Slope Subflooring for linear drain

[kvn27]

Hi Guys, I've got to build a new shower pan from the bare studs. There are 3 points to keep in mind for this build. 1. I want a linear drain, with only one direction for drainage. 2. It is a non-standard shower size 3. I am trying to eliminate the need for a full heavy dry set mortar bed.

In order to address these 3 things, I was thinking about using the Linear Drain version of the Kerdi-Shower system until I found out their prefab foam base will not fit the size of my shower. So my next logical thought was to slope the Subfloor in one direction to the linear drain. I would need to Shim/sister the joists, and raise one end so that I get the 1/4 inch per foot drop needed., then lay down my 3/4 plywood and the rest. So here's my idea for the layers below. Please let me know if you can think of any concerns with this plan

Tile
Thinset
More Waterproofing Membrane (Most Likely Kerdi)
Thinset
Durock
Waterproof membrane(rubber liner or Kerdi/thinset or Laticrete/Redguard)
3/4 Plywood
Shim/sistering to create 1/4 per ft drop
Joists

My only REAL concern is this - I have been told that Dry Set Mortar Beds will "give" so that when the house swells/flexes, it will reduce the likelihood of popped tile or open grout lines.

Do you think that the 3/4 PLUS the Durock will keep it from flexing and causing issues? OR should I just go with Schluter Ditra on top of the Durock for added security?

Anyone out there experienced doing something like this?

Thanks for your help in advance,
Kevin

[cpad007]

What is the size of your shower such that Schluter's ramp wouldn't work for you? Their largest, I think, is 55" x 55".

USG has come out with a membrane system that competes with Schluter and I understand that they will build you a custom shower pan to fit what you need. It might be an option.

[jadnashua]

In theory, if the sloped subfloor is sufficiently strong, and you put a compatible substrate down on top of it, you could just use Kerdi on that substrate. Kerdi cannot be installed directly on plywood, and on the floor, you'd probably want to use a cbu embedded and attached per the manufacturer's instructions.

If you called Schluter, they MIGHT let you use a sheet of KerdiBoard thinsetted directly to the subfloor and laid like a huge tile. They may also have some thoughts on the minimum thickness panel that would work under those circumstances. KerdiBoard can be seamed and waterproofed easily with Kerdiband with either KerdiFix or thinset to hold the edges in place if the available sheet sizes couldn't do it in one piece. Once seamed properly, they are as waterproof as an unseamed sheet. I think that the Kerdi sheet would add more overall strength than the KerdiBoard, but that is an unfounded assumption...their tech support should be able to advise you with facts.