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.
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.
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 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?
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 →