Subsurface Guidelines for Ceramic Tile Installations
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.
ANSI, in AN-2.4.3, includes a cautionary note regarding wood-based panels. Some of the materials not recommended due to their inherent nature -- essentially, a propensity for expansion and contraction when moisture is present -- are: particle board, composite panels (veneers attached to wood cores), wafer board, oriented strand board (OSB), lauan plywood, and plywood that does not make use of fully waterproof adhesive for bonding the plies, exposure durability rating of exposure 1 or exterior. Even these panels are limited to residential ceramic tile installations.
Suitable backings when ceramic tile is being installed using dry-set or latex-Portland cement mortar are: masonry, concrete, lean Portland cement mortar, cementitious backer units, brick, ceramic tile, marble and dimensional stone. In dry areas only, gypsum board may be used.
I must note that there have been new product additions to the backings family since the ANSI guidelines were published in 1999. These new backings are addressed in the Tile Council of America (TCA) 2003-2004 Handbook for Ceramic Tile Installation. For example, there is a Fiber Cement Underlayment (ASTM C-1288) TCA walls interior detail W244-03, one for Coated Glass Mat Water-Resistant Gypsum Board (ASTM C-1178) TCA walls interior detail W245-03, and Cementitious-Coated Foam Backer Board TCA walls interior detail W246-03 (this is a new detail).
The Coated Glass Mat Water Resistant Gypsum Backer Board is also shown as a new floor detail. This detail, F-151-03, is recommended over structurally sound 7/8-inch tongue-and-grove plywood and is limited to the installation of 8-by-8 inch or larger tile.
The Fiber Cement Underlayment detail F144-03 for flooring is also new. Yet another new flooring detail is F170-03, Fiber-Reinforced Gypsum Panel Backer Board.
I cannot overstress the importance of properly preparing your substrate. When failures occur, it is difficult to point to the tile or setting material as the culprit. This leaves the backing material or the quality of the installation as the primary factors in a failure.