Working on a tight budget here, I'm trying to repair a small, walk-in curbed shower on the ground floor of our home in Texas (no basement). The grout in the curb tiles had cracked, the tiles themselves seemed suspiciously flexible and poorly supported, and the grout in the shower wall tiles close to the floor had cracked as well. I decided to remove the tiles on the outside of the curb, and found the lumber under the tiles completely rotted.
I removed the outfacing tiles and rotted 2x4s. After cleaning this out this mess (picture 1), I noticed that the lumber had apparently been fixed by nailing through the vertical section of liner in the original install (probably 40 years previous). These nails would thus have punctured the liner in the curb area. This seems bad practice to me. Further, in pulling the liner back from the curb tiles facing the shower interior for inspection, I noticed standing water. To be clear, this is water on top of the liner, between it and the overlying mortar and shower floor tile (picture 2).
My question: I know that water will find its way through the tiles (and that this is the purpose of the liner), but am wondering why this water apparently has nowhere to go. Can I simply repair the curb with new lumber, replace tiles, regrout and be done with it? Or does the fact that water is sitting there (it doesn't seem to be draining) mean that the shower pan either has an insufficient grade, or that there is some other problem that's preventing its exit? Do I need to rip out the entire floor pan, wall tiles, etc.?
Thanks very much for any wisdom, in advance.