I'm filling in a sunken roman tub and have a couple of question about the best practice for the slab. I'll use sand for the bottom fill up to where I'll start the concrete slab which will be around 4" thick with reinforced with rebar anchored into the walls, then covered with a mud deck, covered by either kerdi or noble's membrane on the floor with probably hydro ban for the walls.
1) What should I place between the bottom sand and concrete slab? gravel?? I'm assuming I should worry about moisture moving in to out of the slab.
2) The shower will be rectangle with noble freestyle drain running longways in the center of the rectangle. Is it OK to just use the mud deck to create the slope from he drain to the walls? I figure it will be 1 1/4" at the drain rising higher to meet the wall and maintain proper slope. If so, would it suffice to use visqueen between the concrete and mud deck for moisture control?