I've got a room with a two-layer plywood subfloor that is fairly flat (not level) but the floor ramps up at the perimeter fairly sharply. I'm going to be using Ditra for the installation (only for uncoupling - not worried about waterproofing). Rather than trying to raise the entire floor to the level of the perimeter, I was thinking of just leaving an 8" gap around the edge where I would not lay down any Ditra. Anyone tried this before?