This case is an elderly couple--they're not capable of doing this stuff.
Some people don't know what to do and want me to teach them. Last week
I ran a dryer exhaust vent for a young attorney who had just bought an
older house. He wanted me to show him how to do it. He gave me a three
hundred percent tip. :-0
The usual cases are people who are intimidated, don't have time, or
don't want to be bothered. I just painted a bunch of inside doors for a
guy. He pays me $40 and hour while he earns more than that doing
whatever he does.
About 25 years ago when the mortar base of my bathroom floor (not the
shower, like you described) started to go bad, I fixed it like this:
Too poor at the time to replace the entire floor, I squared off the
section where the tiles were popping and cleaned out the mortar back
to where it was solid - about 12" x 18". I then piled up various
thicknesses of 1 x material and plywood until I matched the height of
the surrounding tile and screwed it to the subfloor.
Did you know that bathroom rugs are bigger than 12" x 18"? ;-)
When I finally got around to renovating the bathroom I discovered that
the joist spacing where the tiles were popping was 20" OC while the
rest of the floor was 16" OC. This also happened to be right in front
of the sink. So whoever framed the room put the least amount of
support under the area that probably needed the most.
I added an extra joist before replacing the subfloor and retiling.
You can try the Schluter water proofing method under the tiles, but if you
have that much water maybe you should slope the floor down to a drain and
add a "dam" across the door threshold to keep the water in.
*I painted my underlayment plywood with exterior primer and paint to prevent
water from being absorbed into it. Ideally you want to stop the water at
the tile surface. Use caulk around the backerboard edges. Use a grout
sealer or an epoxy grout such as Laticrete Spectra-Lok (My choice).
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