Why do my floor tiles keep cracking?

Reading the crack pattern to tell a moving floor from a void under the tile from a missing movement joint — and why replacing the tile alone never works.

Difficulty: intermediate
Time: 45 min
Read: 5 min

Short answer

Tiles are strong in compression and brittle in bending, so a crack means the tile was bending. The three causes are a floor that deflects, a void under the tile, and a missing movement joint. Where the cracks are tells you which: cracks in one spot mean a void, cracks in lines across the floor mean deflection, cracks at the edges and against fixed objects mean movement.

Replacing a cracked floor tile is a one-hour job that gets done three times before anyone asks why. The tile did not fail — something bent it, and until that is found the replacement will crack too. The good news is that the pattern of cracking is genuinely diagnostic.

What you'll need

a long straight edge
a torch
something to tap with — a coin or a knuckle
access to the floor structure below, if there is any (optional)

Step by step

Map every crack before doing anything.Photograph the floor and mark the cracks. Position and pattern is the whole diagnosis, and it is lost once you start lifting tiles.
Tap around each cracked tile.A hollow ring around the crack means a void under it: the tile was unsupported, something landed on it, and it broke over the gap. If the surrounding tiles ring solid, this is a local bedding fault.
Look for cracks running in a line across several tiles.A crack that continues from tile to tile in a straight line is not about the tiles. It follows something below — a joist, a joint in the boards, a crack in the screed, or a change of substrate.
Walk the floor and feel for deflection.Bounce on it near the middle of the span. A timber floor that visibly moves is deflecting more than a rigid tile can follow. This is the commonest cause of cracked tiles on suspended floors and it cannot be solved at the surface.
Check the perimeter.Cracks and lifted tiles around the edges of a room, at doorways, and against door frames, columns and thresholds usually mean the tiling was laid hard against them with no movement joint, so building movement has nowhere to go.
Check for underfloor heating.A heated floor expands and contracts through every heating cycle, so it needs a flexible adhesive, flexible grout and properly placed movement joints. Rigid materials over UFH crack reliably.
Look at what is under the tiles where you can.Chipboard, floorboards, an unreinforced screed and a solid slab all behave differently. Timber floors need overboarding, stiffening or a decoupling layer; screeds crack at their own joints.
Fix the cause first, then the tiles.Stiffen the floor, fill the void, or cut in the movement joint that was missing. Only then replace the tiles, with full adhesive coverage and a flexible adhesive.

Tips

A single cracked tile in an otherwise perfect floor is usually something heavy dropped on a void. Several cracked tiles is a floor problem.
A crack that appears in the first heating season on a new floor almost always means the substrate was still moving or drying.
If the tiles are cracking but the grout is not, look at the tiles' bedding. If the grout is cracking too, look at movement.
Note whether the crack goes through the glaze only or right through the body — a glaze-only crazing pattern is a different, cosmetic fault of the tile itself.

Common mistakes

Replacing the tile and nothing else — The bending force that broke it is still there. The new tile is in exactly the same position over the same void or the same deflecting joist, and it breaks in the same place, usually faster because the surrounding tiles are now disturbed.
Tiling a bouncy timber floor without stiffening it — Ceramic and porcelain tolerate almost no bending. A floor that feels 'a bit springy' underfoot is moving far more than the tile can follow, and the crack is only a matter of time and traffic.
Running tiling hard into walls, thresholds and columns — Everything in a building moves — thermally, seasonally, and with load. With no perimeter joint, that movement is transferred into the tiling, which cracks the grout first and the tiles second.

If it doesn't work

One cracked tile, hollow all around it

Cause: Void under the tile from poor adhesive coverage — Fix: Remove the tile, clean the bed back, and re-lay with full coverage. Check nearby tiles by tapping — poor coverage is rarely limited to one tile.

Cracks running in a straight line across several tiles

Cause: A moving joint or joist below, or a crack in the screed telegraphing up — Fix: Expose the substrate along that line and deal with what is there — this may mean an uncoupling membrane, a properly formed movement joint on that line, or stiffening the floor.

Cracks and hollow tiles round the edges of the room

Cause: No perimeter movement joint — Fix: Cut a joint at the perimeter, remove the affected tiles, re-lay leaving the gap, and fill it with flexible sealant hidden under the skirting.

Cracking appeared after underfloor heating was first switched on

Cause: Rigid adhesive and grout, or heating brought up too fast — Fix: Re-lay affected areas with a flexible adhesive and grout, and bring UFH up to temperature gradually as the manufacturer specifies after any new floor.

Questions people ask

Can a floor be too bouncy to tile at all?

Yes. Tiled floors need a substrate stiff enough that it barely deflects under load, which is a higher standard than a floor needs to feel solid underfoot. If a timber floor moves noticeably, it needs stiffening — extra noggins, sistered joists, or a thicker overlay — before tiling, and sometimes it is genuinely the wrong floor for tile.

Does a decoupling membrane solve deflection?

No, and this is an important distinction. An uncoupling membrane isolates the tile from in-plane movement in the substrate, such as a screed shrinking or cracking. It does not stop a floor bending up and down. Deflection has to be fixed structurally.

How wide should a movement joint be and where?

The joint positions follow the building — at the perimeter, over structural joints, at changes of substrate, at doorways, and dividing large areas into bays. Widths and spacings are set by the tile adhesive manufacturer and by British Standards rather than by a single number, and heated floors need them more closely spaced.