I have read through some of your book, but there is something that doesn't add up that I hope you can clarify.
In chapter 4 talking about plywood subfloor,you state: "Also position the sheets to leave a gap between them of no less than 1/8 in. and no greater than 1/4 in. These gaps, which should later be filled with tilesetting adhesive to edge-glue the plywood, will allow for seasonal expansion in the wood." If I am using thinset as you recomend, I will be putting a substance that will become hard between the sheets of plywood. I believe this will then make the sheets act as if they were butted tightly together, and therefore the sheets will not have room for the seasonal expansion. Should the joints not be filled with something that will remain flexible, like caulk? This would be similar to the expansions joint you talk about later in the chapter.
I know you are the expert here, so I will take what ever advise you can give, but my wood working background tells me the gap should be kept flexible to keep from having the accumulation of all those sheets expanding.
You are right about moisture expansion, and if you are logical and read the specifications, something does not add up.
The tile industry has said leave a gap, don't leave a gap, fill it, don't fill it. In my opinion, a gap between sheets is important beacause--completely filled with mortar--it transmits movement to the more-important gap: the expansion joint around the perimeter of the floor.
If the sheets are merely butted together, or taped over to prevent them from being filled with mortar, the empty space becomes a place where movement wants to concentrate--and emerge as a cracked or loose tile.
I don't recommend adhering tiles directly to plywood, but rather to a membrane that has been appled OVER the plywood. In such a case, I also recommend that there be a 1/8 to 1/4-inch gap between plywood sheets to be filled with thinset mortar, and a 3/8 (min) to 3/4-inch expansion void around the perimeter of the plywood. This gap is filled with foam filler set at the height of the plywood underlayment. The membrane covers the entire floor, the foam-filled gap, and it laps up the wall to the desired height.
For every job I either install or specify where plywood is used as the setting bed, here is my recommendation:
#1--minimum 3/4-inch exterior glue plywood subfloor.
#2--minimum 3/4-inch exterior glue plywood underlayment.
#3--membrane between tile and underlayment.
To use or not to use a membrane with plywood is not a matter of the installation being in a wet or dry area but that it includes plywood (classified by the ANSI A108 standards as an unsuitable material for the installation of ceramic tile.
Since I view the ANSI A108 standards as absolute minimum, it is essential for my work that each installation be upgraded to assure complete customer satisfaction and performance I can depend on.
A membrane in a dry area? Absolutely! At least until plywood is considered a stable material by the tile industry.