Tile can be placed on any floor surface that meets deflection requirements and to which the adhesive bond will not fail. You should test the adhesive for bond strength on the floor and determine if you need to add an additional layer of underlayment over the existing floor. Reasons that adhesive (including thin-set mortar) may fail include wax, poor surface finish condition, and very slick surfaces.

If the adhesive will function and the floor is reasonably level, installation can proceed. If there is any concern that the bond could fail, use a thin 1/4″ underlayment of Fir plywood or Luuan, glued and nailed to the floor, then proceed normally. Remember to account for floor level transitions at entries and stairways, and you may have to remove baseboards, and cut door moldings and casings to fit the new height.