“Shower pan” is the term that refers to a properly built shower floor structure around a drain. The top layer of a shower pan is Portland cement that’s sloped toward the drain from all sides.
Making a proper shower pan is the most important step in making a solid, water tight shower. It seems easy to do, but unless done properly can lead to disasters with leaks and wandering water.
Begin with the drain as the drain is the only place for water to go in a shower. When the drain clogs, the water fills the shower and run onto the floor. Use a clamping ring drain. The top half works just like an ordinary drain and the bottom half provides backup drainage. Put the supporting flange of the drain directly on the plywood sub floor. It will be kept in place with mortar you install later.
Establish a sloped floor with a thin, latex-modified mortar bed. The minimum recommended sub floor slope is 1/4 inch per foot. After calculating the proper slope, fasten 3/4×3/4-inch float strips around the perimeter of the shower floor to serve as guides when installing the mortar bed.
A flexible, waterproof membrane goes over the mortar base. The membrane is a rubber-like material that’s waterproof. The membrane goes up the sides of the wall a few inches. One of the tricks is properly folding the liner in the corners.
Trowel out the latex-modified mortar. To shape the slope, ride over the float strips and the drain flange with a wood float and compact the mortar by tamping it with the float. Let the sloped mortar bed harden overnight before installing the shower pan membrane.
Most modern shower floors incorporate a flexible liner as a waterproof membrane beneath the tile and mortar. These liners are made of chlorinated polyethylene. The flexible liners conform to the various contours of the wood framing, bending upward at the walls and folding into corners. Wrap the liner completely over the shower curb and fasten on the other side.
When you are sure the fit is right, remove the membrane, loosely screw the membrane clamping bolts into the lower half of the drain and run a bead of rubber sealant around the lower drain flange. Put the membrane over the shower floor and lower it into place. Where the upturned membrane meets the shower curb, install prefabricated dam corners, available from the membrane manufacturer. Test the shower pan. Plug the drain with an expansion-type stopper and fill the pan with water up to the top of the curb. Now drain the water and install the mortar floor.
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