A strong underlayment is the most important part of the tile installation. There are several options for achieving a sturdy floor underlayment.
To tile over an existing floor, it must be flat, solid, and well bonded to the sub-floor. The floor and subfloor should be at least 1-1/8" thick for good support.
If your existing flooring is cushioned vinyl or linoleum, you can either remove it or cover it with a proper underlayment. You can install tile over tile as long as it's level and in good condition. You may have to roughen the surface of tile if it's too glossy.
If you have more than one layer of flooring, or if it's damaged beyond repair, it must be removed and another underlayment installed.
WARNING: Some older resilient flooring may contain asbestos fibers. Do not try to remove or sand this type of flooring without having a trained asbestos inspector determine if asbestos is present. Resilient flooring with asbestos can be covered safely with plywood, or removed by a professional contractor.
For do-it-yourselfers concrete backerboard (Durock® and WonderBoard® are two common brands) is one of the standard materials for floor tile underlayment. Concrete backerboard has a solid concrete core and is faced on both sides with fiberglass. It can't be damaged by water, which makes it ideal for bathroom and kitchen installations.
There are also variations made out of fiber cement (HardieBacker™ and Fiberock®) and specially finished water-resistant gypsum (DensShield®).
Cutting backerboard is a lot like cutting drywall, except that backerboard is much harder.
Using a framing square, score your cut line a few times. You can use a regular utility knife for this, but you'll go through a lot of blades. A special carbide-blade cutter works better.
TIP: If you're using a utility knife, shorten the blade to keep it from breaking easily.
Break the board by applying pressure until it snaps apart along the score line. You'll probably have to cut through the fiberglass on the back also.
Backerboard is installed over a wood subfloor using the same type of thinset mortar (See Choosing Mortar) that you use to set tiles.
Use the flat side of a notched trowel to spread the thinset out where the sheet of backerboard will be set. Then use the notched side of the trowel to comb out the adhesive. Secure the sheets with galvanized nails or specialty backerboard screws about every 8 inches.
Stagger the joints of the backerboard so they don't line up with one another or fall directly over the joints of the subfloor. Leave about 1/8" space between the sheets of backerboard.
Fill these gaps with thinset mortar using a taping knife. Embed fiberglass joint tape into the mortar to cover the seams, then cover that with more adhesive.
Professional tile contractors often put down what they call a mortar, or "mud" bed. It consists of a layer of roofing felt, then a wire mesh, then mortar.
Professionals swear that a mortar bed is by far the best underlayment for floor tile, but it takes a lot of skill to finish it off level and at just the right thickness. We always recommend that do-it-yourselfers stick with a backerboard underlayment.
A stable concrete slab is a great underlayment for tile. Check to be sure an existing concrete floor is flat. Move a straightedge over the surface and mark high and low spots. Low spots can be filled in with a leveling compound and high spots can be ground down with a rubbing stone or grinder.