Settlement Repair
Differential foundation settlement — how it is confirmed, watched, and addressed.
Slab settlement repair: addressing differential movement in a slab-on-grade foundation
A slab-on-grade foundation settles differently than a pier-and-beam one — the slab itself moves with the soil beneath it, which changes both how the problem shows up and what actually fixes it.
Describe what your slab is doing
How slab settlement tends to show up
- Interior cracks in flooring or drywall that follow a diagonal or stair-step patternA slab that has settled unevenly transmits that movement directly into the walls and flooring above it.
- A section of floor that feels noticeably lower or bounces differently than the restCan indicate the slab beneath has lost support in that area — often from a void that has formed underneath it.
- Doors or cabinets that no longer sit level relative to the rest of the roomA common downstream sign of an unevenly settled slab, since everything built on top of it follows the slab's new position.
Why a slab settles the way it does
A slab-on-grade foundation is poured directly on prepared soil, without a crawl space or basement beneath it. When that soil settles unevenly — from moisture cycling, poor original compaction, or erosion — a gap, or void, can form between the underside of the slab and the soil that used to support it. The slab then loses support in that section and settles into the void, which is what produces the diagonal cracking and uneven floor sections that are the classic signs of slab settlement. Addressing it generally means either filling the void so the slab has support again, or, for more significant settlement, lifting and stabilizing the affected section directly. Polyurethane foam injection is one of the more commonly used void-fill methods — see steel piers vs. foam injection for how it compares against a piering-based approach for situations where more of the structure is affected.
Questions people ask
Why does a void form under a slab?
The soil that used to support the slab settles or erodes away, often from water moving beneath the slab or from moisture-driven soil shrinkage — see settlement monitoring signatures by cause for the well-documented mechanisms behind this.
Can slab settlement happen without any visible water?
Yes. Soil moisture changes and erosion can happen well beneath the surface with no visible standing water, which is part of why the cause is confirmed by inspection rather than assumed from what is visible.
What is used to address slab settlement?
It depends on the extent — void filling beneath the slab is common for localized settlement, and more significant or structural settlement may call for a piering-based approach. See the comparison for how the two are actually decided between.
Describe what your slab is doing
Where the cracking or unevenness is, and how it has changed over time, is enough to start with.
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