Slab Cracking: What's Normal and What Isn't
A crack in a concrete floor can look alarming, but most are harmless. The trick is telling a normal shrinkage crack from one that is telling you something more serious is going on underneath.
Why slabs crack in the first place
Concrete shrinks slightly as it cures, and unless that shrinkage is controlled, it shows up as cracking. A few different types are worth knowing:
- Plastic cracking happens before the concrete has even set, usually when water evaporates from the surface too quickly in hot or windy conditions.
- Shrinkage cracking develops later, from a wet or weak mix, not enough joints, poorly timed saw cuts, poor curing, or the slab being restrained so it cannot shrink evenly. In a new floor, drying shrinkage can keep slowly widening cracks for up to two years.
- Structural cracking points to overloading - the ground under the slab giving way from poor compaction, the slab not being designed for the loads placed on it, or rough handling of precast elements.
- Reinforcement corrosion can crack concrete from the inside out. Water reaches the steel mesh, it rusts, and rust takes up roughly three times the volume of the original steel. Cracks running parallel to, and directly above, a reinforcing bar can be a sign this has started.
What counts as "normal"
Fine hairline cracks are common in concrete and usually need no action. Since the Canterbury earthquakes, ground-floor slabs have also had to be reinforced with grade 500E ductile mesh under the Building Code, which helps keep shrinkage cracking fine and controlled rather than wide and random.
As a simple guide, NZS 3101 sets out acceptable crack widths depending on where the crack is:
- Around 0.3 mm for concrete in the ground outside the building.
- Around 0.4 mm for a ground floor slab protected by a damp-proof course, or for other concrete fully enclosed within the building.
Cracks finer than this, that are not growing and are not letting water through, are generally considered acceptable - even if they are not pretty to look at.
Signs it might not be normal
Treat these as a prompt to get a proper look, rather than something to ignore:
- The crack is still visibly widening or lengthening over days or weeks.
- It runs parallel to a reinforcing bar near the surface, suggesting the steel underneath may be corroding.
- Reinforcing has visibly fractured, or the crack goes right through the slab.
- Water is getting through the crack.
- The crack appeared suddenly after an earthquake, an impact, or other unusual event, rather than developing gradually.
If a crack is doing any of these things, monitor it properly - tape across the crack, a feeler gauge checked over a couple of weeks, or a scratch gauge can show whether movement is ongoing or has stopped. If you are not sure what you are looking at, get a structural engineer to assess it before deciding on a repair.
How joints help keep cracking under control
Builders deliberately plan for shrinkage with construction and control joints, so cracking happens along a tidy, predictable line instead of randomly across the floor. Under NZS 3604, an unjointed reinforced slab is limited to 24 m in any direction, and control joints are generally spaced no more than 6 m apart and placed at major changes in the slab's shape. Getting this layout right at the pour stage is one of the simplest ways to avoid random cracking later.
What to do about an existing crack
Fine, stable, hairline cracks in a dry area usually just need monitoring. For anything wider, moving, letting in water, or affecting a structural element, cement or epoxy-based repairs may help depending on the cause - but a structural crack should be assessed by an engineer before it is simply filled and painted over, since sealing a crack does not restore any lost strength.
If you have a crack in a slab, foundation or floor and are not sure which category it falls into, get in touch with Fil Engineers for a proper assessment.
Sources
This article is general information only and is not a substitute for a site-specific engineering assessment. Every crack and every building is different.
