Problems
Sunken concrete
A section of concrete has dropped below the rest.
Sunken concrete is concrete that has gone down because what was underneath it went down first. The slab is a passenger. In most cases the concrete is still perfectly serviceable — it is simply in the wrong place. Concrete can also move the other way, and a slab that has been pushed upward is a different problem needing a different response.
That distinction decides everything about the fix. Concrete that has failed needs replacing. Concrete that has descended may only need raising. Inside a garage that judgement carries an extra step, because a settled garage slab has to be separated from a foundation question first.
Why the ground goes down
The common causes are unglamorous. Fill placed during construction that was never fully compacted continues to settle for years. Water washes fine material out from under an edge and carries it away. A utility trench backfilled quickly consolidates. Soil that shrinks in dry weather pulls away from the slab above. Tree roots take up moisture and change the volume of the soil around them.
What these share is that they are ground processes with their own timelines. Some finish. Some continue as long as the cause continues. Which of those you have is the single most useful thing to establish.
Reading your own slab
Look at whether the slab is still in one piece. A panel that has dropped cleanly, still flat, still whole, is a good candidate for lifting. A panel that has cracked into several pieces and dropped unevenly is a different proposition.
Tap or walk it and listen. A hollow sound suggests space underneath. Look at where the water goes — a slab that has settled toward the house has created a drainage problem on top of the height problem.
Then look for the cause. Where is the nearest downspout discharging? Is there a trench line? Is there a tree? Contractors will ask; having thought about it first makes the conversation better.
What the options are
If the slab is intact and the cause is addressable, lifting it is usually less disruptive and less expensive than replacing it. If it has broken up, or the concrete is deteriorating in its own right, replacement moves ahead. If lifting is on the table, what a lifting quote is priced on is the void underneath, not the size of the slab.
If the ground is still actively moving, neither is the first job. The first job is finding out why.
Signs lifting is worth pricing
- The slab dropped as a unit and is still essentially whole.
- The concrete surface is in reasonable condition.
- You can point at a plausible, fixable cause.
- The drop is measured in inches rather than in whether the slab is still recognisable.
Signs replacement is more likely
- The slab has broken into several pieces.
- The concrete is crumbling, badly spalled or soft.
- Most of the area has gone down, not one part of it.
- It has been lifted before and has come back down.
What to ask a contractor
- What do you think caused this, and how confident are you?
- Is this slab a lifting candidate, and what makes you say so?
- What needs to change about drainage before or alongside the work?
- If you lift it and it settles again within the warranty, what happens?
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 a slab that has dropped 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 here is nearly meaningless — a slab that has dropped is not a surface problem, so judging it by how the concrete looks misses what actually moved.
- Functional is the usual reading: the slab is sound but is now at the wrong height, which changes where water goes and where people trip. That is a support problem with a support answer.
- Structural is the reading to take seriously when the drop is against the house rather than out in the open, when it is accelerating, or when something attached to the building has moved with it.
- The test that separates them is whether the concrete is intact. Sound concrete in the wrong place is a ground story; concrete that has broken up is its own story.
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.
- A photograph from a low angle along the slab's edge, which shows a drop far better than one taken standing up.
- The size of the step at the worst point, measured against something ordinary — a coin, a phone, a tape — held in frame.
- Where every downspout and outlet discharges within a few strides of the affected area, photographed during or just after rain.
- Whether the slab has been lifted or patched before, and roughly when. A slab that has come back down is a different conversation from one that has never been touched.
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 drop is beside the house and a door or window has become harder to operate since it appeared.
- The gap has widened noticeably within a single season rather than over years.
- The slab sounds hollow across a wide area and flexes underfoot when a vehicle crosses it.
- Water now runs toward the building where it previously ran away.
What not to assume
- That a dropped slab means bad concrete. The material is usually fine and replacing it puts new concrete on the same unfixed ground.
- That lifting is always cheaper in the end. It is often cheaper on the day, and it stays cheaper only if the reason the ground moved has been dealt with.
- That because it stopped moving, it has finished. Some ground processes pause seasonally and resume.
- That any contractor who recommends replacement is overselling. On a broken-up slab it is the honest answer.
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.
Lifting methods depend on the slab being sound enough to move as a unit.
Established practice This is how the work is normally done, and it is not seriously contested among people who do it. It usually reflects material behaviour or long-standing method rather than opinion.
Evidence considered
- Lifting works by raising the slab from beneath, which requires the slab to transmit that force across its area.
- A slab already broken into small pieces moves as pieces rather than as a slab.
- The method is well established where the precondition holds and is not seriously contested.
Still to establish
- Slab thickness — partly, on your own
- Crack activity — you can check this yourself
- Void depth and extent
- Soil movement — partly, on your own
Why we say this, and when it may not apply
It is the practical constraint that decides whether lifting is even on the table, and it explains why one contractor may rule it out where another proposes it.
What we had to assume
- That the extent of cracking can be established well enough to judge whether the slab acts as a unit.
- That there is material beneath capable of taking the lifting pressure.
When this may not apply
- Heavily fragmented slabs, where lifting can widen existing breaks rather than restore level.
- Very thin sections, which can crack under the lifting itself.
- Situations where the cause of settlement is still active and would resume afterwards.
What would change it
- Finding cracking more extensive than the surface suggests, once an edge is examined.
- Finding the slab thinner than assumed.
- Establishing that the ground is still moving rather than having settled and stopped.
Established practice does not mean this: It does not mean the practice is right for your property, that any particular contractor follows it, or that following it guarantees an outcome. Established practice describes what is normal, not what will happen at your address.
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 sunken concrete dangerous?
Usually it is an inconvenience and a trip hazard rather than a danger, though a step of an inch or more in a walkway is a genuine fall risk and worth addressing. Where sunken concrete sits against the house and now directs water toward it, the concern is the water rather than the concrete. Nothing on this page can tell you whether a specific slab is safe; that needs someone on site.
Will it keep sinking?
If the cause has finished, no. If it has not, yes. Marking the edge and photographing it over a couple of months is the cheapest way to find out, and it is worth doing before spending money either way.
Can I just fill the gap with something?
Filling a visible gap at the surface treats the appearance and leaves the void. It is a reasonable temporary measure at a trip hazard, but it is not a repair, and it can make it harder to see whether the movement is continuing.
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 →Voids under concrete
Space beneath a slab — often before anything has visibly dropped.
Read more →Concrete leveling
Raising settled slabs back toward their original height instead of tearing them out.
Read more →Leveling vs replacement
When lifting is the smart economy and when it is a deposit on a job you will still do.
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 →