I noticed a tile in my shower was loose, so I figured I would pull it off and reattach it. When I pulled it off I found water had gotten behind the tile and deteriorated the backer board behind it. So now there is nothing to attach the tile back too.
Do I have to replace the entire wall of backer board? Then re-tile? Or is there a way to replace just the lower portion for the tiles to attach too?
I'm sure there is no leak as the board is intact and fine above.
There may be no leak right now, but there certainly could be later - the grout can fail anywhere. Just pull everything down and redo it using a proper backer board. Then you'll never have to worry about it.
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Cementitious backer should be able to take prolonged moisture contact without deteriorationg. But the wall will rot out behind it and there will be mold and all sorts of other issues in a lot of cases. A properly installed tub/shower surround involves waterproofing the backerboard with an applied waterproofing sealant such as RedGuard. Then the tile goes on. Or you can do a membrane system called Kerdi over your backerboard before tiling.
If you've got tiles falling off or coming loose, anything you do will be a band aid. If there's moisture in the wall it is time for a re-do. NEVER rely on tile and grout to keep water out of the wall. It takes more than that.