Noticed some grout voids in shower tile, scraped away to re-grout and found bottom row of tiles had some wiggle. Figured I should assess underlying cause and removed tiles.
What was under the tiles was a dry-papery substance and behind that some course powdery bits, maybe just dried out mastic. House was built in the 70's, not sure of conventions at the time or if it was tiled since.
The gap shown seems problematic, but do I need to cut away material and apply new backerboard, or should I just put a little moisture barrier over this section to protect the gap? I'd like to avoid re-tiling the whole shower for now as will be doing that in the next few years, but also want to fix it properly so that it will last a while. Any thoughts?