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Q.We will be installing ceramic tile flooring in the kitchen of an existing house. The subfloor consists of 1/2-inch plywood over 3/4-inch particleboard (not OSB). The joists are spaced 16-inches on-center. My plan is to install 1/2-inch cementitious backerboard on the plywood and then install the tile. Will this be adequate?

A.Chip O’Rear, assistant executive director of the National Tile Contractors Association, responds: Particleboard, which easily absorbs moisture and is dimensionally unstable, shouldn’t be included anywhere as part of a tile substrate. In this case, the existing plywood and particleboard subfloor layers will have to be removed and a new subfloor installed on the joists.

The best subfloor for a backerboard and tile installation is 3/4-inch plywood, glued with subfloor adhesive and mechanically fastened every 8 to 12 inches in the field, and every 6 inches on the edges. Leave a 1/8-inch gap between plywood sheets for expansion relief. Then install your 1/2-inch backerboard in a freshly combed bed of thinset (to give uniform support), fastening the backerboard with corrosion-resistant fasteners. Since installation requirements vary depending on the backerboard manufacturer, always verify a manufacturer’s requirements before proceeding.