Substrates and Setting Beds

Tile requires a flat and stable surface, especially ceramic and stone tile. You can tile over properly prepared concrete, drywall, and plaster, but other installations call for a substrate. Plywood is not suitable for mortared tiles. It pulls water out of mortar, and its faster expansion increases the potential of cracked joints. Here are the common substrates used with ceramic and stone tile.

Drywall
Drywall, a core of gypsum compressed between two layers of heavy paper, provides an appropriate substrate for wall installations that won't get wet. Drywall is not suitable in wet areas. It is manufactured in different-size sheets (4x8 feet is standard) and thicknesses (1/2 inch is common on walls). In wet areas, remove existing drywall from the studs and install a waterproofed backerboard or a waterproofing membrane and cement backerboard.

Greenboard
Greenboard is a drywall product with paper layers that have been treated for water resistance. It is available in different thicknesses, but 1/2 inch and 5/8 inch are used on walls. You can install it in locations that will receive minimual moisture, but it won't stand up to repeated wet conditions. In any wet location, it's better to use backerboard.

Cement backerboard
Cement backerboard is a cement-based material formed in one of two methods. A cement core is sandwiched between layers of fiberglass mesh, or the fibers are impregnated in the core. Cement board is the most prevalent substrate for both wet and dry installations. It is made in 32- or 36-inch widths, up to 60-inch lengths, and in 1/4- and 1/2-inch thicknesses.

Backerboard made of cement, ground sand, cellulose fiber, and additives does not contain fiberglass mesh. It is lighter than cement board, cuts easily, and includes an imprint to guide fastener placement. It comes in 4x8-foot sheets and 1/4- and 1/2-inch thicknesses.

Glass-mat gypsum backerboard
Not to be confused with gypsum drywall, this material is made of a compressed gypsum core with embedded fiberglass and a water-resistant coating. It is lighter and easier to cut than cement backerboard but not as strong. It is available in 4x8-foot sheets and 1/4- and 1/2-inch thicknesses.

In a typical tile installation, a plywood subfloor provides a stable foundation and supports the weight of the tile. Cement backerboard laid on the subfloor in a bed of thinset mortar is screwed to the plywood and joists. Tile is set in a mortar bed and grouted. Installations over a concrete slab may require an isolation membrane to keep cracks from telegraphing to the tile. Wet locations such as shower enclosures need a waterproof membrane under the backerboard.