Slab Foundation Repair: Cracks, Voids, and Lifting

Slab foundation repair costs $3 to $12 per sq ft for lifting and $6,000 to $30,000 for structural underpinning. The right repair depends on what failed: a crack in a sound slab, a void underneath it, or a footing that has settled. Those are three different jobs at three different prices.

The Three Slab Types

A slab on grade is a concrete slab poured directly on prepared soil, thickened at the perimeter to form a footing. A monolithic slab pours the footing and the floor in one continuous placement. A post-tension slab adds tensioned steel cables that compress the concrete and resist cracking.

Each type fails differently. A conventional slab cracks at shrinkage points and settles where the soil compacts. A monolithic slab moves as one unit, so a void at one corner tilts the whole floor. A post-tension slab resists cracking well but cannot be cut without a cable scan, which limits repair options.

Reading the Cracks

Measure the width at the same point each month. A crack that grows is telling you the slab is still moving, and sealing it before the movement stops wastes the seal.

Lifting and Void Filling

Slab lifting raises a settled slab by injecting material underneath. A sand-cement slurry costs $3 to $8 per sq ft, and polyurethane foam costs $5 to $15 per sq ft. The same injection fills the void that let the slab settle.

The arithmetic is simple. A 500 sq ft garage floor lifted with slurry at $6 per sq ft:

500 × $6 = $3,000

The same floor with foam at $11 per sq ft:

500 × $11 = $5,500

Foam costs more but weighs a fraction of slurry and cures in minutes, which matters on weak soil or when the floor has to be back in service the same day.

Structural Repair

When the slab’s footing has settled, lifting the slab alone will not fix it. The footing needs support from below, which means piers. Push piers run $1,000 to $3,000 each and helical piers run $700 to $2,500 each, and a typical house needs several.

A full underpinning job at $6,000 to $30,000 reflects that count. The slab is then patched where the piers were installed, and the lift restores the floor elevation.

A Costed Example

A 1,700 sq ft house has settled 1 inch at one corner. The engineer specifies six push piers at $1,900 each and slab patching afterward.

6 × $1,900 = $11,400

Add engineering at $900 and patching at $1,200:

$11,400 + $900 + $1,200 = $13,500

That sits in the $6,000 to $30,000 underpinning band and well above a simple lift, which is the difference between supporting a footing and lifting a slab.

Repair Versus Replace

Replacement of a slab is a larger project than most homeowners expect, because it involves removing the floor, protecting or relocating services, and restoring the interior. It is justified when the slab can no longer act as a unit.

Post-tension slabs need one extra step before any repair. The cables are under high tension, and cutting one can release that energy violently, so no contractor should drill or cut a post-tension slab without a cable scan and a plan. The scan typically adds $300 to $700 to the job. It is not optional, and a contractor who dismisses the concern is a contractor to replace.

The Repair Sequence

1. Map the cracks and photograph them with a scale.
2. Tap-test the slab and check for hollow areas and flexing.
3. Confirm whether the slab is post-tensioned before any drilling.
4. Fix the water source that created the void or the settlement.
5. Fill voids and lift, or underpin the footing, to the engineer’s design.
6. Patch the injection holes and restore the floor finish.

When to Call an Engineer

Call one when a crack reaches 1/8 inch, when a floor has dropped more than an inch across a room, when doors and windows bind, or when the slab is post-tension and needs work. The inspection runs $400 to $1,500 and produces the scope that makes the repair price meaningful.

For the type comparison, read slab foundation and floating slab. If the slab is post-tension, post-tension slab explains the scanning requirement, and slab lifting cost covers the injection pricing in more detail.

A repaired slab should be watched for one full year. Note the cracks that were sealed, photograph them in the same light, and compare widths in the spring and again in the fall. If a sealed crack reopens, the cause was not corrected, and the next step is drainage or underpinning rather than another seal.