Slab foundation repair, and the two checks that come first

What is slab foundation repair?

Slab foundation repair addresses a concrete slab-on-grade that has moved because the soil beneath it moved - settling in one area, lifting in another, or both across a season. The repair itself is usually a form of underpinning, but two questions have to be answered before anyone prices it: whether a plumbing line under the slab is leaking and driving the movement, and whether the slab is post-tensioned, because that changes anything involving cutting or drilling.

Signs you need slab foundation repair

• Doors along one side of the house binding at the same corner, while doors elsewhere still swing freely
• A gap that has opened between the slab edge and the brick or siding above it, visible from outside
• Cracks in floor tile or vinyl that run in a straight line across several tiles rather than at one broken piece
• Interior wall cracks radiating diagonally from door and window corners, particularly in a repeating pattern down one wall
• A section of floor that feels warm underfoot, or a hot-water bill that climbed with no change in habit
• The sound of running water when every fixture is off, or an unexplained jump in the water bill
• Patches of the slab perimeter that stay damp long after the surrounding ground has dried
• Seasonal symptoms that appear in a dry summer and partly close after a wet spell, in a clay-soil region

Seeing something not on this list? Browse foundation repair problems by symptom →

How slab foundation repair actually works

1. 1

   Rule out an under-slab plumbing leak before anything else

   A leaking supply or drain line under a slab softens and erodes the supporting soil continuously. A pressure test on the supply side and a static or camera test on the drain side answer this at a small fraction of the cost of underpinning.

   Watch forA pier proposal written with no plumbing test performed or requested. Supports installed into soil that a live leak is still washing out will keep moving, and the warranty conversation that follows tends to go badly.

2. 2

   Establish whether the slab is post-tensioned

   Post-tension slabs carry steel tendons under high tension. They are common in newer construction in some regions, often signposted at the slab edge or in the builder documentation. If the answer is yes or unknown, no one cuts, cores or drills until the layout has been located non-destructively.

   Watch forA crew proposing to saw a trench or core the slab for interior supports on a house nobody has checked. Cutting a tendon is dangerous during the work and is a serious structural alteration afterwards - it is the one mistake on this page with no cheap remedy.

3. 3

   Map the movement with an elevation survey

   Relative floor elevations across the whole plan show where the slab is low, where it is high, and what shape the movement takes. A uniformly low corner and a domed centre are different soil stories and lead to different recommendations.

   Watch forSymptom-led scoping - "there are cracks on this wall, so we will place supports along this wall". The cracked wall is often not the low point; it is just where the finishes gave up first.

4. 4

   Understand what your soil is doing seasonally

   Expansive clay swells when wet and shrinks when dry, so a slab in that soil rides a seasonal cycle. Distinguishing a house riding that cycle from a house progressively settling requires observation across seasons, not a single visit in August.

   Watch forA diagnosis made at the driest point of the year and presented as permanent settlement. In clay regions the worst-looking week of the year is a predictable week of the year.

5. 5

   Try the cheap interventions where they apply

   In expansive-soil regions, stabilising moisture around the perimeter - consistent watering in drought, gutters and downspouts that discharge well away, correcting grade that ponds against the slab, managing large trees drawing moisture from beneath - is inexpensive and sometimes changes the trajectory.

   Watch forThis step being skipped because it generates no revenue. It is not a substitute for underpinning where underpinning is warranted, but it is the only intervention that addresses the cause rather than the consequence, and it has to happen either way.

6. 6

   Specify the support scheme from measurements

   If underpinning is warranted, the scheme states where each support goes, what it must reach, and what the target movement outcome is. Method selection - driven or screwed, exterior or interior access - follows from the loads and the soil, and is covered on the comparison pages rather than repeated here.

   Watch forA per-unit price with the count left open. On slab work the count is the price, and an unspecified count means the number you agreed to is not the number you will pay.

7. 7

   Interior access, and what it does to your house

   Supports placed under interior loadbearing lines require the flooring to come up and the slab to be opened. That means tile out, furniture out, dust everywhere, and a reinstatement question that homeowners routinely discover late.

   Watch forFlooring reinstatement excluded from the contract. Ask who puts the tile back, whether the replacement will match, and who owns the risk that it does not - before, not after.

8. 8

   Re-measure, then keep measuring

   Post-work elevations document what was achieved. Continuing to re-check over the following seasons is what tells you whether the underlying soil behaviour was actually addressed, and it is the evidence any warranty claim will need.

   Watch forNo closing survey. Without one there is no defensible record of the condition on the day the crew left, which is precisely the fact a future dispute turns on.

What does slab foundation repair cost?

Slab repair pricing is driven mainly by the number of supports specified and whether any are placed inside the house, since interior work adds flooring removal and reinstatement. Plumbing tests and soil-moisture correction sit at the low end; a full perimeter underpinning scheme runs into five figures.

See the full slab foundation repair cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →