Problems
Voids under concrete
The slab sounds hollow, or you suspect there is space underneath it.
A void is empty space between a slab and the ground it was poured on. It forms when soil is carried away or compresses, and the concrete — which is stiff enough to bridge a gap — stays where it is. Filling that space before the slab follows it down is its own operation, described in void filling beneath concrete.
That bridging is why voids are worth knowing about. Concrete spanning a gap is carrying loads it was not designed to carry unsupported, and it tends to crack at the point of least support. Finding a void before that happens is the cheap version of this problem. Most voids have water behind them, which is why correcting the drainage is often what stops a filled void from reforming a season later.
How to notice one
Sound is the most accessible clue. A slab with a void under it often sounds hollow when tapped or when walked on in hard shoes, distinctly different from the solid, dead sound of well-supported concrete.
Other signs: a slab edge that flexes slightly under load, water disappearing into a joint or crack rather than running across the surface, soil visibly washed away at an exposed edge, or a small gap where the slab meets a step or wall.
Where they tend to form
Downhill of a downspout that discharges near the slab. Along the edges of driveways and walkways where surface water runs. Over utility trenches. Around the perimeter of slabs where water sheets off a roof without gutters. Near anything that concentrates water at one point.
The pattern is consistent: voids form where water runs, and they form on the timescale that water works on.
What filling one involves
The same methods used for lifting a settled slab are used for filling: polyurethane foam or a cementitious slurry, injected through small holes. When the slab has not yet dropped, the goal is to fill the space and restore support rather than to raise anything — sometimes called void filling rather than lifting.
It is generally a smaller job than a lift, and it is the case where acting early genuinely saves money. It is still worth asking what caused the void, because filling one and leaving the water running produces another.
How a contractor confirms one
Sounding the slab is the first step, and it is the same tapping exercise a homeowner can do — systematically, across the whole area, marking where the note changes. It maps the extent rather than merely establishing that something is there.
Beyond that, a probe pushed into the ground at an exposed edge tells them how soft the material is and how far down it stays soft. On larger jobs some contractors drill a small test hole and look, which is the only way to be certain about depth before quoting a volume of material.
A quote given without any of that is a quote based on the surface appearance of a problem that is entirely below the surface. It may still be roughly right, but it is worth knowing which kind of number you have been given, and what happens to the price if the space turns out larger than assumed.
What to ask a contractor
- How did you confirm there is a void, and how big do you think it is?
- Is this void filling or an actual lift, and how does the price differ?
- Where is the water coming from that made this?
- What stops it forming again?
Before you ask anybody for a price
Four things worth settling first. None of them needs a contractor, and none of them needs your contact details — they are simply what separates a useful conversation about space underneath a slab from an expensive guess.
Cosmetic, functional or structural
We cannot tell you which of these applies to your property — nobody can from a distance. What follows is how the three differ, so you can look at your own with a clearer question in mind.
- Cosmetic does not really apply — a void is invisible by definition, and a slab above one can look completely sound right up until it does not.
- Functional is a void that has begun to show: a slab that flexes underfoot, sounds different when tapped, or has started to crack where it is no longer supported.
- Structural is a void beneath something carrying real load — a drive a vehicle crosses, a slab under an attached structure — where the failure mode is sudden rather than gradual.
- The distinction that matters is not how big the space is but what is above it and what will be asked of that surface.
What to record before requesting proposals
Contractors price what they can see. Arriving with these already noted is the difference between quotes you can compare and quotes you cannot.
- Where the slab sounds different when tapped, mapped roughly rather than precisely — a difference between two areas of the same slab is more informative than either alone.
- Any visible deflection as a vehicle crosses, filmed if you can, because a slab that moves under load is reporting on what is beneath it.
- Where water enters and where it plausibly leaves, since a void is usually the end of a route rather than a spot.
- Whether anything has been injected or filled beneath this slab before, and when.
Keep it in the Project Brief if you would rather not lose track of it — it prints to something you can hand to whoever comes out.
Signs that justify getting somebody out sooner
These are observations, not deadlines. Nothing here is a reason to rush a decision; they are the situations where waiting has a cost worth knowing about.
- The slab visibly moves or flexes when a vehicle or other weight crosses over it.
- A crack has appeared over the suspected area and is opening.
- The surface has begun to dish or dip rather than staying flat.
- The area carries vehicles, or is where people regularly stand or gather.
What not to assume
- That a sound-looking surface means sound support. The whole difficulty with voids is that the concrete above one gives very little away.
- That resurfacing helps. A treatment over unsupported concrete produces a better-looking slab in exactly the same structural situation, and hides the problem for longer.
- That filling the space ends it. Whatever moved the material out will keep doing so unless that route is dealt with.
- That nothing is happening because nothing has changed visibly this year. Material moves gradually and the surface reports late.
How we know what this page says
Each of these is something this page says. What follows is why we say it, what we had to assume, and what would change our mind — so you can weigh it rather than take it on trust. How we grade what we publish.
A hollow sound when concrete is tapped suggests a void but does not establish one.
Site dependent There is no general answer worth giving. The right conclusion turns on conditions at your property — soil, drainage, what is under the slab, what has been done before — and those differ enough that a general rule would be misleading.
Evidence considered
- Sounding is a real technique and a change in tone does indicate a change in what is beneath.
- A thin section over sound base, delamination within the concrete, and an actual void can all produce a similar change.
- Confirming which requires probing or drilling.
Still to establish
- Void depth and extent
- Slab thickness — partly, on your own
- Whether damage is surface or full depth — partly, on your own
Why we say this, and when it may not apply
It is the one test homeowners can do themselves and it gets reported as conclusive. It is genuinely useful and genuinely not conclusive, and both halves matter.
What we had to assume
- That the comparison is against an adjacent area of the same slab rather than against expectation.
- That the surface is dry and clear enough for the difference to be audible.
When this may not apply
- Over a large area of consistent tone, there is nothing to compare against and the test says little.
- Where the surface is already delaminating, the tone reflects the surface rather than the base.
What would change it
- Probing or a test hole establishing whether there is a gap and how deep.
- Establishing the thickness at that point.
- Whether the same tone appears across the whole slab or only in one area.
Site dependent does not mean this: It does not mean nothing can be known, and it is not a way of avoiding the question. It means the information that settles it is at your property rather than in an article.
Common questions
Is a void under my driveway urgent?
Not usually urgent, but worth acting on before the slab cracks into it, particularly under a driveway that carries vehicle loads. Unsupported concrete under load is the situation that turns a void into a crack and a crack into a settled panel.
Can I fill a void myself?
Pouring material into a visible gap at the edge does not fill a void; it fills the part you can see. Reaching the actual space requires injection through the slab under controlled pressure. Filling the edge can also disguise the problem and make it harder to monitor.
Work it out, or judge the quote
Related guides
Concrete settlement
The process behind sunken slabs: why ground loses volume and what makes it stop.
Read more →Sunken concrete
A slab that now sits lower than it should — almost always a ground problem, not a concrete problem.
Read more →Concrete leveling
Raising settled slabs back toward their original height instead of tearing them out.
Read more →Poor drainage around concrete
Where water goes decides how long concrete lasts — and it is the most commonly skipped part of a quote.
Read more →Polyjacking (foam injection)
Polyurethane foam injection: light, fast-curing, small holes — and more expensive per unit of material.
Read more →