Garage floor epoxy paint for shower

Hi All, We've got a bathtub/shower where some tiles were coming loose. I pulled out the loose tiles, ready to scrape the old mortar out, and put new mortar in to re-attach the files. Well, [URL="http://s461.photobucket.com/home/jbeall/tag/BehindDamagedTiles"]this is what I found[/URL]. You can see that the concrete backer board (where it exists) is crumbling to pieces, then there's plywood off to the right, and then there are spots where is no backing at all! The backing is completely missing in the corner (on the righthand side), and between the concrete backer board and the plywood. By the way, the plywood was black with mold--I sanded a lot of it away. But I thought I would pause and come here for advice. Can I mortar this up and put the tiles back, or am I just asking for heartache later by doing that? Do I have to (gulp) rip all the tiles out in this quadrant of the shower, put up new backerboard, and then re-tile it? Thanks for any insights! -Josh

Hi, I have a 1920's house and demoed the bathroom. As the concrete flr had cracked, I built up the flr joists with sister 2x6", and many cross beams. It is now perfectly flat. I will screw down 3/4" outdoor CDX on the joists, on top of that will adhere backerboard. On top of that will adhere 18x18" porcelain tile. Question 1: should I use 1/2 or 1/4" backerboard? Can I get away with 1/4? Now I have a clawfoot tub, but there is extra joist support where the four claw feet will meet the tile. Question 2: what thickness can I expect the flexbond adhesion to end up being? I have the knotched trowel that is 1/4" deep. With all the knotches, will the troweled flexbond end up being 1/8" thick (betweeen the tile and the backerboard)? Many thanks!!!