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.
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