Setting the Ceramic Tile

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Dry floors: Use a level concrete bed (a slab, or at least 1", wire-reinforced concrete on W-or-more exterior plywood) or a minimum of a 1 / 4 " thickness of wood with the upper layer being no less than W exterior plywood. The plywood should be clean before installation; a roughing with coarse sandpaper is helpful.

Wet floors: Use a concrete bed with W-per-foot slope and appropriate drains. In a shower stall, for example, you can lay a chlorinated polyethylene (CPE) waterproofing membrane (see your tile dealer or plumbing supply) on 1 1 / 4 " of plywood, and cover it with a 1 ½” mortar bed sloping toward a central drain. (Grout, thinset mortar, backer board and concrete are permeable to water, so even impervious tile doesn't form a waterproof covering.)

Dry walls: Use at least a 1" thickness of moisture-resistant dry wall or ½” backer board. Nail or screw these materials every 6", leaving a 1/8” to ¼” gap at the joints. Then tape the joints with fiberglass mesh, and fill and smooth with thinset mortar. Many tile installers claim good results using dry wall as a substrate for this application, while others insist that nothing less than backer board will do. (Some traditional tile setters still trowel mortar to walls as a substrate. This is an excellent method but is probably beyond the amateur.)

Wet walls: Use ½” backer board with the CPE waterproof membrane lapping up the walls behind the backer board to a height of at least 18".

Counter tops: Use at least 1 ¼” of plywood, or ¾” plywood under ½” backer board.
If your tile dealer is conscientious, you will probably be urged to use backer board for all wall installations. Pros often get by with alternatives that are less expensive and easier to install, but a knowledgeable dealer should understand that amateurs are more likely to succeed with backer board.

These panels come in a variety of sizes and thicknesses, but ½" X 3' X 5' is by far the most common simply because it's easier to handle—in both weight and price—than bigger sheets. Even a 3' X 5' piece of backer board weighs almost 50 pounds and costs upward of $15. The dealer probably won't call it backer board, but will use a brand name, such as Wonderboard.

Cutting backer board is no picnic. You can scribe it with a special carbide-tipped tool (see your tile dealer) and then snap it much as you would dry wall. This leaves a pretty rough edge, though. So, many installers prefer to cut it with a masonry blade in a circular saw. Goggles and a dust mask are mandatory when shaping this material. Make holes for faucets and the like with carbide-tipped hole saws or jigsaw blades or both.

Adhesives

Materials for bonding tiles to a substrate—we'll call them adhesives as a catchall—are of two general types: thinsets, which are based on Portland cement, and glues, such as latex mastic. Each type has its advantages and appropriate applications, which are summarized in the accompanying chart.

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