I have a common wall between 2 baths. 1 has shower, the other a shower/tub combo. If fixed head is pointed towards common wall, water leaks through to first floor. Both baths being gutted. Have 12x12 1/2" porcelain for floors and 6x6 for walls. Confused by various contractor recommendations (hardibacker or durrock on the walls, using fiberglass mesh tape at joints and Flexbond thinset over the boards). I've heard several recs for subfloor in shower area ranging from durrock, wonderboard, hardibacker, 2 3/4" ply glued with mesh and cement above it.

My major objectives are to prevent future leaks and keep the tiles in place.

Thanks for any suggestions,
Barry