[arnav]

So now for the interesting / question part. You know the shower drain pipe you saw above? It is 4" high above the sub-floor. I want to avoid having to create a 5" high pre-slope mud bed. It is whole lot of concrete to bring up 3 floors up...
I am thinking to create a 3rd (and hopefully) final structure so that the mud bed for the pre-slope start higher. Attached are dry-fitted pics of what I am planning to do.
Note, only one sub-floor layer is shown.

The structure consists of:
- 2 x 3 "joists" parallel to the sub-floor joists below
- 2 "subfloor" layers of 3/4" BC Ply

This will allow the pre-slope to start right above shower drain pipe.

It is actually two separate structures (separated by the pipe). I am thinking to fill the space around the pipe with the pre-slope mud. The pre-slope itself will be 1" at the drain. Similar to the tub's raised platform, this structure is also merely resting on the over-built sub-floor.

Do you see any issues with plan / structure?