After demolishing my old shower, I found nine tongue in groove floorboards that were partially rotten. I've attached a pdf diagram of the project. Looking at the diagram, I was able to cut the old floorboards back to the left. I went back to the sill on the left (not shown) so I can nail the new boards to a good surface there. I can't cut the floorboards back to the joist on the right unless I pull out the bathtub in an adjoining bathroom.
So, here's my idea:
1. Replace tongue in groove floorboards so the new boards (on left of diagram) butt up against the old ones coming from the right.
2. Screw another layer of tongue in groove boards into both the old and the new floorboards (working from the crawlspace below). Offset these so each lower board can be screwed into two upper boards. This means I'll have 10 boards below and 9 boards above.
3. Screw 2x4 nailers into the joists to provide some support to the lower layer of tongue in groove.
I know that you might suggest I do the above in reverse order (i.e. 3-2-1), but I'm afraid that doing it that way would either result in a gap between the top and bottom floorboards (if I put the nailer a little too low) or would result in a bulge in the upper floorboards (if I put the nailer a little too high). Also would be tempting to install two concrete piers with posts and a new joist running between the old ones. However, there's too many pipes in the way to do this and the seam between the old and new tongue in groove is not straight (i.e. I would need multiple new joists). I can't make the seam straight unless I start cutting boards out from under the bathtub.
a. Is there a better way to do this?
b. If this proposal looks ok, should I use tongue in groove for the lower layer or maybe treated 2 x 6's?
c. After all of the subfloor is repaired, should I cover the tongue in groove with pressboard (the floor that I didn't rip up has pressboard now) or should I rip out all of the old pressboard and replace it with plywood?
I've hired a good plumber to install a new shower and toilet so I'm hoping dealing with rotten wood will be a one time thing. I'll be installing a new vinyl floor.