Here is some information I have borrowed from the TCNA: Maximum Allowable Deflection for Substrates Installed by Other Trades: Ceramic tile installations require the floor areas over which tile is to be applied to have a deflection not greater that 1/360 of the span when measured under a 300 lb. concentrated load (see ASTM C627). This deflection limitation differs from the L/360 deflection limit under uniform load prescribed by building codes. It is the responsibility of the project owner (or owner’s builder or design professional), not the tile installer, to insure the substructure does not deflect more than this maximum allowable deflection criteria. As tile is a “finish” applied to and relying upon the underlying structure, an inadequate substructure can cause a tile failure. In many cases, problems in the substructure may not be obvious and the tile installer cannot be expected to discover such. -TCNA (Tile Council of North America - Handbook for Ceramic Tile Installation 42nd Edition) In my experience a problem or a failure of a tile installation has always been quickly laid at the feet of the installer. Though I can't argue that the ultimate responsibility for the "quality of workmanship" is solely the installers responsibility I will argue that an installer is not an engineer and shouldn't routinely be expected to recognize a structural deficiency. Fortunately our friends at TCNA support this theory and have published the above for all to know. Thank you TCNA, it's been a long time comming.