Not every crack in a slab needs the same treatment, and getting that distinction wrong in either direction causes problems — either paying for unnecessary structural investigation on cosmetic crazing, or filling over a genuine full-depth crack and building a floor finish on top of a problem that's still there.
A proper repair starts with establishing which kind of crack you actually have, then treats it accordingly.
Repairing surface crazing
This is a straightforward, low-risk repair once crazing is confirmed to be shallow and surface-only. It's largely a cosmetic and weatherproofing job rather than a structural one.
- Clean out the fine cracks and fill with an appropriate crack filler or sealant
- Seal the slab surface if a floor finish or coating is planned, to stop moisture tracking through the fine cracks
Investigating and repairing a full-depth crack
This is the step that separates a genuine repair from a cosmetic one. A full-depth crack filled and covered without investigation can continue moving under a floor finish, and by the time it shows again, it's under tiles or flooring that then need lifting.
- Confirm crack depth and whether it runs through the full slab thickness
- Assess whether the crack is static or still moving, which may require a monitoring period
- Investigate sub-base compaction and reinforcement position in the affected area where settlement is suspected
- Repair the crack appropriately once the cause is understood — options range from structural crack injection to localised slab replacement depending on severity
- Never simply fill and cover a full-depth crack without understanding why it's there
Preventing recurrence on future pours
Movement joints should be introduced at appropriate centres and at re-entrant corners on any future pour, so shrinkage stress has somewhere to relieve itself instead of cracking the slab randomly. Sub-base preparation should be checked and properly compacted in even layers before the pour, particularly on remedial work where the original preparation is the suspected cause. Curing should be managed properly — covering the slab or keeping it damp in hot or windy conditions reduces the risk of plastic shrinkage cracking.
How you know the repair has worked
Filled surface crazing should stay stable with no fresh cracking pattern developing nearby. For a full-depth repair, the key test is that the crack shows no further widening over a monitored period before any floor finish is committed on top of it — that's the point at which it's genuinely safe to build over.
Common questions
Can a full-depth crack be repaired without breaking out the whole slab?+
In many cases, yes — options such as structural crack injection or localised repair can address the crack without full replacement, depending on what the investigation finds. Full replacement is reserved for more severe or widespread failure.
How long should a full-depth crack be monitored before repairing?+
It depends on the suspected cause, but allowing time to see whether the crack is still moving, ideally through a change in ground conditions such as a wet spell, gives a more reliable picture than a quick one-off inspection.
Will the repaired slab be as strong as the original?+
A properly investigated and repaired crack, addressed at the correct depth and cause, can restore adequate performance for most uses, though the approach needs matching to what caused the original cracking rather than applied generically.
Get your slab repair quoted properly
We assess crack depth and cause before quoting, so you're not paying to fill a problem that's still moving underneath.