Where this starts
Tile is strong in compression and brittle in bending. It cracks when the thing beneath it moves, deflects, or fails to support it — so the useful question is never about the tile, it is about what is underneath and what changed.
THE LINE IS THE DIAGNOSIS. Get down and trace the crack. A crack that runs on through the grout and continues into the next tile in a straight line is a crack in the SUBSTRATE that has propagated upward — the tiles are following a line that already existed or has just formed below them. A crack confined to one tile, not continuing across its grout joints, belongs to that tile.
DEFLECTION IS THE COMMONEST SUBSTRATE CAUSE on a timber floor. Tile will not tolerate the bending a floor can happily survive structurally: a floor well inside every strength and deflection limit can still flex enough to crack a rigid finish over it. That is why tiling over joists calls for a stiffer floor than the building regulations alone require, and why a decoupling membrane exists at all.
MOVEMENT JOINTS ARE THE SECOND, and their absence is invisible until it is not. A tiled field expands and contracts — with temperature, with underfloor heating cycling, with a slab still shrinking — and if the field is large, or runs through a doorway, or meets a wall with no perimeter gap, that movement has nowhere to go and the weakest tile takes it. Cracks that appear in a line near a wall or across a threshold are usually this.
THE THING THAT DECIDES THE REPAIR is whether the cause is still active. Replacing a cracked tile over a crack that is still moving gives you a new cracked tile, usually within a year. A dated photograph and a second look in a few months is worth more than an immediate repair.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A crack in the substrate propagating upward
Most commonA straight crack running across several tiles and through the grout between them, often continuing under a threshold or along a line in the slab.
How to check: Whether the crack continues past the tile's own edges. A line that ignores the tile joints was not started by the tile.
A floor that deflects too much for a rigid finish
Most commonCracking on a timber floor, often in a line along a joist or between them, frequently with grout cracking and hollow-sounding tiles nearby.
How to check: Whether the floor is timber and how it feels underfoot. Tile tolerates far less movement than a floor is allowed to have.
No movement joints, or the field too large
CommonCracks in a line near a wall, at a doorway, or across a long run, sometimes with tiles tented up at the crack rather than dropped.
How to check: Whether there is a gap at the perimeter under the skirting, and whether the field is broken at doorways.
A void under the tile
CommonA single cracked tile that sounds hollow when tapped, with adhesive coverage visible only in ridges or dabs when it is lifted.
How to check: Tap around the crack. A hollow under a tile leaves it unsupported and it breaks under a point load.
Underfloor heating cycling
CommonCracking that appeared after heating was installed or after the first heating season, sometimes following a pipe or cable run.
How to check: Whether there is heating in the floor and whether it was commissioned gradually. A screed brought up to temperature too fast moves sharply.
An impact
Less commonA single tile with a radial or star-shaped crack from one point, with sound neighbours and no line continuing anywhere.
How to check: Whether the crack radiates from a point. Something was dropped, and it is a replacement rather than an investigation.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the whole first pass. A line crossing tiles is the substrate; a crack stopping at the joint is the tile, and the two lead to completely different work.
- 2
A hollow under a tile leaves it unsupported, which both explains a single crack and warns that the neighbours are next.
- 3
Tile tolerates far less deflection than a floor is permitted to have, so a floor that is structurally fine can still be the reason the finish cracks.
- 4
A tiled field with nowhere to expand puts that movement through the weakest tile, and cracks near walls or at thresholds are usually this.
- 5
Replacing a tile over a crack that is still moving produces a new cracked tile, and only a dated record shows whether anything is still happening.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a straight crack crosses several tiles, since the substrate has moved and the tiling is reporting it rather than causing it.
- Where the floor is timber and feels lively underfoot, because stiffening it is structural work and retiling over it is not a repair.
- Where cracking followed the installation or first use of underfloor heating, which is a commissioning and movement-joint question.
- Where tiles have tented or lifted at a crack rather than dropped, which means the field is in compression and is a movement-joint failure.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.