Cracks that run in lines across several tiles are due to movement in the substrate. There are structural problems that need attending to. Sure, you could cover the floor with vinyl or carpet and hide the symptoms, but there is still an issue that needs to be fixed.
What can cause the problem?
• Excessive movement in the substrate will cause a crack in the tiled surface that runs through several tiles in a roughly straight line. • If the tiles are laid over a slab of concrete and a crack appears in the tiled surface, then that slab is moving and there is a structural issue. • If the crack occurs at the meeting point between two slabs that were laid at different times, then there should be an expansion or movement joint at that junction in the substrate. Cracking here is due to that movement or expansion joint being carried through the tiles. • If the cracking occurs along the junction of two different surfaces, such as where a timber floor and a concrete floor meet, a movement joint is necessary and must be extended through to the surface of the tiles.
What could prevent the problem?
If the tiles were laid with suitable flexible adhesive and/or Crack Suppressant Membrane along with correct Primer and labor, the cracks wouldn't appear in the tiles. Keep in mind that even when a flexible adhesive is used, the cracking, shrinkage, or movement can be more than that which the adhesive or roll-on membranes can compensate. That's why anti-fracture mat membranes has a necessary role in professional hands.
So what can I do now?
Firstly, it needs to be determined if the cracking is only superficial and due to the slab shrinking and curing, or if there are structural issues, or if movement joints were not extended to the surface through the tiling.
If the substrate cracking is superficial, and the concrete slab new, then the tiler has undoubtedly not used suitable flexible adhesives and/or any membrane. The tiles should be replaced and correct underlayment used. If the expansion joints were not treated correctly, then that will need to be corrected.
The damaged tiles need to be replaced and movement joints extended to the surface. This is also the case if the concrete slab was old, but had obvious established cracks of 2mm or wider and the tiler did not make allowance for this movement.
Source: Beaumont, AU