There should be no 1/4" anything in a tile installation sandwich. Regardless, I think it would be a good idea to get down to the planks and refasten/repair as necessary. Chances are the planks are not t&g and the nails may have worked a bit loose by now.
What kind of shape is the 1/2" you're looking at? What was on it? I might consider leaving what you've got, just refastening if I thought everything was in good shape and didn't mind the extra height.
But let's see what other info you give.
I was asking for an answer. Did we ever talk about the framing yet?
We went in circles a little cuz in your first post I think you went into too many subject. You asked about; plywood types & thickness, concrete board, Ditra, thinset mortars, Lash, trowels, backbuttering, and large tile sizes. We can help with all those, but not all at once. So, let's start with the joists.
Size, spacing, unsupported span to the inch. Species and grade if at all possible. Basement, crawl space?
The 1/2 ply is in excellent shape.. Pretty solid and level. Looks to me to be only a few years old (i bought the condo in 09) and the previous owners were only here 2 yrs. The old flooring was linoleum tile which came up easier than I expected and didn't leave much, if any, adhesive. I sanded the floor to clear up any residue.
Sry for jumping around topics. Since this is my first tiling job and a much delayed project, Im trying to get alot of info quick. Originally, I was told I could tile directly over the plywood with no problem. Then someone at the tile place suggested laying mesh over to get a better bond to the tile... which lead me to this site and research on the proper surface for tiling- and everyone was against tiling on wood.
I live on a ground floor condo built in the 60s..no basement, no crawl space. I don't know the exact dimensions or condition of the joists and subfloor. My question about the thickness was to get a better idea of the levels associated with a normal floor. As I mentioned, the height difference is pretty apparent so I wanted to know if there was a better option (i.e. cement board directly over the lowest level subfloor/ planks). This doesn't seem to be an ideal set up tho. So, the I think I will refasten the 1/2 ply which would give me more than enough support to lay the cement board then tile. I will have to figure out a solution to transition the different level of flooring. Any advise is very much appreciated!