Problems
Cracked concrete
There is a crack and you do not know whether to worry.
Concrete cracks. It shrinks as it cures, expands and contracts with temperature, and has very little tensile strength, so it relieves stress by cracking. Control joints exist precisely because cracking is expected: they are deliberate weak lines that decide where it happens. What separates an ordinary crack from one worth spending money on is whether it is still changing, which is the subject of cracks that keep growing.
So the question is never whether there is a crack. It is what the crack is doing. Width, direction and whether it is still moving point at different causes, and each pattern has its own page in the concrete problem library.
The four things worth checking
Width tells you least on its own, though very wide cracks are worth more attention. Vertical offset — whether one side sits higher than the other — tells you a great deal, because it means the two sides are no longer supported the same way. That is movement, not cracking.
Growth matters more than size. A stable half-inch crack and a growing quarter-inch crack are different problems, and the growing one is the more interesting. Water matters because water in a crack moves the material underneath it and, in freezing climates, expands inside it.
- Is one side higher than the other?
- Is it getting longer or wider?
- Does water run into it, or across it?
- Is there settlement visible nearby?
Patterns and what they usually mean
A crack that follows a control joint is the joint doing its job. A single straight crack across a panel is very common and usually a shrinkage or temperature response. A fine map or spiderweb pattern across the surface — sometimes called crazing — is typically a finishing and curing effect confined to the top layer.
Branching cracks, cracks that pass through several panels along a consistent line, and broken-off corners are the patterns that more often indicate something underneath. None of these is a diagnosis; they are reasons to look further.
How to monitor properly
Draw a pencil line across the crack at each end and date it. Photograph the crack next to something for scale, from the same position. Repeat in a month or two, and particularly after heavy rain.
This costs nothing and answers the one question that changes the decision. Contractors take it seriously when a homeowner arrives with dated photographs.
What to ask a contractor
- Do you think this crack is stable or still moving, and what are you basing that on?
- Is a repair here worth doing, or is it treating a symptom?
- What would you fill it with, and how will it look afterwards?
- Is there anything about drainage you would change first?
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 crack you are trying to read 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 covers the fine surface lines that appear in almost all concrete as it cures and ages. They do not go through the slab, the two sides sit level, and they are unchanged year to year.
- Functional is a crack wide enough to catch a shoe or a wheel, to let water through to the ground beneath, or to have opened enough that debris now sits in it.
- Structural is where the two sides no longer line up — one side higher than the other, or the crack passing through something load-bearing rather than through flatwork.
- Height difference across the crack is the single most informative thing to check, and almost nobody checks it. Run a finger across: if you can feel a step, the ground moved, not just the concrete.
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.
- One photograph with something for scale laid across the crack, and one taken along its length so its path is visible.
- Whether the two sides are level or stepped, noted in words as well as shown in the picture.
- The date. A crack photographed twice, months apart, answers questions a single image never can.
- Whether it follows a straight scored line in the surface or wanders across it — those are different events with different causes.
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 two sides of the crack sit at different heights and that difference is growing.
- The crack runs from flatwork into something structural, or continues up a wall.
- Water disappears into it and the ground alongside stays wet long after rain.
- It has widened enough since you first noticed it that you no longer need a photograph to be sure.
What not to assume
- That every crack is a defect. Concrete is expected to crack; joints exist to decide where.
- That filling it fixes anything beyond appearance and water entry. A filler does not restore what the crack was relieving.
- That a wide crack is worse than a narrow one. A narrow crack with a height difference is telling you more than a wide flat one.
- That a crack which closes up seasonally has resolved. Movement that reverses is still movement.
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 crack does not on its own establish that anything has settled.
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
- Concrete shrinks as it cures and cracks in response, independently of what is beneath it.
- Restrained movement, thermal cycling and loading each produce cracking without settlement.
- Settlement cracking tends to be accompanied by vertical displacement, which shrinkage cracking usually is not.
Still to establish
- Crack activity — you can check this yourself
- Vertical displacement — you can check this yourself
- Joint condition — you can check this yourself
- Age and history — you can check this yourself
Why we say this, and when it may not apply
Cracking has several ordinary causes that have nothing to do with ground movement, and treating every crack as settlement leads to repairs aimed at the wrong thing.
What we had to assume
- That the crack can be seen well enough to tell whether the two sides are at different heights.
- That the observation is of the crack itself rather than of a joint doing its job.
When this may not apply
- Where cracking is accompanied by clear vertical displacement, movement is a stronger possibility.
- Where cracks have widened measurably over time, something is ongoing.
- Where cracking appears in a pattern that follows a slope or a low point rather than the slab geometry.
What would change it
- A dated measurement showing the crack has changed width or displacement over time.
- A straightedge reading showing the two sides sit at different heights.
- Finding the crack ignores the control joints rather than running along them.
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.
Common questions
How wide does a crack have to be before it matters?
There is no single threshold that applies across every situation, and any page that gives you one is oversimplifying. Behaviour matters more than width: a narrow crack that is growing, or that has a height difference across it, is more informative than a wide one that has been static for a decade.
Should I fill cracks myself?
Filling a crack to keep water out is a reasonable homeowner job on stable cracks in flatwork, and in freezing climates it is worth doing. What it does not do is address movement — and filling a crack makes it harder to tell whether it is still moving, so photograph and mark it first.
Do cracks mean the concrete was poured badly?
Not necessarily. Some cracking is inherent to the material. Cracking that appears very early, runs in a pattern unrelated to the joints, or is accompanied by surface problems can point at mix, placement, curing or jointing decisions — but plenty of well-executed concrete cracks too.
Work it out, or judge the quote
Related guides
Common concrete problems
Start here: match what you are seeing to what it usually means and what it usually needs.
Read more →Concrete settlement
The process behind sunken slabs: why ground loses volume and what makes it stop.
Read more →Spalling concrete
Surface breaking away in chunks, often with aggregate showing — usually water, freezing and salt.
Read more →Repair or replace?
The central question: when repair holds, when it is money spent twice, and how to tell.
Read more →Sealing and coatings
Sealers, epoxy and garage-floor systems: what they protect against and what they do not.
Read more →