer or an epoxy coating. This reduces water intrusion that can cause future movement. Ensure proper drainage around the garage by grading the soil away from the foundation and installing gutter extensions.

When to Call a Professional

If cracks exceed 1 inch, show rapid growth, or are accompanied by structural shifting of walls, a structural engineer should assess the foundation. Professional contractors can install steel piers or carbon‑fiber reinforcement for severe settlement.

Maintenance Checklist

Regularly inspect the slab for new cracks, monitor moisture levels, and keep the surrounding soil dry. Reapply sealant every 3‑5 years to maintain protection.