Thanks for the reply Jaz. I'm glad you chimed in because I had already bought a sheet of 3/4" pressure treated ply to replace the planks. The signs at Lowe's describing the purpose for each board and differences between them were terrible, they provided zero help in choosing the right board for the job. So I'll take that back and exchange for OSB exposure 1. If I remember right, those OSB boards said something like "wall/roof" and not floor so I skipped over them. I looked at every sheet of plywood in Lowes for a good while and none of them mentioned anything about being subfloor rated from what I recall. I was looking for that.
I was going to replace a couple of the planks instead of the entire subfloor but I couldn't find the right size planks and unsure about the type of wood to use. The planks are 5.5" x 3/4" thick, eastern spruce. All I saw at Lowe's were 1x6s, which would be too high in order to match the exact same height of my planks. Although at this point I am prepared to replace the entire darn subfloor, I was going to replace only the sections of the planks that are damaged but I had a question about how flat the first subfloor layer (sitting directly on the joists) needs to be. My solid planks are still cupped and warped a little, and a bit squeaky. Even if the first subfloor layer is not completely flat, wouldn't a 3/4 layer of ply above that provide a pretty solid and flat surface to tile on? I know the right answer is probably no, just tear it all out and replace everything, but I am curious about how flat the first subfloor layer needs to be below my 3/4" ply and cement backer board.
The plan for now is to have either planks or(probably) 3/4" ply for the first subfloor layer, then 3/4" ply on top of that, then 1/4" cement backer board. This should get me to the same floor height I had before, to meet the toilet flange. Yes I would definitely plan on thinset, taping and fastening the cement board to the ply below. I'm assuming both the subfloor layer replacing the planks and the next layer of ply on top of that should be OSB exposure 1 then, right? And I bought deck screws to fasten the plywood to the joists where I'm replacing the slats, does that sound like the right fastener for the job?
Now to jump back to the shower. My old tile didn't go up to the ceiling, there was a 2 foot section of drywall between the ceiling and the top of the tile. I'm tiling all the way up to the ceiling now. I have a couple questions about the edges of the tile and whether or not the tile can overlap the backer board and sit on some drywall too. It appears my old tile was doing this at the edges of the shower tile. As you can see from the picture below, where the drywall ends and the mudbed for the shower tile begins (where it used to be anyways), the old tile was overlapping and was set on the drywall. I assume it's ok to do the same thing and tile all the way to the edge of that 2" strip of drywall. Would this be correct, or do I need to tear that strip of drywall out and replace with backer board as well? Same goes for the section of drywall running across the top of the shower, I will be tiling that too. Just curious if that needs to be cement board as well or if that can just be tiled over the drywall since it's a small area. I know I should probably just replace it with cement board but I am curious and just want to know what can and can't be done for the future.
Also, what are your recommendations for waterproofing the area below the shower pan and the shower walls? There will be no benches or horizontal areas in the shower other than maybe a soap holder somewhere on the wall.
Cement board - Let me know what you think about these choices. For the floor I was planning on using the 1/4" hardie baord. Looks easy to work with and light weight. For the shower walls, 1/2" durock.