Types of Cracks in Concrete and When to Call an Engineer
Most cracks in outdoor concrete are surface cracks from shrinkage or heat movement, and they can be repaired and recoloured. Cracks with a step between the two sides, cracks that keep growing and cracks next to sunken areas suggest movement, and those need a qualified engineer before anyone touches the surface. This guide helps you tell the difference from what you can see.
Send us a photo and the m²Why does concrete crack at all?
Concrete cracks because it is strong in compression but weak when pulled. As it dries after being laid it shrinks a little, and as it heats and cools through the seasons it moves; if that movement is held back anywhere, the slab relieves the strain by cracking.
So a cracked driveway is not automatically a failed driveway. Joints are cut precisely to control where these cracks happen, and when joints are missing or too far apart, cracks appear between them instead.
The question that matters is not whether your concrete has cracked, but why, and whether it is still moving.
What are hairline and crazing cracks?
Hairline cracks are very fine lines, often straight, that run across a slab without any height difference. Crazing is a network of fine, shallow cracks across the surface, a bit like the glaze on an old plate.
Both usually come from the way the surface dried or from surface stresses, and both are typically cosmetic. On a sunny terrace or pool surround they often become more visible as the surface fades and dirt settles in them.
These are the cracks that disappear best under a repair and two-coat recolour, as described on our concrete crack repair page.
What are shrinkage and thermal cracks?
Shrinkage and thermal cracks are wider than hairlines and often run straight across the narrow width of a driveway or path. They appear where the slab needed a joint and did not have one, or where a joint was placed too far away.
The edges usually stay level, and once the crack has formed it may open and close slightly with the seasons without getting progressively worse. That pattern is generally a surface matter that can be repaired.
Where these cracks run off the end of a joint or follow a broken one, the joint itself may be the underlying issue; see expansion joint repair.
What do settlement and structural cracks look like?
Settlement cracks happen when the ground or base under part of the slab moves, usually down. The tell-tale signs are a step across the crack, a slab that tilts or rocks, or a hollow sound when you tap the concrete.
Structural cracks are those that affect something carrying load, such as a slab over a garage, a retaining structure or a foundation. Their size, shape and direction tell an engineer a great deal, which is exactly why they need an engineer to read them.
How can you tell which type you have?
You can make a first, rough judgement from four observations: the width, the level of each side, the pattern and whether it is changing. None of this replaces a professional opinion, but it tells you who to call first.
| Observation | Usually points to | Who to call first |
|---|---|---|
| Fine, flat lines or a crazed network | Surface or drying cracks | A repair contractor |
| Straight crack across the width, sides level | Shrinkage or thermal movement | A repair contractor |
| Crack along or off the end of a joint | Joint problem | A repair contractor |
| Step between sides, tilting or rocking slab | Settlement or base movement | An engineer or surveyor |
| Crack widening over time or reopening after repair | Ongoing movement | An engineer or surveyor |
| Matching cracks in walls or steps nearby | Possible structural issue | An engineer or surveyor |
How do you check whether a crack is still moving?
The simplest way is to record it. Take a dated photo of the crack with a ruler or tape measure across it, and mark the ends of the crack discreetly so you can see whether it extends.
Repeat the photo after some time has passed and compare. If the width or length has changed, or the two sides have moved out of level, treat it as active and get professional advice before any repair.
If you are abroad, a property manager can take these photos for you. Sending the series with your enquiry helps us, or an engineer, give a much better answer.
Why does the type of crack matter before repair?
A surface repair works on a stable slab and fails on a moving one. If a moving crack is filled and painted over, it reopens, the new coating cracks along the same line, and the money spent on the surface is lost.
Worse, a neat coat of colour can hide the progress of a problem that should be monitored. That is why we will not quote a cosmetic repair for a crack that looks structural in the photos.
Our article on red flags before repairing concrete turns these signs into a short checklist, and when repair is not enough explains what usually happens next.
What happens once an engineer has seen the crack?
If the engineer confirms that the cracking is old and stable, or once any recommended work underneath has been done, the surface can be repaired like any other. At that stage, cracks are cleaned out, filled and recoloured with the rest of the area.
If the engineer recommends replacing the slab, that is building work outside our service. We would rather send you in the right direction than paint over the wrong problem.
For stamped surfaces that have been cleared for repair, see imprinted concrete repair.
Frequently asked questions
Are hairline cracks in a concrete driveway normal?
Fine hairline cracks are very common in outdoor concrete and usually come from shrinkage as the concrete dried or from heat movement. On their own they are generally cosmetic. Keep an eye on them: if they widen, lengthen or develop a step, get them checked.
When are concrete cracks serious?
Cracks are serious when one side sits higher than the other, when they keep getting wider, when the slab around them has sunk or rocks, or when similar cracks appear in nearby walls. Any of these points to movement and should be seen by a qualified engineer or surveyor.
What is the difference between structural and surface cracks?
Surface cracks affect only the top of the slab and do not change its level or strength in a meaningful way. Structural cracks relate to movement or failure in something that carries load or in the ground underneath. Only an engineer can confirm a structural diagnosis.
Can a contractor tell me if my crack is structural?
A repair contractor can tell you whether a crack looks like the surface cracking they repair, but should not give you a structural diagnosis. We are not engineers, so when a crack looks like movement, we recommend an engineer or surveyor rather than guessing.
Why is my driveway cracking across the middle?
A crack across the width of a driveway is often a shrinkage or thermal crack where the slab needed a joint. If the two sides are level and the crack is not growing, it is usually a surface repair. If it has a step, ask an engineer.
Should I fill a crack myself while I wait?
We do not give do-it-yourself instructions. Filling a crack before it has been assessed can hide whether it is moving. If you are worried, photograph it with a measure across it and get advice from a professional first, then decide on the repair.
Can tree roots crack a driveway?
Roots can lift or crack concrete, especially near the edges of a driveway or patio. Root damage often shows as raised sections or cracks radiating from one area. It is a cause that needs dealing with before a surface repair, and may need professional advice.
Do cracks in imprinted concrete mean the same thing?
Yes, the same rules apply. Stamped concrete cracks for the same reasons as plain concrete, though the pattern can make cracks harder to see. Look for steps across the crack and changes over time, just as you would on a plain slab.
What photos help assess a crack remotely?
A close-up with a ruler or tape measure across the crack, a photo along the crack at low angle to show any step, and a wide shot of the whole area. Repeat photos taken some time apart are especially useful for showing whether a crack is moving.
Can a repaired crack come back?
If the slab is stable and the repair is done properly, the crack should stay closed for as long as the materials perform according to their data sheets. If a repaired crack reopens, it is a strong sign of ongoing movement and should be investigated.
Do you carry out structural surveys?
No. Structural surveys, calculations and design are outside our service. We repair surface cracks on stable outdoor concrete and imprinted concrete, and we recommend a qualified engineer or surveyor whenever the photos suggest anything more serious than surface damage, such as steps, sinking or growing cracks.