Void filling is the process of pumping a grout or polyurethane material under a concrete slab to fill the empty space left when the soil shrinks and pulls away from the foundation. In Central Texas it typically runs $8–$25 per square foot of affected area, depending on void size and access.
Quick Facts
| Cost | $8–$25 per sq ft of affected area (typical Central Texas range) |
| Timeline | Most residential jobs finish in 1 day |
| Best for | Slabs with hollow spots or soil separation, not major structural settlement |
| Warranty | Warranty coverage depends on the repair type, site conditions, and the terms of the signed agreement |
Why Voids Form Under a Slab Here
Round Rock sits on Taylor Black Clay, part of the Blackland Prairie. This clay swells when it’s wet and shrinks when it dries. After a dry summer, the soil under a slab can pull down and away from the concrete, leaving a gap. The slab is now bridging over open air instead of resting on soil.
That gap is the void. Left alone, the unsupported concrete can crack, and settled areas can keep moving with the next wet-dry cycle. Filling the void puts support back under the slab so it isn’t spanning empty space.
Void filling addresses that missing support. It is not the same as lifting a badly settled foundation with piers, and it won’t fix a slab that has dropped several inches. When there’s real structural settlement, the right answer may be piers, and we’ll tell you that plainly based on the elevations.
Signs You May Have a Void
You usually can’t see a void directly, but the slab tells on itself:
- Floors that feel hollow or “drummy” when you walk or tap on them
- A section of tile or flooring that flexes or sounds different from the rest
- New cracks in a slab that was fine last year
- Gaps opening between the slab edge and the surrounding soil
- Exterior concrete (patios, walkways) that has started to rock or settle
These signs often show up after a dry stretch followed by heavy rain, which is the normal Central Texas pattern.
How Void Filling Works
Here’s what you’ll see and hear on site:
- We look first. We check the slab and the settled areas to understand what is actually happening. If the movement is structural, void filling alone is the wrong tool, and we’ll say so.
- Small ports are drilled. We drill a series of dime-sized holes through the slab into the void below.
- Fill material is pumped in. A grout slurry or expanding polyurethane is pumped through the ports until the void is filled and the material reaches refusal — meaning it stops taking more.
- Ports are patched. The holes are filled and finished flush with the existing slab.
- We talk through the rest. Long-term performance depends on water management, so we’ll cover drainage, gutters, and grading that affect the soil under your slab.
Void Filling Cost
Pricing depends on the size of the void, the type of fill material, and how easy the area is to reach. These are typical Central Texas ranges, not a flat quote.
| Variable | What moves the price |
|---|---|
| Affected square footage | Larger voids need more material and time |
| Fill material | Cement grout is lower cost; polyurethane costs more but is lighter and cures fast |
| Void depth / volume | A deep or wide gap takes more fill to reach refusal |
| Access | Slabs under finished flooring, cabinets, or tight spaces cost more to reach |
| Severity | A simple hollow spot is far cheaper than a slab that also needs piers |
Typical residential range: roughly $8–$25 per square foot of affected area. A small isolated void may land near the low end; a large area with high-density polyurethane sits higher. We give you a clear, written estimate before any work starts.
Void Filling vs. Piers
These solve different problems, and honesty here saves you money.
- Void filling is the right call when the slab is still at a reasonable elevation but has lost soil contact underneath. It restores support without a major lift.
- Piers are the better call when the foundation has actually settled and needs to be stabilized and lifted back toward its original position. Piers are driven to refusal to transfer weight to deeper, stable soil.
Sometimes a job needs both: piers to stabilize the settled areas, and fill to close remaining voids. We’ll base the recommendation on the elevations, not on what’s more expensive.
Local Proof
Financing Options
Foundation work is an unplanned expense, and we understand that. Financing is available, subject to credit approval, and we accept all major credit cards. We also offer discounts for military, first responders, firefighters, law enforcement, nurses, retired teachers, and seniors. See our financing page for details.
FAQ
Is void filling the same as foundation repair?
It’s one part of it. Void filling restores soil support under a slab. If the foundation has also settled, that’s a separate issue that may call for piers. We evaluate both before recommending anything.
Will filling the void lift my slab back up?
Not meaningfully. Void filling is designed to support the slab and close the gap, not to lift a settled foundation. For an actual lift, piers are the appropriate method.
How long does void filling take?
Most residential jobs are done in a single day. Polyurethane cures quickly, so you can usually walk on the area soon after.
How do I know if I actually have a void?
Hollow-sounding floors, new cracks, and gaps at the slab edge are common signs. The only way to confirm it is to have us take a look and check what’s happening under the slab.
Does void filling stop foundation problems for good?
No repair stops all future movement. Void filling helps reduce the risk of continued settlement in the affected area, and pairing it with good drainage and water management supports long-term performance.
Do you offer free estimates?
Yes. Estimates are free for homeowners and commercial properties. We’ll give you a plain-English explanation of what we find and clear repair options.
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Serving Round Rock and the greater Austin area — Cedar Park, Georgetown, Leander, Pflugerville, Hutto, Taylor, and surrounding Williamson County.
Related: Foundation Piers · Slab Foundation Repair · Drainage & Water Management
Ready to find out what’s under your slab? Schedule an inspection or request an estimate — call (512) 746-7223 or send us your questions.