Tile is persnickety stuff. Because you're dealing with issues of moisture, movement, and grip-ability, tile will work with certain types of subfloor--and will go dramatically wrong with other subfloor materials. Let's look at what's acceptable, what's not.
Can Be Used As Tile Subfloor
- Cement-Based Backer Board: Backer board is made for tiling. Backer board goes under various brand names, such as DensShield, Hardiebacker, WonderBoard, and so on. Tile backer board should always be your preferred ceramic tile underlayment.
- Exterior Grade Plywood: Exterior plywood accepts tile, without swelling up like interior-grade plywood. Since cement board is cheap and easy enough to lay down on plywood, it makes little sense to skip this step. However, if you would like to install tile on the plywood subfloor, it is possible. In short, you would put down two layers of plywood. Screws from the top layer must not run all the way down to the joists.
- One Layer Resilient or Vinyl Flooring, Sheet: Not only must this must be sheet vinyl flooring, (wall to wall), but it must be full contact, as well. Vinyl flooring glued or stapled only at the edges will not work. To test if your vinyl floor is full contact, use a utility knife to cut a small slice at the center of the floor. Use the blade tip or sharp screwdriver to try to lift up the vinyl. If it does not lift, it is full contact vinyl. Overly glossy, cushioned, and tile vinyl floor will not work as tile subfloor. For more, see our article about installing tile over vinyl flooring.
- Concrete Slab: Yes, concrete is a good ceramic tile underlayment. But be careful of the expansion joints. If you tile over the expansion joints, the tile will crack. To reduce the possibility of cracking, use a material called variously an Anti-Fracture Membrane, Crack Isolation Membrane, or Crack Suppression Membrane.
As a side note, it's worth mentioning that you can apply ceramic tile directly to drywall on vertical surfaces, in limited areas. Specifically, it is acceptable to tile directly on drywall for a kitchen or bathroom backsplash that is no more than a couple of feet high. Also, note that the drywall cannot be painted. Bondera
tile mat is a good non-thinset adhesive method of attaching tile to drywall.
Cannot Be Used As Tile Subfloor
- Interior-Grade Plywood: This plywood will swell upon contact with water.
- OSB Wood: Like interior plywood, OSB does not perform well with water.
- Multi-Layer Resilient or Vinyl Flooring, Sheet: One layer of vinyl floor is fine. More than one layer isn't.
- Resilient or Vinyl Flooring, Tile: Tile vinyl flooring has countless seams that will allow for moisture intrusion.
- Drywall, Large Expanses: Because drywall has a paper outer layer, large applications of ceramic tile will eventually weaken and pull off that paper.