Because it is considerably stronger than the combination of plywood and backerboard, a double layer of 3/4-inch plywood covered with a membrane is the author's first choice for thin-bed tile floors. For extra strength on stone installations, or wherever deflection is an issue, the two plywood layers should be screwed and glued before the membrane is installed.
On wood construction, all movement joints must extend from the top of the subfloor to the top of the tile.
To achieve maximum strength and smoothness, 95 percent to 100 percent of the entire plywood surface must be covered with ANSI Type 1 or 2 waterresistant wood glue. At the perimeter of the floor, to avoid squeeze-over into the movement-joint slot, the author keeps the glue about 1/2 inch from the wall.
The plywood underlayment is partially installed. Note that glue was spread a few inches beyond the footprint of the plywood sheets. As subsequent boards are secured, any exposed glue not covered within five minutes or so should be removed with a scraper and damp sponge.
Using a stand-up screw gun with quick-loading 22-screw clips is substantially faster than shooting individual screws. It's also much easier on the knees.