Ceramic tile flooring is beautiful, long-lasting and easy to maintain, but it also can be a disappointment if it is not installed correctly. In addition to thinking about the hardness and thickness of the tile, the glaze, and the mortar and grout, it is also important to consider what subfloor and underlayment to use to prevent the tile from slipping and cracking. Plywood is one option.
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Ceramic Tile Underlayment Requirements
Ceramic tile is rigid and somewhat brittle, so it requires a solid underpinning. A plywood underlayment must be laid over a floor and subfloor that is at least 1-1/8 inches thick. Interior-grade plywood, particleboard, waferboard and masonite are not strong enough to support ceramic tile. Plywood underlayment for ceramic tile should be 3/8-inch thick, exterior-grade C-C, and not fired treated. The plywood underlayment should be fastened to the subfloor with drywall screws. Because plywood will expand and contract, you should leave a gap of 1/2-inch between the wall and the plywood along the exterior edges, and a 1/4-inch gap between the sheets of plywood.
Disadvantages of Plywood Underlayment
The Ceramic Tile Institute of America considers laying ceramic tile directly on plywood, other types of wood, or particleboard to be a highly suspect procedure. The institute says new kinds of mortar make it possible to achieve a tight bond between the plywood and the tile, but the inherent instability of plywood and other wood products could be a problem. Plywood will expand, contract, swell and shrink depending on the moisture content of the wood, relative humidity, and other factors. Ceramic tile, on the other hand, is fairly rigid and not subject to changing shape because of moisture. Consequently, the movement of a plywood underlayment as it responds to moisture will cause the grout between the tiles to crack and could cause cracks to the tiles themselves. To quote the institute: "Plywood is not a stable material and to directly install tile over it is an invitation for failure."
Floor Covering Installer Magazine states that plywood is a perfectly suitable material for the subfloor under ceramic tile, but not for the underlayment for a ceramic tile installation. (Note that the subfloor is below the underlayment, which is what comes into direct contact with the tile.) If you are not willing to risk using plywood as an underlayment, you still have some options. You could cover the plywood with concrete board (also called backer board), which is very stable, or you could cover it with a product called ditra, which is a special plastic material that separates the ceramic tile from the plywood underlayment and acts much as a concrete board would.
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