Slab Cracks Repair: Seal, Lift, or Replace

Slab cracks repair depends on whether the slab is moving. A stable crack is sealed for a few hundred dollars. A crack over a void needs the void filled and the slab lifted at $3 to $8 per sq ft. A slab broken into moving pieces needs replacement at $8 to $18 per sq ft. The crack tells you which case you have if you measure it and watch it.

The Four Crack Types

Shrinkage cracks are narrow, random, and appear as the concrete cures. They are cosmetic and rarely need more than sealing. Settlement cracks are wider, often diagonal, and appear where the slab has lost support underneath.

Structural cracks run through the full thickness and may have a height difference across the two sides. Map cracking is a network of fine cracks that often signals a surface problem such as an alkali reaction or improper finishing.

Measuring the Crack

A crack with a vertical offset is more serious than a wider crack without one. The offset means the two sides have moved relative to each other, which is a support problem rather than a shrinkage problem. Run a finger or a straightedge across the crack; if it catches, there is an offset, and the slab needs more than sealant.

Sealing and Injection

Surface sealing uses a flexible polyurethane or silicone sealant in a cleaned crack, often with a backer rod to control depth. It runs $3 to $8 per linear foot for small jobs and is a homeowner-level task for a driveway.

Injection uses epoxy or polyurethane resin under pressure and runs $400 to $1,200 per crack. Epoxy restores structural continuity in a dry crack; polyurethane foams and stays flexible in a wet one. Injection is the right repair for a crack that is stable but leaking.

Lifting and Void Filling

When the crack is caused by a void, filling the void and lifting the slab addresses the cause. Slurry costs $3 to $8 per sq ft and foam costs $5 to $15 per sq ft.

A 300 sq ft patio lifted with slurry at $6 per sq ft:

300 × $6 = $1,800

The same patio with foam at $12 per sq ft:

300 × $12 = $3,600

Foam costs more and weighs much less, which matters when the soil underneath is weak.

Replacement

Replacement is the answer when the slab can no longer act as a unit. It costs $8 to $18 per sq ft depending on thickness, reinforcement, and access, and it involves removing the old concrete, preparing the base, and placing new concrete with control joints.

A 400 sq ft driveway replaced at $12 per sq ft:

400 × $12 = $4,800

That is more than lifting but less than living with a slab that keeps cracking after every repair. Replacement also gives you the chance to improve what is underneath: a compacted base, a vapour barrier, and control joints that the original slab may have lacked.

The Repair Sequence

1. Map the cracks on a sketch and record widths with dates.
2. Tap-test for hollow areas that indicate a void.
3. Check for vertical offset across each crack.
4. Fix the water source that created the void or the settlement.
5. Seal, inject, or lift according to the diagnosis.
6. Cut control joints if the slab lacks them.
7. Recheck after one freeze-thaw or wet-dry cycle.

Control Joints Matter

Most slab cracks appear where a control joint should have been. A control joint is a saw cut that gives the concrete a planned place to crack, and it is far cheaper than repairing random cracks later.

If a slab is being replaced or newly poured, insist on control joints at intervals of roughly 2 to 3 times the slab thickness in feet. A 4-inch slab wants joints every 8 to 12 feet. That single detail prevents most of the cracking a homeowner will otherwise be repairing in five years. The joints also need to be sealed after curing, because an open joint lets water reach the base and the base is what supports the slab. Sealing a joint costs a fraction of injecting a crack and a small fraction of replacing a slab.

When to Call a Contractor

Call one when a crack is wider than a quarter inch, when there is a vertical offset, when the slab sounds hollow, or when a repair has failed twice in the same place. Those signs point to a support problem rather than a surface problem, and the repair cost moves from hundreds to thousands.

Get two quotes and ask each contractor to explain the cause before the price. A contractor who says the crack needs sealing without looking underneath is quoting a surface repair for what may be a support problem, and the second crack will appear within a year.

For the slab context, read slab crack and concrete slab foundation repair. If the slab is part of the house foundation rather than flatwork, slab foundation repair covers the structural side.