When we started sawing out the grout to regrout – tiles started falling off revealing two materials to which tile had been adhered. Upper wall material is green. Lower wall material is white. White and green wall materials don’t blend smoothly – and tile was originally applied right over the mismatched wall surface so that you can see straight tile then a slant back section then straight again on a new plane. Possibly it was just a tub with a half wall/shelf and they added the upper wall for a shower. Shower over tub with walls and ceiling including roofline slant all tiled (old farmhouse second floor bathroom). Worst area is the wall on which the faucets and shower are attached – that is where we started. Roofline slant seems to disallow a shower enclosure – but I could be wrong about that. Not sure how to proceed from here (need expert advice). Hate to tear off all the tile (most of which feels firmly in place) IF there is a way to make a long term fix without doing so.