Tile isn't a waterproof surface and neither is the mudbase, so the membrane must be sloped toward the drain to keep water from collecting. To add the slope, first a layer of modified thinset is spread on the subfloor as a bonding agent.
Spread a layer of dry-pack mortar (four parts sand to one part Portland cement, and enough water so that it compacts in your hand), pitching it toward the drain at a slope of about 1/4 inch per foot. Pack and screed the mortar to eliminate flat or hollow spots.
The final step before cutting and fitting the membrane is applying an elastomeric sealant, such as NobleSealant 150, around the drain assembly. This will create a seal between the membrane and the drain.
Always use a waterproofing membrane specifically meant for shower floors. When measuring the membrane, be sure there’s enough to go up the walls at least 2 inches above the threshold around the perimeter of the shower. There also needs to be enough material to wrap over the top and down the outside face of the threshold. Place the membrane in the shower with the same overlap on all sides.
In the corners, neatly fold the membrane flat against the framing. Attach the membrane to the framing with nails, at least 2 inches above the top of the threshold.
To connect the membrane to the drain, carefully cut the membrane from around the drain-assembly bolts, always cutting toward the middle of the drain.
Cut out the center of the membrane by letting the knife follow the inside ring of the drain. Then press the membrane into the sealant that was spread earlier.
Slip the clamping ring into place and tighten the bolts incrementally, alternating sides until they are all tight.
The membrane needs to be cut where it transitions from the vertical plane of the walls to the horizontal plane of the threshold. This area demands close attention to keep it waterproof. The first step is to cut the membrane tight to where the top of the threshold meets the jamb framing, allowing the membrane to wrap over the top of the threshold.
Carefully fold the flap inside the shower and attach it to the framing with a nail 2 inches above the threshold. Nail the membrane to the outside of the threshold. No nails should be driven on the inside or through the top of the threshold.
Apply a generous bead of sealant to the joint underneath, as well as to the overlapping piece and then press it into place to seal the corner.
When the membrane is installed and all the joints are appropriately sealed, insert an expansion plug into the drain and dump about 10 gallons of water into the shower pan. Let the pan sit filled with water for three to four hours. Carefully inspect the drain and the floor below the shower for leaks before proceeding to the next step.