Whether you're laying down a new tile floor or installing a shower, you may use a sheathing material such as backerboard. Although most backerboards currently available won't fall apart when exposed to moisture and are somewhat water-resistant, they aren't waterproof. To prevent moisture from seeping through to the wood or metal studs beneath the tile, a barrier or sealant must be used on porous materials.
To Seal or Not To Seal
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Contrary to popular thought, tile and grout are not waterproof, and some moisture will penetrate even if sealant is used. This makes it crucial to protect the materials underneath from water and vapor damage. Some materials, such as fiberglass-coated gypsum board, allow you to dispense with using any kind of sealant or even a moisture barrier to keep things dry underneath the tile, except for the areas where the board is screwed into the studs. However, if you choose to use concrete backerboard, which is much stronger and more durable than gypsum board, a water vapor membrane must be placed underneath it or a sealant applied on top of it.
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For the floor and sides of showers, as well as tile floors in other areas of the home, using a membrane underneath the backerboard may be your best bet to make absolutely sure moisture doesn't get through to the studs underneath. If you decide to go this route, no sealant will need to be applied on top of the backerboard, as this could trap moisture between the two layers of waterproofing.
Should you decide to use sealant over backerboard, there are a few points to keep in mind. Liquid sealants must be applied thickly enough over the entire surface to provide complete coverage, which means multiple coats. As with any porous surface, liquids can be soaked up and leave some areas unprotected unless several layers are laid down. When this sort of system is used in the shower, plan on treating the entire area to prevent water from getting behind the walls or beneath the floor. If sheeting is used, seams must be fitted together in such a way that leakage doesn't occur, and must be bonded appropriately to the underlying surface. Certain systems anticipate these problems and have developed products to address hard-to-treat areas such as corners and drains, so check to see what's available. For shower benches or Jacuzzi frames constructed from wood with backerboard screwed on top, membranes like these can provide plenty of protection from moisture.