A.Michael Byrne, director of the Ceramic Tile Education Foundation, responds: I wouldn’t risk granite tiles over plywood. The best base to use is a backerboard made specifically for ceramic tile. This could be a cementitious backer unit, like Durock or Hardibacker, or a glass mat water-resistant gypsum board, like Dens-Shield.
The backerboard should be laid over a 3/4-inch exterior-grade plywood substrate, attached with a leveling bed of thinset mortar in between, and secured with Hi-Lo S Rock-On screws or galvanized roofing nails. Avoid the use of drywall screws.
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