Concrete Void Filling: Restoring Support Beneath Settled Slabs

A void is the empty space that opens up when the soil beneath a slab settles, washes out, or pulls away from the underside of the concrete. In the Carolinas the usual driver is the soil. In Piedmont clay around Charlotte and the Triangle, the soil swells when wet and shrinks when dry, and that cycle pulls it away from the slab. In Sandhills sand near Fayetteville and Pinehurst, fine particles wash out and loosen the support. Water draining against a slab from a downspout and poorly compacted backfill are common contributors statewide. We confirm the cause during the inspection rather than guessing.

They are closely related and often happen on the same job. Void filling means filling the empty cavity beneath a slab so the concrete is supported from below again. Concrete leveling means raising a slab that has sunk back toward its original height. Filling the void is what re-supports the slab, and when the same injected material is allowed to build pressure it can also lift a slab that has settled. Some slabs only need the void filled because they are still close to level, while others need both. We explain which applies to your slab during the inspection.

Depending on the slab and the soil beneath it, we fill voids with either a cement-based slurry or a two-part polyurethane foam. The slurry is a dense, flowable cement-based mix, while polyurethane is lighter and expands to fill the cavity, which can be an advantage over soil that is already loose or washed out. Neither material is automatically the right choice. During your no-pressure inspection we explain which one fits your slab and the conditions underneath it, and why.

Not by itself. Void filling re-supports the slab and removes the hollow gap, but it does not change the deeper soil behavior or drainage problems that allowed the void to open in the first place. If the cause is something like water draining against the slab or unstable subgrade, we address that as part of the recommendation, sometimes with grading or drainage corrections and sometimes with a deeper structural solution, so a new void is less likely to form beneath the slab.

When the slab is a good candidate and the underlying cause is addressed, void filling can provide long-lasting support. The honest answer is that longevity depends on the soil and drainage beneath the slab. In active Piedmont clay that keeps swelling and shrinking, or where a downspout is still washing out the subgrade, an unaddressed cause can allow a new void to form over time. That is why we diagnose the cause first rather than just filling the cavity and leaving.

Cost depends on the size of the slab, how large the void is, the number of injection points needed, the fill material used, access to the area, the soil conditions, and whether any drainage or soil correction is recommended alongside the work. Because those factors vary from one slab to the next, we provide pricing after a no-pressure inspection rather than a flat number online. The inspection is how we give you an accurate, honest estimate for your specific situation.

Let's take the first step toward a healthy home.

A local specialist will inspect your foundation, walk you through the findings, and send a clear estimate. no cost, no pressure.