From your description you say
the stud can be seen behind the
crumbling "plaster". Okay we need
to ascertain what the material is
behind the tile. Plaster or drywall
or concrete backer board. Lets rule
out some things here. If it's plaster
and its crumbling you have one of two
problems. 1) Moisture is causing the
plaster to crumble and "effloresce"
thus making the ability for the tile
difficult to maintain adhesion to the
wall. 2) Termites!! yep them nasty
little buggers that eat wood paper
and moisture barriers. Once a termite
eats the paper moisture barrier used behind scratchboard or wooden plaster lathe its just a matter of time before
the plaster disintegrates and the tile
falls off the wall. Now if Drywall
was used it has disintegrated cut and
dried and it could be moisture, or
termites and moisture combined.
There is no quick easy fix for this.
My suggestion is to make your decision
to renovate or repair the backer behind the tile. If it has gone bad in one
spot then its a matter of time before it all starts to fail.
Best Renovation is to Demo the shower!
remove all the tile, then whatever is backing the tile to the studs. Check
for leaks, moisture accumulation, rot
or termite damage. Use an icepick and
drive it into the studs and look for
paper-like tendencies in the stud. This
is indicative of dry rot or termite/carpenter ant infestation. Look
for light brown dusty, sand like powder.
If you see this you have a secondary
problem that could be indicative of a
much more destructive problem, TERMITES!
If you just find material failure, check
and remove all nails or screws. If you
do not have at least an 1 1/2 of wood
on either side of the corner studs add
a 2 x 2,3,4 inch nailer to catch the
corners for the backer board product
used. Now you must decide on the cost
vs wear/lifetime of the backer product used. Moisture proof Drywall used to be the norm costing about $5. to $8. more
than regular Drywall. If cost and peace
of mind is the issue use Duroc Cement
board. It lasts almost forever and gives
an excellent adhesion for thinset tile
cement. Install the Tile working from
a struck line using the showerhead center as mill edge line for running
tile try to work full tiles if it isn't
possible work from the Curtain side of the shower hold an 1 1/2 " for the tile
edge border and then work full tiles to
toward the corners. If you don't fall
on full tiles in the corner do not install the partial tile until you have
run the opposite wall to the other corner. You will find that this will
minimize wasted tile. Remember to Install spacers as you go to maintain
equal distance between tiles. Run all the full tiles. Leave out tiles where the plumbing fixtures make penetrations
through the tile. Cut tiles or drill
and nibble out for plumbing valves. Let
tiles set 24 hours before grouting. Let
grout dry and sponge off excess. Caulk
all corners and material changes such
as tile to Painted surfaces. Let set
up 24 hours. Paint any new surface
changes to fit your color patterns.