Waterproofing Showers, Part 4: Sheet-Applied Membranes Done Right

The shower is the assembly that faces the most water management details in home construction. The penetration power of shower water is far greater than natural rain when soaps, oils, gels, and other surfactants are present. Water-management details, including sheet-applied membranes, are therefore crucial in a shower.


What You Need for Shower Waterproofing Using a Sheet-Applied Membrane

In the last twenty years, sheet-applied waterproofing membranes have become more popular, with several manufacturers on the market.

A waterproofing system can be cobbled together using a combination of brands, but I recommend going the complete-kit route for the full benefit of the manufacturer’s warranty.

Make sure you read and follow the instructions included with the kit, as well as the warranty. Many of these companies provide great warranties, and some are even better if you use one of their certified installers.


Doing It Right: Correct Installation for Sheet-Applied Waterproofing Membranes

In order to properly install a sheet-applied waterproofing membrane, the following things must be considered:

  • Install approved waterproofing backer-board on wall framing no larger than 16 inches on center. Fasten with appropriately sized, noncorrosive fasteners in the proper nailing pattern, usually 6 inches on center.
  • When installing a custom tile pan, make sure it is pre-sloped; some manufacturers offer pre-sloped foam panel kits. In other cases, dry-pack mortar must be used to create the slope.
  • Fit built-in seats with a fall of at least 14 to 12 inches per foot towards the drain.
  • Apply the membrane to the substrate using the manufacturer’s recommended thinset (some call for unmodified thinset) and a grooved trowel, such as a 1/8-inch square-notch trowel.
  • Prefabricate corner pieces for all inside and outside corners and for the seat, niches, and curb. You may need to find a supplier who makes these, because not all waterproof membrane manufacturers do.
  • Make sure that custom escutcheons made for pipe and valve penetrations are installed properly.
  • Make sure the membrane is properly integrated with the manufacturer-approved drain. Make sure that all seams and joints have a minimum 2-inch overlap, and make sure there are no air bubbles under the membrane.
  • When the shower is going on a prefabricated pan or the walls of a bathtub, lap the membrane over the flange of the pan or tub and seal it to the flange with a manufacturer-approved sealant.

If the shower cannot be used before installing the tile or solid surface finish without leaking, it is not ready for tile installation or solid surface finishing. The shower can be used with tiles and water tight waterproof membranes. Although it might not be attractive, it would be functional.