Movement

• Foundation movement remains one of the main reasons the grout or tiles laid over a concrete foundation crack, says Quandamn Home Inspection. A properly designed and cured foundation withstands rises in pressure when the soil around it freezes or becomes saturated with water. Weak foundations with improper footings shift under this extra pressure. The floor tiles embedded in a stiff mortar can't move with the foundation, so the pressure builds until cracks develop. Look for cracks in walls that rest on the foundation as another sign of movement.
  
  

Cracks

• The forces building up inside concrete foundation as it cures causes cracks that may not appear until a month after the pouring of the foundation, according to Ask the Builder. These cracks take even longer to appear on the surface of the slab. However, once you lay an inflexible tile floor over existing foundation cracks, the slight movement at the crack makes it appear in the floor above as well. Even the best foundations develop a few cracks, but badly-built foundations crack enough to leave gaps that can snap tiles.
  
  

Foundation Repair

• Don't assume that repairing your foundation fixes the cause of floor tile cracking or prevents it. The process of installing piers below or around a foundation actually causes tile cracks in many cases, according to Foundation Guard. Piers balance the entire weight of the foundation and the structure above it on a few small points. This puts extra pressure on the slab and creates more small cracks that disturb your carefully laid tile flooring.
  
  

Prevention and Repair

• Small hairline cracks are harder to prevent because the slightest shifting of the concrete causes them. Preventing large cracks that break tiles in half or loosen them requires the installation of a crack isolation membrane that absorbs the movement of the foundation, says Ask the Builder. Reinforced concrete also resists cracking, but this only works if you're pouring a new foundation. Repairing your cracked tile floors with epoxy or grout works temporarily, but it doesn't stop foundation problems from causing the problem again.