A.Michael Byrne responds: If the tiles in question will be used to cover a serving counter that is normally dry, then they can be set directly over plywood with an epoxy mortar or an organic adhesive. But if a sink is part of the installation, plywood should not be used as the base for tiles.

The American National Standards Institute (ANSI) specification for tile characterizes plywood as dimensionally unstable and not ideal backing for ceramic or stone tiles. Nevertheless, many thinset mortar manufacturers produce tile adhesives specifically for use with plywood.

In my work, if plywood is specified in plans and must be used as the base for tiles, I get the architect or GC to include a waterproofing membrane. I recommend either a sheet membrane, such as NobleSeal TS (The Noble Co., P.O. Box 350, Grand Haven, MI 49417; 616/842-7844), or a trowelapplied membrane, such a Laticrete 92-36 (Laticrete International, One Laticrete Park North, Bethany, CT 06525; 800/243-4788).

A better approach than using plywood, however, is to use cement backerboard, which is made specifically for tiles. Cement backerboards provide a tough base for ceramic or stone tiles, and are unaffected by water or moisture.

For moderate protection against moisture penetration, the backerboard can be laminated directly to a plywood base with latex-modified thinset mortar. For a commercial or active residential kitchen or bathroom countertop, however, I recommend combining the backerboard with a waterproofing membrane as the base for the countertop tiles.

Michael Byrne is a tile consultant and specialty installer based in Burlington, Vt.