How to Waterproof Under Thinset


If you're planning a do-it-yourself job of tiling the shower or the kitchen, and you know there will be plenty of water on the tiles, it is essential that you properly waterproof the substrate before laying thinset mortar and setting the tiles. There are various ways to waterproof under thinset mortar, but one of the simplest and surest ways is to first install a waterproofing membrane, then use cement-based backer board for the tile substrate.

Things You'll Need

  • Vacuum
  • Hammer
  • Waterproof membrane or roofing felt
  • Staple gun
  • Cement backer board
  • Utility knife
  • Drill
  • Backer board screws
  • Silicone sealant tubes
  • Vacuum the work surface to remove any dust or particles, and use a hammer to level any protruding nails.

  • Spread sheets of waterproof membrane or roofing felt over the area to be tiled, overlapping joints of the membrane by about 4 inches. Use a staple gun to secure it to the floor or wall studs.

  • Make mitered "hospital corners" for creating a tight, 90-degree corner in the membrane, where needed.

  • Lay sheets of cement backer board over the membrane, leaving around 1/4-inch gap between boards. Use a utility knife to score the boards to the desired size and snap them loose.

  • Secure the backer board to the wall or floor with backer board screws driven through the boards every 8 inches on center.

  • Bead a line of silicone sealant into the gaps between the backer board and leave it to cure as indicated on the sealant tubes. Using tubes, rather than canisters, allows greater precision of application. You are now ready to apply thinset and begin tiling.

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