Simple Bath Floor Questions

Riddler

The old house I am renovating has 1" T&G fir plank flooring directly over 2" x 10" rough sawn joists spaced 18" OC. In the new small bathroom that has just been roughed in, I was considering laying down 5/8" sheathing over the plank flooring and then installing ceramic floor tile over that.

My first question is, should I count on installing a layer of cement board (1/4" perhaps) on top of the sheathing before the tile goes down? My second question, which is related to the first I suppose, is what additional "thickness" should I plan for due to the "finish floor" (i.e., the added thickness above the sheathing)? Can I keep it to 1/2" total with ceramic floor tile?

R&D Tile

For 18" on center joists, I wouldn't go with anything less than 2 layers of plywood, 3/4" then 1/2", then a membrane or backerboard, a membrane such as Ditra will keep the height down some, I wouldn't tile directly over the plywood.

Riddler

Thanks R&D. Just to confirm, with the membrane option available, I should have no trouble keeping the thickness of my finished ceramic title floor to 1/2"?