Subfloor question

[bbcamp]

Not trying to get into an argument with the folks at Schluter or Dobri, but your situation is kinda outside the norm. 1/2" subfloor is not sufficient on it's own, and is not quite thick enough for screws (or nails) to hold without stripping. The 3/4" plywood may be stiff enough to prevent the screws from pulling it down flush with the old subfloor. Given that 3/4" plywood is sufficiently thick for a porcelain tile installation all by itself, it seems that if you fasten it to the joists, with no regard to the 1/2" plywood being there other than using longer deck screws, your floor would meet Schluter's standards. I see no down side to this method.

[Dobri]

Rob, i think Bob answer your question. It's up to you.

[UCbandit22]

Thank you all for the suggestions....I will probably try to screw the 3/4 down on the joists and add the Ditra from there. The one thing that I never mentioned is that 3/4 of the whole downstairs of my house is tile and all of it has a 1/2 sub and 1/2 ply on top of that. I just bought the house last year but none of the tile is cracked or coming up, it' sjust outdated. So i will probably be okay..hopefully....

Thanks again everyone!