What a Slab Foundation Actually Is
A slab-on-grade foundation is a single concrete pour placed directly on prepared ground. The perimeter and the interior load lines are thickened into what is called a grade beam, and that beam carries the walls above. There is no crawlspace, no joists, and no accessible space between the ground and your floor.
That construction has two consequences for repair. First, the slab and the floor are the same thing, so movement in the slab is movement you walk on. Second, everything beneath it is reached either from outside the perimeter or by opening the floor, which is why access is such a significant part of any slab repair conversation.
Our slab foundation repair page covers the full service. This guide focuses on what the work itself consists of.
Why the Slab Moves
The slab is supported by the ground, and across most of San Antonio that ground is expansive clay that changes volume with moisture. When moisture around the perimeter varies, as it always does, the support varies with it.
Movement shows up in three broad shapes:
- Differential settlement. One area drops relative to another as the soil beneath it loses volume or bearing capacity.
- Center heave. The interior rises, often because moisture is accumulating underneath, sometimes from an under-slab plumbing leak.
- Edge lift. The perimeter rises as clay around the edges swells after significant moisture.
The direction matters enormously, because underpinning addresses settlement and does not address heave. Establishing which one applies is the first job of an evaluation.
The Core of the Repair: Underpinning
Where settlement is the finding, the repair transfers load off the unreliable surface soil and onto something deeper.
Piers are installed beneath the grade beam. Each one is driven or pressed down until it reaches material capable of carrying the load, and a bracket transfers the structural weight onto it. Once the piers carry the load, seasonal movement in the surface clay no longer moves that part of the structure.
That is the mechanism in full. The variations are about which pier system, how many, where, and how deep, and those come from the evaluation rather than from a preference. Our guide on how slab foundations are underpinned with piers covers the mechanics in detail.
Typical Project Scope
A slab repair project usually consists of:
- Evaluation and profile. Elevation measured across the whole plan, plotted, with site and drainage conditions recorded.
- Scope and plan. Pier count, marked placements, system selected, depth assumption stated.
- Excavation. A pit at each pier location to expose the grade beam.
- Pier installation. Driven or pressed to refusal, with depth and pressure recorded per pier.
- Bracket and load transfer. Brackets fixed, load transferred onto the piers.
- Controlled lift. Synchronised hydraulic lifting in increments, with readings between stages.
- Backfill and grading. Pits backfilled, perimeter regraded to fall away from the structure.
- Documentation. Post-repair elevation record, pier log, warranty terms.
What Is Typically Included and Excluded
| Usually included | Usually excluded |
|---|---|
| Excavation at pier locations | Landscaping reinstatement beyond backfill |
| Pier supply and installation | Irrigation line repair or relocation |
| Bracket attachment and load transfer | Flooring replacement over interior work |
| Controlled lift | Cosmetic repair of existing cracking |
| Backfill and rough grading | Plumbing repair where a leak is found |
| Pier log and post-repair profile | Drainage work unless separately scoped |
The exclusions column is where proposals differ most and where the least is usually written down. It is the part worth reading twice.
A note on scope honesty
Cosmetic repair being excluded is not a contractor cutting corners. Cracks are the symptom, and closing them before the structure has settled into its new position means doing it twice. What matters is that the exclusion is stated rather than assumed.
What the Repair Does Not Address
Movement outside the piered area. Piers support what was measured as needing support. The rest of the foundation continues to sit on the original soil.
Moisture conditions at the surface. Drainage and grading still matter, and where they are contributing, correcting them belongs in the conversation.
Plumbing. If a leak beneath the slab is driving the movement, underpinning does not stop it. That is why a hydrostatic plumbing test is recommended where the profile or the home’s age suggests it.
Existing cosmetic damage. Separate finishing work, usually best done after the structure has settled.
Is Replacement Ever the Answer?
Rarely. Replacement means demolishing the slab, temporarily supporting the structure, and pouring a new foundation. It is vastly more disruptive and more expensive than underpinning, and it is warranted only where the concrete itself has failed structurally rather than where the ground beneath it has moved.
In almost every residential case we assess, the concrete is sound and the ground is the problem. Supporting the sound concrete from a better bearing point is the proportionate response.