A crack that runs in a line across several tiles is movement in the slab or subfloor coming through, not a tile problem. Replacing the tiles without addressing the movement gets you the same crack in the same line within a year or two.
Read the pattern before you do anything
The pattern tells you the cause more reliably than anything else.
- One cracked tile, on its own. Usually impact — something heavy dropped — or a void under that tile from poor coverage when it was set. A single-tile repair is legitimate here.
- A straight line through several tiles, running through the grout and the tile alike. A crack in the slab underneath, telegraphing up. This is the one that matters.
- Cracks along a wall or a doorway. Often a missing movement joint. Tile assemblies need a soft joint at every perimeter and at changes of plane, and they very often do not get one.
- Tiles sounding hollow, grout crumbling, no crack yet. Debonding — the tile has let go of the substrate. It will crack soon.
The Florida version of this
Post-tensioned and conventional slabs both move — with moisture, with soil conditions, with the seasonal wet and dry cycle we get here. A hairline shrinkage crack in a slab is normal and is not a structural problem. Rigid tile bonded directly to that slab has nowhere to go when the crack opens a fraction of a millimetre, so it cracks in sympathy, in the same line.
This is why crack-isolation membrane exists. It is a sheet or liquid layer between slab and tile that lets the slab move underneath without dragging the tile with it. On a slab-on-grade floor in this climate it should be considered standard, and it is one of the first things cut from a cheap quote.
What a proper repair looks like
Lift the cracked tiles, look at the substrate, and establish whether the crack is stable. Then isolate it — membrane over the crack, extending well past it — before re-setting tile. Skipping that step is why so many tile repairs fail in the same place.
When it is not cosmetic
Get someone to look properly if the crack is wider than about a sixteenth of an inch, if the two sides sit at different heights, if it is still growing month to month, or if it runs alongside doors that have started sticking and drywall cracks at window corners. That combination is movement in the structure, and tile is just the thing showing it to you first.