I have a similar problem to this in my bathroom. I am installing a new tub and 10"x10" ceramic tiles in a 5.5'x11' bathroom The 11' span has another partition under it, 3 ft. from the exterior wall.
Previously, I had 1" square tiles on the floor.
In removing the existing tiles, I found a 5/8" plywood underlayment on top of a 1/2 plywood sub-floor on top of 2"x10" joists. I replaced the 1/2" plywood sub-floor lying between the outermost joists with 1/2" exposure 1 plywood. I'm worried about this half inch subfloor but the walls sit between two joists so I can't remove the wood up to the walls to replace the 1/2" with 5/8".
I'd appreciate help on coming up with options and the pros and cons of the top ones. Here's what I can think of.
Option 1: USG makes 5/8" Durock nextgen cement board but I can't find anyone who distributes it - I've got a call into USG to see if I can buy it locally anywhere but I'm not optimistic. If I find it, I could replace the 5/8" plywood I removed with that but I don't think it would be as strong as the plywood was.
Option 2: Use plywood for the under-layment using 5/8" AC plywood from the local store or underlayment plywood, duplicating what I removed. I read here and elsewhere that there is such a thing as underlayment plywood - the local lumber yard doesn't have it - I'd have to search for that.
Option 3: Judging by something I read in this stream, the floor should be between 1 1/4: and 1 1/2". The floor I removed was only 1 1/8 so you're saying I should increase that anyway. That means, I should put 5/8 plywood atop the 1/2" plywood and add 1/4" cement board. Then I'd have a strong floor and the correct underlayment material. This would cause me a problem with the toilet flange and the water line to the toilet tank. Would it cause a problem having 3 layers plus the tile? If not a major concern and the toilet would be easy to deal with, this sounds like my best option, right?
Option 4: Option 2 with 1/8" Ditra. Not sure how this would affect my toilet height issue.