Cross Section of Typical Shower Membrane Installation

If you have ever gone swimming inside an above ground pool, you will easily understand the concept of a membrane shower pan liner. The vinyl swimming pool liner does a great job of keeping water inside the pool. You want your shower pan liner to act in the same manner and keep water within the confines of the liner and subsequently direct it to the shower drain.

The drawing below shows an interesting detail that many plumbers and tile setters fail to do.

Note how the top of the clamping ring drain assembly is flush with the top of the sub-floor plywood. In order to achieve this, the plumber (or jobsite carpenter) needs to rout out a recessed area in the actual sub-floor. Most plumbers don't carry routers in their truck, unfortunately! I used to go to this detail since I had all of the tools. This is an important step. If you don't do this, about 3/16 inch of standing water will always be present in the pan.

Note the top of the adjustable shower drain inlet that rises above the subfloor. This part of the drain assembly screws up and down in order to compensate for the thickness of the cement mud base and the thickness of the finished tile. It is critical to have this drain part in place when you pour the cement mud base. For sake of illustration, I have not shown the cement base or the pea gravel that sits on top of the CPE liner.

Before the mud hardens, you screw it up and down to cut threads into the soft cement mixture. I always make sure that the cement mud mixture is poured below the top of the finished drain an amount equal to the finished thickness of the ceramic tile. You really don't want to crank the drain up and down once the cement mud has dried. Don't forget to install the washed pea gravel above the drain holes in the drain assembly. If you forget this, you may develop a leak.