Red Flags: When Cracked Concrete Needs an Engineer, Not a Repair
Stop and call an engineer or surveyor before any repair if you see a step across a crack, a section that has sunk or rocks, a crack that keeps growing, rust stains or exposed steel, or matching cracks in nearby walls. These are signs of movement or deeper damage, and a surface repair will not fix them. If none apply, your cracks are probably a surface matter that can be repaired and recoloured.
Send us a photo and the m²Is there a step across the crack?
Run your hand or your shoe across the crack. If one side is higher than the other, the slab has moved, and that is a red flag.
A flat crack, even a wide one, can be a surface matter. A stepped crack, even a narrow one, should be seen by a qualified engineer or surveyor before anyone repairs it.
Has any part of the slab sunk, tilted or started to rock?
Look for water collecting in places where it used to drain, a driveway edge that has dropped away from a wall, or a section that moves when a car drives over it. Tapping with a stick can also reveal hollow-sounding areas.
These signs mean the ground or base under the slab has shifted or washed out. Our guide on when repair is not enough explains what usually needs to happen next.
Is the crack getting bigger?
A crack that widens or lengthens over time is active. The simplest way to check is a dated photo with a tape measure across it, repeated after a while.
A crack that was repaired properly and has reopened along the same line is the same red flag. It shows the movement has continued since the repair.
Can you see rust or steel in the concrete?
Brown rust stains coming from cracks, or steel bars visible where the concrete has broken away, suggest that the reinforcement is corroding. As steel rusts it expands, which pushes the concrete apart from inside.
This is especially important on slabs over garages, on ramps and on anything that carries load. A qualified professional should decide what needs doing before any surface repair.
Are there matching cracks in walls or steps?
If the crack in your driveway or terrace lines up with cracks in a boundary wall, a retaining wall, steps or the house, the cause may be wider than the slab. That is a matter for an engineer or surveyor.
The same applies to doors and gates that have started to stick or drag across the paving, which can be another sign that something has moved.
What if none of these red flags apply?
Then your cracks are probably surface cracks: hairlines, crazing or flat shrinkage cracks in a stable slab. Those are exactly what our crack repair service deals with, together with small potholes and broken joints.
The guide to crack types explains each kind in more detail if you want to be sure.
| Sign | Red flag? |
|---|---|
| Fine, flat hairline cracks | No, usually surface |
| Wide but level crack, not growing | Usually no |
| Step across the crack | Yes |
| Sunken, rocking or hollow slab | Yes |
| Crack growing or reopening | Yes |
| Rust stains or visible steel | Yes |
| Matching cracks in walls or steps | Yes |
What red flags should you watch for in a repair quote?
The concrete is not the only place to look. A quote that never mentions cleaning, that proposes painting over cracks without repairing them, or that dismisses a stepped crack as cosmetic is a warning in its own right.
- No mention of cleaning or surface preparation.
- Cracks to be painted over rather than repaired.
- No question about whether cracks are growing.
- Durability promises with no reference to product data sheets.
- Pressure to start immediately without seeing the whole area.
Our article on what can go wrong with concrete repair explains why each of these matters.
What should you send us if you are unsure?
Send photos of the cracks, including one taken low along the crack to show any step, a wide shot of the area and anything unusual nearby. We will tell you honestly whether it looks like a repair job or something for an engineer first.
Frequently asked questions
What is the biggest red flag in a cracked driveway?
A height difference across the crack, where one side sits higher than the other. It shows that part of the slab has moved. Whatever the width, a stepped crack should be assessed by a qualified engineer or surveyor before any surface repair is considered or quoted.
Are wide cracks always serious?
Not always. A wide crack with level sides that is not growing can still be a surface matter. Width is less telling than a step across the crack, growth over time, or sinking around it. Those signs matter more than how wide the gap looks on its own.
How do I check if a crack is growing?
Take a dated photo with a tape measure or ruler across the crack and mark its ends discreetly. Repeat the photo after some time and compare. If the width or length has changed, or the sides have moved out of level, treat the crack as active and get advice.
What does a hollow sound under concrete mean?
A hollow sound when you tap the slab can mean there is a void beneath it, often because the base has settled or washed out. It is a sign that the support has changed and should be investigated before any repair is done to the surface above it.
Why are there rust stains coming from my concrete?
Rust stains usually mean the steel reinforcement inside the concrete is corroding. Corroding steel expands and pushes the concrete apart, which can cause cracking and spalling. A qualified professional should assess it, especially on slabs over garages or ramps and other load-bearing parts.
Should I worry about cracks in my wall and driveway together?
Yes, cracks that line up across a driveway and a wall, steps or the house can indicate a wider movement issue. That needs an engineer or surveyor to assess. A surface repair on the driveway would not address the cause and could hide useful evidence.
Can you repair a crack that has a step in it?
Not until the cause has been assessed. A step means the slab has moved, and filling and painting it would only hide the problem for a while. Once an engineer has confirmed the slab is stable or the cause has been dealt with, surface repair can be considered.
Are hairline cracks a red flag?
Fine hairline cracks on their own are usually not a red flag. They commonly come from shrinkage or heat movement and are a surface matter. They become a concern only if they start to widen, develop a step, or appear together with sinking or cracks in walls.
What red flags should I look for in a contractor?
Watch for quotes that skip cleaning, propose painting over cracks without repairing them, never ask whether cracks are growing, or promise durability with no reference to product data sheets. A contractor who ignores signs of movement is not saving you money in the long run.
Do you provide engineering assessments?
No. Structural assessment and design are outside our service. We look at your photos and tell you honestly whether the damage looks like surface repair work. When we see red flags, we recommend you speak to a qualified engineer or surveyor before we quote anything.
Can my property manager check for red flags for me?
Yes. Ask them to photograph the cracks with a tape measure across them, take a low-angle photo along each crack to show any step, and note any sunken, rocking or hollow-sounding areas. Repeat photos after a few weeks help show whether anything is moving.