My husband and I just installed a 3x5 cast iron Kohler shower pan (K-9055) on top of level half-inch cement board. After finishing the drain installation, we realized that the pan moves very slightly (less than 1/8 of an inch) when someone steps on the corners against the back shower wall. Kohler instructions (see link below) indicate the walls connect to the shower pan with a bead of silicone caulk allowing for movement between the floor and the wall. Are these pans supposed to have some give/flex to them? Or will its slight movement eventually leak or damage the tile walls? Do we need to do something to stop this slight movement before we install the walls and tile?

http://www.us.kohler.com/onlinecatalog/pdf/1069570_2.pdf

My guess is that the cement board shouldn't be under there, it can give with weight on it