If you're looking to waterproof the floor, run the Ditra up to the shower pan, and cut it nice and uniform, leaving about an 1/8" gap, and then caulk it. Also, pick up a roll of Kerdiband to seal up the seams in the Ditra.

We recently got a Cultured Marble shower base. The advantage is that they can be created in custom colors. Though it is too early for me to start raving about it (because I have heard that maintainance is tricky) -- I am happy about the fact that they are single piece, solid surface (no leaks, no grout lines) and need no base preparation (vis-a-vis an acrylic base that needs the underside be prepared properly to avoid breaking). The downside is that they can be twice as expensive as the Acrylic ones. With custom sizing they will be even more so. You might want to get quotes from a couple of places. Do Google on "cultured shower pan" in your area. Hope this helps,

Thank you all for the informative posts! I think we may need to go "back to the drawing board" and re-think our plan…… Our current home is a fairly well built 20 yr old "spec" home in Boise, Idaho. I think the first thing we need to do is find out what the underlayment or decking is, and if it is substantial enough as-is, or if it will need to be changed. I can see this spiraling out of control quickly! What I had in mind (and I probably used the wrong term) was a pre-fab shower base, which we would have installed by a plumber. My husband would then install the proper cement board and tile the walls. We have had a shower like this in a previous home--although it was a smaller shower, so flexing might not have been much of an issue. We are trying to be budget conscious and also simplify the installation procedure. But we certainly won't take any shortcuts that could lead to issues later. And--the information about "the weight of an average North American user" made me smile!! Thanks again--I'm sure I'll be back with more questions!

Ditch the linear mosaic entirely and use the 2x2 in the niche. The 12x24 for the shower walls. Test that gray tile for dirt, lint, and high maintenance before committing to it. You might just want to use the Carrara for everything tile.