The first thing that you have to establish is whether the floor onto which you will install the plywood is level. If the floor is level and solid, then 12 mm (1/2 in.) or even 9 mm (3/8 in.) plywood will suffice. Once you screw down the plywood, you can then glue on the ceramic tiles and you should not encounter any problems. The reason for this is if the floor is rigid and completely level, the floor will absorb all the weight-not the newly installed plywood. If, however, the floor is not level or flexes too much, and you install the plywood, it will bend here and there, with loose and cracking ceramic tiles the end result. If the hardwood floor beneath is not completely level and solid, you should use 15 mm (5/8 in.) tongue-and-groove plywood. Before installing it, you should also ensure that the joists below the floor are properly reinforced.