In Ceramic Tile & Stone Industry News tile expert witness Donato V. Pompo of Ceramic Tile And Stone Consultants writes on tile job problems:
Common failures are due to substrate cracking and excessive deflection, moisture problems (that can lead to mold), bond failure, and lack of flatness or slope. Thankfully, remedies exist for correcting substrate problems, but it is important to first evaluate the substrate and take any corrective action as part of the floor preparation process.
Is the substrate made of a suitable material allowing for the application of Tile & Stone? Concrete substrates are always best because they are the most stable and are not significantly affected by exposure to moisture. There are limitations to adhering to lightweight concrete, as it tends to have an unstable surface, but remedies for even this situation exist. You can bond to wood, wall board, and steel, but with limitations.
Is the substrate structurally stable? Deflection in the substrate must not exceed L/360 for ceramic tile or L/720 for stone. Excessive deflection is a big problem for installations over wood sub-floors and suspended concrete slabs. Structurally, these substrates need to be designed by a qualified engineer to ensure compliance with industry standards for tile and stone.