Hard tiles are deceptively fragile. Virtually any kind of tile you can lay -- ceramic, porcelain, brick or stone -- can crack or come loose if it's not set on a proper underlayment. This is because tile, unlike wood flooring, has no "give'' that allows it to move with its surroundings; if it moves, it will crack. That's why it's important to have an underlayment that is rock-solid, and also free of moisture, which can cause the mortar that holds the tiles in place to loosen.
A concrete slab floor can be an excellent surface for installing tile, provided the concrete is solid and free of significant cracks and surface moisture. Cracking is almost inevitable in concrete slabs, and to prevent cracks from telegraphing to the tile, many installers use a decoupling membrane. This flexible synthetic membrane is applied over the crack before the tile is laid, bonding to both the concrete and the tile, while separating the tile from movement in the concrete.
Cementboard, a cement-based sheet material, is the standard underlayment for most tile applications over wood floors. Cementboard sheets range in thickness from about 1/4 inch to 1/2 inch and can be cut like drywall, by scoring and snapping the panel. The panels are bonded to the subfloor with thinset mortar and fastened with nails or cementboard screws. The panel joints are then covered with mortar and cementboard joint tape. Cementboard is not waterproof or impermeable, but it resists breaking down when in contact with moisture, making it a more durable underlayment than plywood or other organic subflooring and underlayment materials.
Sometimes cementboard isn't a practical choice because of how much it raises the level of the floor. If you have a good plywood subfloor, it's possible to lay tile over it without cementboard, by using a special polyethylene membrane designed specifically for floor tile. The plywood should be very solid and free of any splintering or rot. The membrane comes in 1/8-inch thick rolls and is bonded to the subfloor with thinset mortar to provide a stronger tiling surface and moisture resistance. Once the membrane is set, more mortar is spread on top of it to set the tiles.
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