Which thin set do you use over plywood?

Answer 1

No. A subfloor needs to be 1 1/4" thick. Ceramic needs an extremely stable substrate. When thinset is added to the floor the water in the mixture weakens the plywood. Any flex in a subfloor will cause a tile to crack. Traditionally tile was placed on a mudbed. It was a cement mixture laid on the plywood at least 3/4" thick. Mudbeds are not for a do it yourselfer, since if the cement isn't mixed right or the floor is prepped incorrectly you can have a mess quickly. Today, a great many pros and the majority of DIY's use a cement board. There are several brand names on the market, Wonderboard, Hardibacker, Durock, etc. You apply a layer of thinset with 1/4" x 1/4" square notch trowel. The board is put over this and screwed into place. Some can be nailed, but screws are more reliable The seams are taped and mudded with a special mesh (similar to drywall tape) and thinset. The ceramic is then applied to the top of that with another layer of thinset. Depending on the thickness of the tile and original subfloor, you should end up with something around 1 1/2" to 1 3/4". There is a new generation of "cement board" but they can be quite costly. Schuter makes a product called Ditra that looks like a waffle pattern. You lay thinset on both sides. It will shave off a little in the height but is about 3x more expensive to use then cement board.