Stone or clay-based tile has no "give'' to it, meaning it will break if it moves at all. That makes it important to have a solid, immobile underlayment. The only truly perfect tile floor underlayment is a thick slab of concrete, but in most cases that's not going to be available. The closest practical alternative to that today is cement board, which is a big improvement over the plywood underlayment of a generation ago. In addition to strength, cement board is moisture-resistant, making it ideal for wall applications as well.
The surface that the cement board goes on has to be clean, dry and solid. The floor has to be stripped to plywood subfloor, with all the nailheads sunk and the seams even. Remove the floor trim. For walls, either plaster or drywall is fine, but make sure there's no gloss or grime on the surface. If the wall is very shiny from gloss paint, use a belt sander to buff it out. For floors, the cement board can screw to the subfloor anywhere, but for walls, you'll have to find the studs. Use an electronic studfinder to mark them.
Cement board generally comes in 3-by-5 sheets of various thicknesses. For floors, use 1/2-inch thick board to ensure strength; for walls, 1/4 inch is fine. Carpenter's glue is an acceptable adhesive, but some prefer thinset mortar, spread with a notched trowel. Either way, sink galvanized 1-1/2-inch screws as well. For walls, follow the studs, putting a screw every foot or so; for floors, put one every square foot. Position the boards in courses, staggering the seams. Cement board cuts like drywall, by scoring it with a razor knife and snapping it. Tape the seams as you would drywall, using mesh tape and spreading thinset mortar over it.
The cement board itself has to be smooth and free of any ridges at the seams before you tile. Run your hand over the surface to find any high areas, bumps or ridges, and use a vibrating sander to take them down. (Go easy with the sander, using it only where needed.) Once everything is flat and smooth, lay out your tiles with marked lines, using a level and pencil for walls and a chalk snapline for floors. You'll secure the tiles with thinset mortar, spread on top of the cement board with a notched trowel.
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