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 highly moist areas, they need different kinds of a substrate than with other tile applications. By way of comparison, tile on a relatively dry kitchen floor can be laid on plywood; tile on a shower surround cannot use bare plywood as substrate.
The answer is a ready-made building product called a shower backer board. Technically called a cementitious backer unit (CBU), well-known brand names include Durock, Denshield, Hardiebacker, and Wonderboard. All are made of 100-percent inorganic materials that will not rot, shrink, delaminate, or decompose.
Acceptable shower backer boards for both showers and bathtub surrounds include:
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 on the 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 on the studs. No plastic sheeting goes behind the cement board. A liquid membrane, such as Redgard or Hydroban, is applied to the cement board. After drying, tile is installed.
Cement Board and a Sheet Membrane
Cement board is installed directly on the studs with no plastic sheeting behind it. Sheet membrane such as Schluter Kerdi is applied to the cement board with thinset. After the thinset has dried, tile is applied to the sheet membrane with thinset. Sheet membrane plus cement board (and the Schluter Kerdi brand, in particular) is one of the most popular methods of creating tight, dry shower units.
Membrane-Faced Board Only
A certain type of shower backer board is available which integrates the properties of the cement board and the moisture barrier. Georgia-Pacific's Denshield is one such product. An acrylic coating forms a sealed moisture barrier on top of a glass mat backer board. As with the other applications, thinset mortar is then applied to the surface, followed by tile and grout.
Backer Board on Drywall Instead of Directly on Studs
All of the previous acceptable options are installed directly on the open studs. But if you happen to already have drywall in place, you can install these tile backers on existing drywall. Removing the drywall is preferable but not necessary. However, in most cases, you may want to remove the drywall anyway to access plumbing and to better control the thickness of the installation. Finally, removing existing drywall gives you a chance to assess and correct any water damage to studs and flooring.
Unacceptable Shower Backer Boards
- Drywall: Drywall as the only backer board for tile is not an option. The chance of moisture infiltration is too high to risk this. It takes 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. 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. Most 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.