I have two problems in the bathroom (fixable ones, anyway, I hope).
The shower wall is grouted ceramic tile. Apparently because of some settling, the grout in one corner has cracked pretty badly and is falling out.
Should I chip the remaining grout out and re-grout it, or should I use some sort of caulk instead?
Other problem:
The floor is also a ceramic sort of tile (approx. 1 foot squares) with about 1/4" of grout between. Near the bathroom door, apparently the subfloor has flexed a little and the grout is deteriorating. Is there a way to repair this without pulling up the whole floor and shoring up the subfloor? (It seems like a pretty small flexing, but it’s doing quite a job on the grout.)
Thanks!
Where tiled walls meet one another, and likewise for the tile/tub joint, a premium grade kitchen and bath caulk is preferred to allow for expansion and contraction of the supporting substrates. For a repair job to have longevity, first remove all of the old material, ensure that surfaces are clean and free of bond-breakers such as soap scum, old caulk or grout, etc. Also make certain that you are not trapping moisture in the joint by allowing it to air for a day or so. Buy the best stuff you can find-typically around $5 or 6 per tube at the big box stores-I’ve had good results with GE and DAP. Finally, allow proper time for curing before wetting the new joint.
Regarding the floor, flexing and tile don’t get along well. Latex-modified grouts will forgive a tiny amount of motion, but a properly prepared subfloor-I prefer cementitious tile backer boards set in a combed bed of mud-is the only way to preclude grout and/or tile failure.
That’s exact what I was going to say. 
Peace,
mangeorge
What more is there to say? Thanks, folks!
Mr. Ujest just cleaned and re-grouted our tub after ten years.
We have hard water, even with a water softener, the rust stains on grout is rather groady. ( I clean with either with baking soda and then spraying on vineagar, then scrub with an old tooth brush. Or I just chemical it up with " The Works." Depending on if I am in my Earth Mother Save The Planet Mode or Fuck It And Your Little Dog Too mode.
My only advice, besides to have him do yours 
, was that when you are finished, you seal the grout by wiping lemon oil over it.
Advice was given to us by the Tile Guy who did our inlaws bathroom.