Severely damaged tile and underlayment

I am taking care of a friend's rental house while he is in FL. The first story bathroom has a tub/shower combo. The hole into which the tub was placed was about 3 inches bigger than the tub was long. The original installer built a little tile shelf at the back of the tub. The shelf is about 6 inches high and about three inches deep. I got a call about water damage in the downstairs bath from water leaking from the upstairs shower. I quickly saw that the tiles on the top of the aforementioned shelf were not connected to the wall behind them and the little wall at the front of the shelf. Water had gotten thru this shelf, totally destroying the underlayment (seemed like drywall!), run into the joist space between floors and done considerable damage to the downstairs sheetrock. I removed all the tiles from the horizontal surface as the vertical ones were intact. There is a 2x4 as framing in this tiled shelf, but the underlayment was toast.

What repair procedure should I use for this? I was going to use low expanding foam to fill the small gap between the little vertical wall and the six inch high section of totally unsupported tile, hoping the adhesive qualities of the foam would help stabilize the wall. I was then going to get a small piece of concrete backerboard to use as the underlayment for the vertical shelf, and then use thinset to replace the old tiles, and then grout. Is there any special cleaning I need to do to the old tiles, of can I leave the old adhesive on? Is there a better solution to stabilize the little vertical wall than foam? I really don't want to take any more tile out than absolutely necessary, but will if it is the only way.

I left the wall open and told the girl who live there not to use that shower (there are 2 other full baths and one 1/2 in this house) for a day or two while a fan funs and dries it out. The framing is not rotten and is not substantially hurt by the water. Some suggestions would really be appreciated as I have never done tiling/tile repair before. Thanks, Bob

 

Your going to be chasing that leak unless you do it right!

When Gypsum board(Totally the wrong stuff for a wet area!) gets wet it is land fill! It will grow mold quickly.

A total Demo, and rebuild using the right cement backer board or Ditra membrane and waterproofed.

The cheap fix, is not going to be the least expensive in this case. You'll be fixing it again.

NOTE: You may find it has been leaking for some time and the subfloor needs some repacing also!!