(Also see Substrates)
Exterior-grade plywood is also known as exterior-glue plywood. Its crossply construction and water-resistant glue make exterior-grade plywood the industry’s best choice for tile installations over a wood substrate. Tile installations over plywood are limited to interior floors and countertops in dry areas only.
Floors: Exterior-grade plywood floors should consist of a double layer of plywood, glued and screwed together. Each layer should be a minimum of 5/8″ or 3/4″ thick, to equal a minimum of 1-1/4″ total thickness. The floor should be very solid and stable with no deflection (bounce or movement). Any deflection issues should be corrected prior to installation. Plywood should be free of paint. If plywood has been painted, paint should be mechanically removed. Old flooring glue should be mechanically removed or may be scraped down to a thin residue (depending on the type of glue). Minor low spots in plywood can be corrected by filling these areas with a cement-based patching compound such as MAPEI Planipatch. If leveling is needed, MAPEI recommends the use of a pourable leveler such as MAPEI Self-Leveler Plus. MAPEI Primer T should be used to prime the plywood before the use of a pourable leveler.
All exterior grade plywood substrates should be clean, dry and free of dust, dirt, oil, grease, paint, tar, wax, clear coats, loosely bonded toppings, any primers that are not MAPEI brand, old adhesive residues (unless otherwise recommended by MAPEI), and any other substance that may prevent, reduce or affect adhesion or performance.