The short answer
Most houses in Texas sit on slab-on-ground foundations, according to the Texas Section of the American Society of Civil Engineers, and newer Austin houses are typically slabs; 43% of the city’s homes date from 2000 or later. When a slab here has a problem, it’s usually one of three:
| Problem | What you notice | Usual cause | Usual fix |
|---|---|---|---|
| A settled edge or corner | Stair-step cracks in brick at one end, doors that drag, a floor that falls toward an outside wall | Clay under the edge drying and shrinking, often near trees or after a long drought | Piers under the beam along the low area, then a lift |
| Center heave | Cracks across interior walls and ceilings, doors that stick in the middle of the house, a hump you can feel | Water collecting under the slab: a plumbing leak, poor drainage, or clay rewetting after a drought | Find and stop the water first; piers don’t fix this |
| A plumbing leak | A wet spot, a high water bill, a sewer smell, warm floor tiles | A broken supply or drain line under the slab | A licensed plumber, then a foundation assessment |
Telling these apart is the job of the inspection and the floor elevation survey. A plan that proposes piers for a heaving slab is answering the wrong question.
Why slabs move on Austin clay
Texas engineers rate a clay’s swelling with its plasticity index (PI); the Texas Section of ASCE treats soil as expansive at a weighted PI greater than 15, and the clays across eastern and northern Austin are the classic case. An environmental inventory filed with the city for a site on East Rundberg Lane maps its soil unit as Houston Black (56%) and Urban land, and rates the Houston Black part’s shrink-swell potential very high. On the city’s southeast edge, the developer of Easton Park reported highly expansive clay soils, and its builders and their foundation engineers asked for lots sloped no more than 2% to cope with them.
A slab sits on top of that clay like a raft. The Texas Department of Licensing and Regulation sums up the cycle as heave when wet, settle when dry. The edges feel it first, because the soil there dries and rewets faster than the soil under the middle of the house.
West of the escarpment, slabs more often sit on thin soil or fill over limestone. Those houses can still crack, usually where fill was placed on a slope and settled, or where runoff from rock keeps one side wet.
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Exterior and interior piers
Exterior piers go in along the perimeter beam. The crew digs a hole beside the house at each pier location, installs the pier down to firmer soil or rock, and uses jacks to lift the beam back toward its original level. Shims or a cap transfer the load, and the holes are backfilled.
Interior piers support the interior beams of a slab when the middle has dropped. They’re reached either by cutting small openings through the floor or by tunneling under the slab from outside. Breakouts disturb flooring; tunnels disturb the yard and take longer. Either way they cost more than exterior piers.
The pier type, pressed concrete, steel push, helical or drilled, matters less than whether it reaches ground that doesn’t swell and shrink with the seasons. The Foundation Performance Association warns that shallow piling can continue to settle as moisture changes. The foundation piers page compares the types.
Spot lifts for flatwork
Driveways, patios, sidewalks and garage slabs that have sunk are a different job. Mudjacking or slabjacking pumps a cement slurry under the slab to raise it; HomeAdvisor puts it around $500–$1,300. Polyurethane foam does the same with expanding foam and costs $5–$25 per sq ft according to Angi. Both sit in the active clay, so they lift a slab without anchoring it below the zone that moves. Contractors use them mostly on flatwork; if one is proposed for the house itself, ask why it was chosen over piers.
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The plumbing under the slab
Plumbing and slab movement are tangled together. A leaking drain line can soak the clay under part of the house and lift it; a slab that moves can crack a pipe. Before a lift, ask whether the plumbing will be tested, and after it, test again, because raising a slab can stress lines that were fine before.
- Who does it: under Texas law, plumbing under the slab needs a licensed plumber (state plumbing license required).
- Permits: the City of Austin’s Express permit page notes that a plumbing permit may be required depending on the foundation work.
- Cost: Fixr puts an average slab leak repair, including leak detection and tunneling, at $2,500.
- Insurance: damage from a sudden leak is the one foundation problem Texas policies may cover. The Texas Supreme Court addressed it in a ruling from 1998; read your own policy and call your insurer before permanent repairs.
Cracks in the slab itself
Hairline cracks in a slab are common and often harmless: concrete shrinks as it cures. The ASCE Geo-Institute’s homeowner guide treats floor cracks 1/16 inches across or wider as worth attention. A crack where one side sits higher than the other says more than a flat one. Tile that cracks in a straight line across a room is often following a slab crack underneath. Photograph it, mark the ends with a pencil and a date, and check again after the next dry spell.
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What slab repair typically costs
| Item | Typical cost | Source |
|---|---|---|
| Concrete slab repair | $1,000–$10,000 | HomeGuide |
| Piers | $1,000–$3,000 per pier | HomeAdvisor |
| Piers on a typical slab job | 5–15 piers | Olshan (contractor figure) |
| Slab leak repair, average | $2,500 | Fixr |
| Foundation repair in Austin, average | $6,815 | Angi |
The cost page explains why pier counts and per-pier prices vary.
Questions for a slab repair plan
- Is this settlement or heave, and what in the survey shows it?
- How many exterior and how many interior piers, and how will the interior ones be reached?
- How deep will the piers go, and how is the stopping depth decided?
- Will the plumbing be tested before and after, by a licensed plumber?
- What drainage or watering changes does the plan assume?
- Who restores flooring, landscaping and flatwork, and who pulls the permit?