Installing over Wood - Is it possible to tile a wood stair case without having cracks in the grout and tile after a few years?

ANSWER - It is always best to attach ceramic tile or stone tile to a concrete substrate that is very structurally sound and with little deflection. It is possible and legitimate to bond tile to an exterior grade plywood subfloor as long as it is structurally sound, does not have any deflection greater than L/360 for ceramic tile or L/720 for stone tile. The wood can not be subjected to any moisture. You need to use an epoxy adhesive or an EGP polymer modified or latex modified thin-set that meets ANSI A118.11 to bond the tile to the exterior grade plywood. It would be better to apply a moisture barrier and cementitious backer board to the stairs first, but it still needs to meet the deflection requirements. Good Luck!  more