Ceramic and porcelain tile, more than almost any other surface material, needs a stable, flat, flex-free substrate. Even the slightest amount of movement in a building structure can telegraph to the tile and crack it. In addition, because showers and bathtubs are wet areas, they need a substrate that will not be damaged by moisture, just in case a crack in the tile or grout lets water through to the backer.
Technically called a cementitious backer unit (CBU), well-known brand names of tile backer include Durock, Denshield, HardieBacker, and WonderBoard. All are made of 100-percent inorganic materials that will not rot, shrink, delaminate, or decompose when exposed to moisture. There are several acceptable applications of cement board in the shower.
Cement Board and Plastic Sheeting
In this highly convenient, inexpensive, and popular application, a moisture barrier of 4- or 6-mil-thick plastic is installed directly over the wall studs. Then, the cement board is installed on top of the plastic sheeting. Screws secure the cement board to the wall studs. Tile is laid onto the cement board with thinset mortar or mastic.
Cement Board and a Liquid Membrane
In this application, cement board is installed directly onto the studs. No plastic sheeting goes behind the cement board. A liquid waterproofing membrane, such as RedGard or Hydro Ban, is applied to the cement board and allowed to cure. Then, tile is installed over the membrane.
Cement Board and a Sheet Membrane
Cement board is installed directly on the studs with no plastic sheeting behind it. Waterproof sheet membrane such as Schluter Kerdi is applied to the cement board with thinset adhesive. After the thinset has dried, tile is applied to the sheet membrane with thinset. Kerdi can also be installed directly over standard drywall in a shower because, when installed correctly, it creates a continuous waterproof barrier.
Membrane-Faced Board Only
One type of tile backer board is made with a water-resistant facing on both sides of the board. Georgia-Pacific's DensShield is one such product. The facing serves as an integrated waterproofing membrane, so you don't need to install a separate layer of plastic behind the tile backer or a sheet membrane over the backer. As with the other applications, thinset mortar is then applied to the board's surface, followed by tile and grout.
Unacceptable Shower Backer Boards
Several traditional tile installation methods used materials that are no longer considered acceptable for shower applications.
- Drywall: It takes just a little moisture for drywall's paper facing to disintegrate and turn moldy. Even a tiny amount of water introduced through a crack or hole in the tile will expand once it hits the moisture-hungry paper facing and gypsum core of the drywall. Because paper is an organic product, it will quickly become moldy.
- Greenboard: Greenboard's acceptability as a shower backer board is debatable. Greenboard is only slightly more water-resistant than plain drywall. Greenboard has drywall's same gypsum core and paper facing. However, the facing is impregnated with waxes that shed water better than conventional drywall's paper. Many local building codes do allow for greenboard as a tile substrate in showers. But with many non-organic shower backer boards available as alternatives, there is little reason to use greenboard.
- Plywood: Plywood alone cannot be used as a substrate under tile in showers. Some homeowners believe that painting or priming plywood will render it suitable to use as shower/tub backer board. This is not true. Since plywood is often used as an underlayment for floor tile, some do-it-yourselfers may believe that it can also be used in shower wall applications.