Plywood is not moisture-resistant, and it expands and contracts at a different rate from tile. So you need to isolate the tile from the wood subfloor with a layer of waterproofing and crack-isolation membrane. This will allow the tile and the subfloor to move independently—a key to long-term performance.
Carefully installed ceramic tile is an expensive floor finish. Its durability and beauty are the mark of quality, but tile is prone to cracking if it’s not well supported. So when you invest in tile, spend on structural support to match.
Design the floor structure for minimal deflection. “L/360,” equivalent to a half-inch in 15 feet for a fully loaded floor, is the limit for joist deflection. Deflection of the subfloor between the joists under a point load should be even less—perhaps as slight as L/480 or L/1080 for brittle stone tile. To get that kind of stiffness, frame with 2x10 joists or better at 12 inches o.c., and install a double layer of ¾-inch plywood attached with deck screws. Stagger the plywood joints. Use subfloor adhesive to glue the lower course of plywood to the framing, and spread carpenter’s wood glue (such as Titebond 3) before installing the upper course.
Over the plywood, adhere a layer of waterproofing and crack isolation membrane using contact cement. Noble Co. (www.noblecompany.com) and Schluter Systems (www.schluter.com) each offer suitable materials for this purpose. Study and learn the supplier’s recommended methods for use—the details matter.
Finally, apply the ceramic tile using thinset mortar. Make waterproof movement joints at wall and floor intersections. For comprehensive details on best-practice tile work, consult Michael Byrne’s book, Tiling for Contractors.
Latex-modified thinset mortar is well-suited for applying tile over membrane. Consult with your supplier to be sure the mortar is compatible with both the tile and the membrane you’ve chosen. And be sure to properly detail any joints where tile walls or floors intersect.
Cement backerboard over plywood subfloor is an acceptable underlayment for ceramic tile flooring. But using a double layer of ¾-inch A/C structural plywood is even stiffer. Glue and screw the plywood to the floor joists, and also apply glue between the two layers of plywood.
L/360 is the official deflection standard for floors meant to support tile. Depending on the span, 2x10 joists at 12 inch o.c. can achieve that. Using deeper 2x12 joists is a more efficient way to stiffen the framing, but closer joist spacing also helps limit subfloor deflection between the joists (an important concern with tile).