What matters is width, depth, pattern and whether anything is moving. A crack with a step in it, a crack that runs the full width of the slab and keeps widening, or a crack with movement you can feel underfoot is in a different category.
Harmless shrinkage cracking
Plastic shrinkage cracks form in the first hours while the concrete is still curing, usually because the surface dried faster than the body of the pour on a windy or hot day. They are short, shallow, often parallel and they do not go through the slab.
Drying shrinkage cracking follows in the first weeks as the remaining water leaves. It shows as fine map cracking or the occasional longer hairline. On a domestic floor that is going to be screeded, tiled or covered, this is a non event.
- Short shallow surface cracks appearing within a day of the pour
- Hairline map cracking during the first weeks
- Cracks appearing at a re entrant corner, for example beside a door opening
- Straight cracks that follow a construction joint
Cracks worth investigating
Width is the first test. Anything above about three millimetres in a domestic slab deserves a look. So does any crack where the two sides are at different heights, which means one part of the slab is being supported differently from the other and points at the sub base rather than the concrete.
Cracks that keep growing months after the pour, cracks that follow the line of a buried drain or a service trench, and cracks accompanied by hollow sounding areas when the floor is tapped all suggest settlement below the slab rather than shrinkage within it.
What causes the serious ones
Poor compaction of the sub base is the biggest single cause. Hardcore tipped and levelled but not compacted in layers will settle under load and take the slab with it. Backfilled trenches inside the slab footprint behave the same way if they are not compacted properly.
Missing or wrongly positioned reinforcement is next. Mesh that is meant to sit in the upper third of the slab and is instead lying on the damp proof membrane does nothing useful. Third is a slab poured too thin for the loads on it, and fourth is a mix that had water added on site to make it easier to place, which raises shrinkage sharply.
Repairing a cracked slab
Fine shrinkage cracks are simply filled before the covering goes down, usually with a resin or a flexible filler depending on the finish above. Where a screed is going on top, cracks are taped or filled and the screed carries the finish.
Wider stable cracks are cut out to a V, resin injected and stitched with steel bars set across the crack where movement needs restraining. Cracks caused by settlement under the slab are the ones that need the underlying cause addressed first, which may mean breaking out a section, compacting properly and repouring. Filling a settling crack achieves nothing except a straight line that reopens.
If you have concerns about a slab we have not poured, we are happy to look. We do our own concrete formwork, so we know what a well made slab should look like from underneath as well as on top.
Common questions
How wide is an acceptable crack in a domestic slab?
As a working guide, hairline cracks up to around half a millimetre are cosmetic, up to about three millimetres is usually tolerable in a covered domestic floor, and anything wider or with a step in it should be assessed.
Will a crack in the slab damage my tiles?
It can. A moving crack telegraphs through rigid finishes. An uncoupling membrane or a resin repair before tiling deals with a stable crack, but a moving one needs the cause fixed first.
Should the slab have had movement joints?
Domestic slabs in ordinary rooms often do not need them, but large pours, long thin areas and slabs with underfloor heating benefit from properly positioned joints so shrinkage happens where you decide rather than where it wants to.
How long before I know the cracking has stopped?
Most shrinkage movement is done within the first three months. Mark each end of a crack with a pencil line and date it, then check in six weeks. No growth means stable.
Can a cracked slab be repaired without breaking it out?
Frequently, yes. Resin injection and stitching handle a lot of cracks. Breaking out is reserved for cases where the ground under the slab is the problem.