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
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
For a counter that will get heavy use, you should also
consider using a trowel-applied or sheet membrane over the
cementitious backerboard and extending up into the backsplash
area (Dens-Shield needs no further waterproofing).
As for grout, my recommendation is for a natural
cement-colored grout mixed with a latex additive (or use a
polymer-modified grout). Width is up to you. I usually
recommend narrow joints (1/8 inch or less) because
they’re easier to clean than wide joints.
Use a heavy-duty sealer as well (not a cheap silicone
pour-on), and most important, include caulked expansion joints
around sinks, cooktops, chopping blocks and at the joint
between the top and the backsplash (see illustration).