We recently demolished this shower inside a semi-custom production home built in the 90's. On the left side you can see where the original builder of this home decided to install tile directly onto sheetrock. Those tiles fell off the wall during demo like butter. Interestingly enough, on the right side, they installed the tile over green-rock which did not allow any moisture to get through to the layer of sheetrock below. In fact, we could not even break the tiles off of the green-rock and the entire wall of tile stayed in tack. The fact that the builder used green-rock on at least one side shows that this builder knew at least one acceptable construction method and chose to cut at least one corner.
