Void filling beneath concrete

If the slab is still at the height it should be, filling the space underneath it is the repair — not lifting. Lifting is what you ask for when a slab has dropped and has to come back up. Filling is what you ask for when it has not dropped, and you would like to keep it that way.

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. The two materials used to do it behave differently in wet ground: polyjacking against mudjacking.

If the slab has already cracked, filling the void underneath it does not close those cracks — support and surface are separate repairs, and an estimate can cover either or both. Where a crack has stopped moving, crack filling is the surface repair; where it is still moving, crack sealing is, because it stays flexible.

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

What moves the number

No figures. This site publishes no prices because it has no sourced, dated pricing data, and a national average would be wrong on most streets. These are the variables to compare estimates against instead.

• How much space is actually under there. It is estimated before the work and known only during it, which is why what happens to the price if more material is needed belongs in the conversation before anybody starts.
• Whether the scope is filling only, or filling and lifting. They are separate outcomes and they are frequently written as one line.
• The number of injection points, which follows from the extent of the hollow area rather than from the surface area of the slab.
• Which material is specified. Polyurethane and cementitious grout behave differently, price differently per unit, and suit different ground.
• Access for the equipment, and how far material has to be run from wherever it can be parked.
• Whether the water path that created the void is inside the scope or excluded from it.
• How the holes are patched and to what standard, which varies between contractors and is rarely described in advance.

Evidence worth having before you ask for estimates

Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.

• A photograph of the chalk outline you get from tapping, so the extent of the hollow area is on record before anybody quotes and can be compared with what an estimate assumes.
• Photographs of where roof water and surface water reach the slab, and where they go from there. This is the half of the job most often left out of a price.
• A photograph of any cracking inside the hollow area, because a slab that has already cracked is a different conversation from one that has not.
• A note of whether anything has been injected under this slab before, and roughly when. A void that has come back is evidence about the cause rather than about the previous repair.

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.

Who tends to do this work

• A concrete leveling specialist
• A drainage contractor or landscaper who does grading

Two trades are listed because this repair has two halves that are rarely done by the same person. The injection is a leveling specialist's work — the same equipment and the same judgement as lifting, with the pressure deliberately held below the point where the slab starts to move. What created the void is almost always a water path, and redirecting water is a drainage contractor's or a landscaper's work. An estimate that covers the first and is silent on the second has priced half the problem.

ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.