Tiles keep cracking / popping up - why?

The four causes

• Movement. No expansion or control joints, or tile run tight to the walls with no perimeter gap, so the slab and the tile fight and the tile loses.
• A crack in the slab telegraphing straight up. This needs an uncoupling or crack-isolation membrane, not more thinset.
• Poor coverage. Hollow spots under the tile from spot-bonding instead of properly combing and back-buttering.
• The wrong mortar. Large-format porcelain needs a large-format mortar, because the flatness tolerances are different.

How to tell which one you have

Tap across the floor with a knuckle or a coin. A hollow sound is coverage. A crack that runs in a straight line across several tiles, in line with a crack you can find at the perimeter or in the garage, is the slab. Tiles failing only at the walls, or along one long run, is movement with nowhere to go.

Doing the repair once

1. 1Establish the cause before lifting anything.
2. 2If it is the slab, install an uncoupling or crack-isolation membrane over the affected area.
3. 3Leave a perimeter gap at every wall, covered by the skirting, and put soft joints where the floor changes plane or passes through a doorway.
4. 4Use the mortar the tile manufacturer specifies, and comb it properly.

If it is happening in a wet area

Cracked or hollow tile in a shower floor is a different problem with a shorter fuse, because water is already getting in: why showers leak behind tile.

If you are replacing the floor throughout rather than patching, the slab preparation matters just as much for the next material: LVP over a concrete slab.

What it costs to ignore

Cracked tile is rarely just cosmetic once it is underfoot. Loose tiles break at the edges, grout fails, and in a wet area water starts reaching the substrate. The repair grows from a few tiles to a room.

It also reads badly to a buyer, who assumes movement in the slab whether or not that is what happened. If you are selling, fixing the cause and the tile together is one of the more worthwhile items on the pre-listing list.

Matching tile you can no longer buy

Before lifting anything, check whether you have spares in the garage and whether the line is still made. If neither, a repair becomes a visible patch, and it may be worth replacing a whole run or using the opportunity to change the floor.

This is also the argument for buying ten per cent extra at the time of any tile job and keeping it. It costs very little and it is the difference between a repair and a room.