Ceramic & Porcelain Installation
Tile needs a substantial subfloor for support. If you are going over concrete, however, a subfloor is not needed. The concrete may just need some skim coating to make it smooth, etc. When going over a wooden subfloor or an above grade installation, you will need at least a 1 1/2” subfloor before the ceramic can be installed. Otherwise, you may have a problem with the subfloor moving due to the ceramic tile being so heavy.
Using a cement board rather than plywood as your subfloor is highly recommended. Cement board also gives you a water resistant installation, so it should always be used on bath floors and also inside showers on the walls and around bath tubs that are also used as showers. Cement boards are available in /4” and 1/2” thickness.