No, you're confusing the Kerdi membrane with a conventional shower construction. When using Kerdi, you only have a one layer sloped bed. Then the Kerdi, then the tile. You need their special drain to bond the membrane to. Unless you are building a commercial steam shower, the preferred medium on the walls is plain drywall. Kerdi is totally waterproof, and therefore, the drywall stays dry. You need to read the Kerdi handbook, and view the installation instruction videos on www.schluter.com. Another good place for help is www.johnbridge.com. The advantage of any surface membrane systtem is that the main structure of the shower is totally isolated from ever getting wet. Do it wrong, and you trap moisture, though. Understanding the methodology and following through is crucial to any shower build, but maybe more so when using a surface membrane since it assumes the stuff underneath will never get wet. It won't, if you do it right.