Ceramic tiles are among the most rigid materials for flooring. They require a subfloor that is designed to account for this, without any fluctuation and movement.
Two common methods are used for supporting tile flooring: cement backerboard and plywood reinforcement.
Cement backerboard offers great water resistance. Backerboard must be used over sound framing and subfloor to offer greater resistance to flexing. Backerboard should be adhered to the subfloor with cement mortar and nailed or screwed to the subfloor.
Plywood will add strength to the existing floor. New plywood will help tie together the old framing members. It is recommended that the subfloor is at least 5/8-inch plywood and the overlay is at least 5/8-inch exterior-grade plywood. No particleboard should be used.