How to Install Mortar Shower Pan Membrane LinerOnce you've framed the shower stall, it is time to install the pan liner and cement backer board to create a water tight seal.Remove the drain cap and tape over the drain to keep mortar and other debris from entering the drain. Put down a layer of felt paper to help prevent moisture from wicking down through to the subfloor. Place a layer of metal lathe over the felt paper. Trowel in a layer of mortar that it has a one quarter inch slope towards the drain. Once the mortar dries, place a layer of felt paper along the walls of the shower stall. Attach cement board with the rough side out along the top half of the walls. Install the shower stall membrane in the bottom of the shower so that it overlaps in the corners, covers the shower curb and is approximately six inches up the side of the walls. Cut out holes for the drain, the drain bolts and the weep holes. Seal the underside of the membrane near the drain with silicone. Nail the membrane that covers the curb only on the outside. Attach the remaining cement board leaving one inch clearance from the bottom. Caulk all seems. Place some pebbles near the weep holes to protect the openings. Trowel in a layer of mortar that has a one quarter inch slope towards the drain. Mortar the curb. Remove the tape from the drain.