Slab Foundation Repair: How a Sinking or Cracking Slab Is Fixed

A slab can sink, dome, or crack — and the right fix depends entirely on why. A slab-on-grade foundation, the San Antonio norm, moves for two main reasons: expansive-clay soil swelling and shrinking with moisture, and a hidden sub-slab plumbing or sewer leak that saturates and heaves the soil or washes it out into settlement. The structural repairs are well understood — deep piers for a settling slab, foam or slurry for sunken flatwork, sealing for cracks. But the sequence is what protects your money: if a leak moved the slab, the leak comes first, before any pier goes in the ground. This page is the bridge from the leak side of the problem to the structural side. For the leak itself — detection, repair, and how it moves a house — start at our slab-leaks pillar; here we cover what happens to the slab.

What moves a slab foundation

A slab-on-grade foundation can sink, crack, or dome from two main drivers, and they often work together.

The first is expansive-clay soil movement. Much of San Antonio sits on high-plasticity smectitic clay that swells when wet and shrinks when dry. Because that moisture almost never changes evenly around a house, one part of the slab heaves up while another settles down — differential movement, the precise pattern that cracks slabs, brick, and finishes. A foundation rides uniform movement with little harm; it is the unevenness that does the damage.

The second driver is a sub-slab plumbing or sewer leak, and this is where the slab problem and the leak problem become one. A chronic leak delivers concentrated water to a single patch of soil. In expansive clay that wetted patch swells into a localized heave or center-lift dome you can feel as a high spot in the floor. A severe drain or sewer leak does the opposite — a steady flow erodes and washes soil into the broken line, removing support beneath that spot so it settles. Either way the slab moves unevenly.

The two drivers also feed each other in a two-way cycle: the same swell-and-shrink action that moves the soil stresses buried pipes and pulls joints apart, and the leaking pipe then feeds more water into the soil. Soil movement breaks the pipe, and the pipe moves the soil — recognizing that loop is what dictates the order of every repair below. For the full mechanism — how a supply leak versus a sewer leak moves a slab, and why older San Antonio homes are most exposed — see our plumbing-leaks cause page.

Fix the leak first

This is the cardinal rule of slab foundation repair, stated flatly: if a leak caused the movement, fix the leak before you touch the structure.

The reason is mechanical, not cautious. Deep piers transfer load to stable soil below the active zone, but the slab still bridges the surface clay around them. If a sub-slab leak keeps wetting that clay, the soil keeps swelling and washing out — and the slab moves again, even though the piers are seated perfectly. You will have paid to stabilize the structure while leaving the cause running underneath it.

The way to confirm a leak is in the picture is a hydrostatic plumbing test — a plumber plugs the sewer at the cleanout, fills the under-slab drains to slab level, and watches whether the water holds; a drop means a drain-side leak. That test belongs before structural work, because a positive result reorders the whole plan: the plumbing is repaired and the soil restabilizes first, the elevations are re-surveyed, and only then is any remaining movement underpinned. Often the leak repair is most of the fix — with the water source gone, the clay settles and far less underpinning is needed than the first estimate assumed.

Repairing a sinking or settling slab: piers

When a slab has genuinely settled — not just cracked cosmetically, but dropped out of level — the engineered repair is deep underpinning. The principle is simple: the restless, moisture-driven soil is concentrated in the active zone near the surface, roughly 8 to 15 feet deep in San Antonio, with competent, dimensionally stable soil below it. Underpinning drives supports down through the active zone to that stable stratum and transfers the building's load onto them, bypassing the soil that caused the problem.

Three pier types do this work:

• Steel push piers — galvanized steel pipe sections driven straight down using the weight of the house as the reaction force, until they reach refusal on competent strata. Best for heavier homes over reachable firm bearing.
• Helical piers — a steel shaft with welded helical plates screwed in by a torque motor that supplies its own driving force. Best for lighter structures and soft or expansive soils, because installation torque verifies capacity in real time.
• Drilled or concrete piers — shafts taken down to firm bearing in poured concrete, a Texas-classic approach in expansive clay.

Once every pier reaches firm bearing, the crew either stabilizes the slab where it sits or jacks it back toward level before locking the brackets off. That decision belongs with your engineer: stabilizing carries near-zero collateral risk, while chasing maximum lift raises the odds of cracked finishes and stressed plumbing.

This page stays at the overview level on purpose — the deep how-each-method-works dives live elsewhere. For the full treatment of push piers (anatomy, drive-pressure verification, when they beat helical), see our steel push piers guide; for every underpinning method side by side, the foundation repair methods comparison.

Sunken flatwork: slab jacking and foam

Not every sunken slab is a settling house, and the distinction decides the method. Flatwork — a garage floor, patio, sidewalk, driveway apron, or AC pad — carries little structural load. When it drops because a void opened beneath it, the fix is not deep underpinning but filling the void:

• Mudjacking (slab jacking) pumps a heavy cement-and-soil slurry through 1–2 inch holes to float the slab back up. Inexpensive and well suited to large, heavy slabs.
• Polyurethane foam (polyjacking) injects a light, expanding resin through smaller holes that lifts the slab in seconds and resists washout. Costs more, cures faster, weighs almost nothing.

The hard line: these are flatwork tools, not underpinning. Filling the void under a loaded, settling perimeter footing treats the symptom for a season and can mask movement an engineer needs to see — it does not transfer the load to stable soil the way piers do. The honest test is one question: is the concrete I'm leveling carrying my house, or not? If it is flatwork, foam or slurry is a sound, cost-effective fix; if it is the slab your home sits on and it is settling, that is a pier question. For the full mud-versus-foam breakdown and the cases where each fits, see our slab jacking and mudjacking guide.

Slab cracks

A crack in a slab is a symptom, and the repair depends on what kind of crack it is. Cosmetic cracks — fine shrinkage cracks from the concrete curing, with no displacement and no leak — can be sealed and forgotten. Structural cracks, with displacement or active movement, are the slab telling you something moved it.

Two sealing methods cover the crack itself. Epoxy injection restores a dry, structural crack in poured concrete to monolithic strength — the cured epoxy is stronger than the surrounding concrete. Polyurethane injection seals a leaking crack to stop water, staying flexible to accommodate minor movement. Both are legitimate repairs for the crack.

But here is the limit that matters: sealing the crack does not fix the cause. If the slab is still moving because of expansive-clay swell or an unfixed leak, a sealed crack will reopen or a new one will form beside it. Seal the crack only after an engineer has confirmed the underlying cause is addressed. For the method-by-method detail on epoxy versus polyurethane and when each is right, see our crack repair guide.

What it costs and insurance

Deep underpinning of a settling slab runs roughly $1,000–$3,000 per pier, with a typical project landing around $15,000–$30,000 depending on the number of piers the engineer specifies, the depth to firm bearing, and access. Partial underpinning of one corner or wall is less; a full perimeter is more. Budget an independent engineer's report at $500–$1,500 and a permit on top. Sunken-flatwork leveling is a separate, far cheaper category. All of these are 2026 industry-estimate planning ranges, not quotes — slab pricing varies by region, depth, and access, so your only path to a real number is on-site bids written against an engineer's spec.

Insurance turns on one distinction: sudden versus gradual. A sudden, accidental event — a burst supply line — may be a covered peril, in which case the policy can pay to tear out and replace the slab to reach the pipe and to repair the resulting water damage. What it generally will not pay for is the pipe itself, gradual leaks, corrosion and wear-and-tear, or earth movement and settling — the exclusion that knocks out the large majority of Texas slab claims by cause. Because coverage hinges on cause, insurers scrutinize these claims hard: per the Insurance Information Institute (Triple-I), water damage is one of the most frequent and costly homeowners claims, so the carrier looks closely at whether a leak was sudden or ongoing. That is why one document decides both your claim and your repair scope — a sealed engineer's cause-of-loss report. For the full coverage breakdown and the cost detail behind each line, see our slab-leaks pillar and its cost and insurance sections.

FAQ Note

The FAQ below answers what San Antonio homeowners ask most once a slab has started to sink or crack — how a sinking slab is repaired, what slab piers are, why a slab cracks, whether it can be lifted, the leak-or-slab-first question, cost, insurance, and whether a crack can just be sealed. For the leak side, start at the slab-leaks pillar and the plumbing-leaks cause page; for the structural methods, the push piers guide and methods overview; and before you commit to any structural work, an independent engineer's report.

Get Matched With a Vetted San Antonio Foundation Specialist

If your slab is sinking, doming, or cracking — a high spot you can feel in the floor, a stair-step crack in the brick, doors that have started to stick — the right first step is a diagnosis, not a pier quote. We'll match you with a vetted San Antonio specialist who fixes the leak before the structure and works to an independent engineer's spec, not a sales script. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for the right sequence above all: any sub-slab leak confirmed and repaired first, elevations re-surveyed once the soil restabilizes, and underpinning scoped by a sealed engineer on only what is still out of tolerance — because on a slab home, fixing the foundation before the leak just guarantees you pay again.