Fortunately we are seeing less copper pans being used for the shower pan when installing a new shower floor. Traditionally plumbers have installed the shower pan which most often was preformed from copper and many homeowners were reluctant to move forward to the newer rubberized membrane material being installed by a tile contractor.
Many times the copper shower pan is installed and the rough plumbing completed with little care given to the pan when working in the area. A screw or nail dropped then stepped on in this area could result in a pin hole which could lead to a shower floor leak. That is why the shower pan should always be flooded to check for any holes and cracks before proceeding with mudding the base of the shower floor and applying the tile to the shower floor
Copper is a poor choice for the shower pan because there is no pre pitch in the pan. This means the water in the pan needs to seek a certain level in order to drain through the weep holes rather then drain naturally. Water in the pan combined with cement on the base of the shower floor will have a negative reaction with the copper. Also the shower curb is not protected correctly when using copper because the pan cannot be molded and folded over to protect the horizontal surface of the curb. Most curbs installed in this way will rot out very quickly. Often when going through a shower renovation and the shower is taken apart, the curb is rotted, especially the lower part of the shower jamb. Copper pans also never seem to fit correctly, if the floor is not level or the walls are not square. They always seem to kink in the corners or at the face of the shower stall.
At one time when all showers were mudded this wasn’t a concern because the walls were blown out roughly ¾ inches. Then cement board started being used and this created a problem as the walls were not straight and you would have to shim your studs to get a plumb wall. The problem usually occurred on the fixture wall because the rough plumbing was already completed. If not corrected you end up with different size tiles leading to the top of the wall. Some installers fill this area with thinset which might correct the plumb situation but now the wall is out of square.
By using a waterproof membrane for a shower pan, you can address many of these issues. Membrane material can conform to any shape of your shower stall design and can be folded over to protect the shower curb and jamb. By having a tile contractor install the membrane (rather than the plumber) and installing the base of the shower soon after, the membrane is not exposed to any traffic which may result in any damages or holes. You also eliminate the blame game in having a tile contractor doing the entire base of your shower; therefore if the shower leaks only one person has worked on the shower floor, installing the membrane, installing the cement floor base and then installing the tile floor. Membrane material is also compatible with your cementitious base which will help to eliminate any negative reaction in the future. Membrane will also conform to your pre pitch shower floor allowing your weep holes to function correctly.