It is probably a good time to address ANSI's General Requirements for Subsurfaces as detailed in AN-2. It is pointed out in AN-2 that the quality of a ceramic tile installation is directly proportional to the stability, permanence and precision in installing the backing or the base material. Some backing materials are prone to deterioration due to moisture exposure, thus making the entire job susceptible to failure.
AN-4.1 recommends backing materials of concrete, Portland cement mortar, cementitious backer units or masonry. Further, when a backing material can be damaged by water, or is not continuous or is unstable, a membrane is required to isolate the backing material from the bonding agent.