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.
That distinction decides everything about the fix. Concrete that has failed needs replacing. Concrete that has descended may only need raising.
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 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?
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 →