When we bought this house in 2003, it was not exactly our dream home, but we needed a place to live, and it was one of the few reasonably priced houses for sale in the area. (Reasonable for Oxford; it was far less quality and aesthetics compared to our former home in Texas, yet at a higher cost). There were certainly things--like the shower--that gave us pause in terms of looks, yet we had no idea that the problems were more than skin deep.
It was actually a piece of good fortune that a water leak in the shower wall occurred. When we pulled the wall away to repair what we thought was just a leak in a pipe, we discovered major problems. It was clear that we would have to rebuild the shower in order to repair the damage in the structural supports for the walls. If it looked like this on the outside, we could only imagine beneath it.
When we began to tear out the tile, we discovered the reason: Will had built the floor of the shower by pouring mortar on top of wood--no shower pan liner. I don't imagine I have to tell most folks what happens if you subject wood to moisture over a period of 3 years.
When we rebuilt, we used the Kerdi system--the choice of pros. Underneath this waterproof kerdi liner is a waterproof plastic floor, and that is what sits on top of the wooden floor, Will. It prevents moisture from seeping through the grout and reaching the wood. You can "seal" grout, but it isn't waterproof.
After removing the crumbling remains of the floor under the mortar, we were horrified to see what we had been standing on. Prior to getting this removed and replaced, I actually did step through it in one place, and ended up with one leg up to my hip and one shoulder under the house, and no way to get myself out of the hole. I do confess, it was pretty funny, even though painful, and Randy had to come haul me out as there was no way for me to gain purchase on my own. Score: Will, another one. Susan, 0.
Yes, you use the kerdi liner on the walls on top of the shower board, and use "thin set" to attach the tiles. You also start one tile row from the bottom so you can have it level, using a board as the guide and support for the tile. That row on the floor goes in last.
Here's a concept, Will: lay the floor tile flat, and space evenly in between them.
This is our shelf. Build the shelf correctly, like a shelf, not a hole in the shower board, support it, cover it with the waterproof liner--did I mention we love the Kerdi system?--and then tile over it.
Note the mortar all over the tiles, and the unevenness.
You see, Will, there is a reason that building supply stores sell these little spacers for tile. It allows you to avoid just "eyeballing" it when you tile, and it makes the tile line up evenly.