Repair methods
Void filling beneath concrete
My slab sounds hollow but it has not really dropped. What do I do?
This is what void filling is for. A slab spanning an unsupported gap is being asked to work as a bridge rather than as a slab on ground, and that is how cracks form under load even where the elevation still looks right.
Filling the void restores support without necessarily changing the height, which makes it a preventive repair rather than a corrective one.
What the method is
Material — commonly polyurethane foam or a cementitious grout — is injected through small holes to occupy the space between the underside of the slab and the ground.
The intent is contact and support rather than lift. Injection is controlled to fill without generating enough pressure to raise the slab, which is a different technique from lifting even where the material is the same.
It frequently accompanies lifting, because a settled slab usually has a void beneath it, but the two goals are distinct.
What it does not do
- It does not correct elevation. A slab that has settled stays where it is unless lifting is also intended.
- It does not repair existing cracks, though it can reduce the loading that produced them.
- It does not rebuild a base or improve soil.
- It does not stop the water that created the void, so recurrence is possible if drainage is unchanged.
What you can safely check yourself
- Sound the slab by tapping and chalk the boundary where the note changes. That map is the extent of your suspected void.
- Note whether the slab flexes or moves underfoot or under a vehicle, which suggests a significant unsupported span.
- Note where water is entering the base — a joint, a crack, an edge, a downspout.
When professional evaluation matters more
- The slab flexes noticeably under load, which indicates a substantial unsupported area
- Cracking is developing in the hollow-sounding area
- The void has recurred after previous filling, pointing at an active water path
Questions to ask whoever comes out
- How are you determining the extent of the void?
- Are you filling only, or also lifting? Which do I actually need here?
- How will you avoid lifting the slab if lifting is not wanted?
- What is creating the void, and is addressing that in your scope?
- How will you confirm the void is filled rather than just that material went in?
Where estimates for this commonly differ in scope
These are the places two prices for apparently the same work stop being comparable.
- Filling and lifting are separate goals frequently sold as one line item.
- Extent is estimated rather than known, so ask what happens to the price if more material is needed.
- The water path that created the void is usually outside the scope.
Usually worth planning a repair
Conditions in this band do not usually become emergencies, but they rarely improve on their own and repair costs tend to rise with the affected area.
Who tends to do this work
- A concrete leveling specialist
- A drainage contractor or landscaper who does grading
ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.
Common questions
Why fill a void if the slab has not sunk?
Because an unsupported slab is carrying load as a bridge rather than resting on ground, and that is how cracks appear in concrete whose elevation still looks fine. Filling the void before the slab cracks is considerably cheaper than repairing it afterwards.
How do I know there is a void?
Tapping and listening is the practical homeowner test — a hollow note where a solid one should be. Chalking the boundary of the hollow area gives you an extent map. It is genuinely indicative rather than conclusive, since sounding tells you where the change is but not how large the gap is.
Will filling the void stop it happening again?
Only if whatever made the void has been dealt with. Voids under slabs are usually created by water moving fine material out of the base, and that process continues unless the water is redirected. Filling without addressing drainage is treating the consequence.
Where to go next
Worth comparing
What drives the cost
Methods that may apply
Want a straight answer about void filling?
Both decision tools are free and need no contact details. If you would rather have somebody local look at it, you can ask us to review whether we can match you — and we will say plainly if we cannot.