Actually, your mistake was quite simple: You assumed mortar board was the same as mortar. And, a check as to the condition of the wood floor was not made. By skipping the inspection, you failed to insure that the floor framing members (the skeleton like series of wood members that support the flooring) were strong enough to resist deflection (sagging or bending) when walked on, and secondly, that the same problem did not exist at the subfloor itself. Unlike carpet, wood and linoleum floor coverings, ceramic tile is quite brittle. So, when the tile covered wood floor deflects (moves), the brittle tiles crack.

SOLUTION: Eliminate localized deflection (floor movement). The most common method is to apply a layer of mortar (mixture of cement and sand-like concrete without the rocks) between the subfloor and the tile. Steel reinforcing bars embedded in the mortar lend to its overall rigidity. Mortar board may provide the smooth surface needed to accomplish a professional looking tile job, but it will not necessarily eliminate movement. In tract homes, it is very common to see ceramic tile laid directly onto "beefed-up" wood floors. But in finer custom homes, the subfloor is dropped 2 inches or more in areas where ceramic tile will be placed to allow for a thick (strong) mortar bed. By the way, the mortar bed also doubles as a smoothing surface. A mortar bed applied after the fact is usually only about 1 inch thick, and has the disadvantage of resulting in a tile surface that is slightly higher than adjacent floor coverings. Even so, it is worth the lasting quality it provides. Since practicality requires that the mortar bed be kept to a minimum thickness in retrofit floor tile installations, it is wise to have the floor members (subfloor and joist) checked for stability and rigidity.