Showers are undoubtedly the most moisture-rich environment in the house. Water from the showerhead runs down walls, pools up in the shower pan, and remains long after the shower is done. So, it's critical that the shower be waterproof.
Whenever installing tile in any area of your house, you need a special substrate or base layer. In showers, the standard substrate is a tile backer like cement board or some type of sheet or liquid membrane that protects the shower stall's underlying structure from water.
What Cement Board Is
Cement board is a mineral-based board, usually 30 inches by 60 inches, that is used as a lower, dimensionally stable surface for installing ceramic tile, porcelain tile, or stonework. Cement board comes both in 1/4-inch-thick and 1/2-inch-thick versions.
Ceramic and porcelain tile, more than almost any other surface material, need 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, in case a crack in the tile or grout lets water through to the backer board and the shower stall's studs.
When water leaks behind shower tile, the results can be devastating. The leak rarely stops on its own, so more water leaks through the tile, weakening studs and creating mold.
All tile backer boards are made of 100-percent inorganic materials that will not rot, shrink, delaminate, or decompose when exposed to moisture. When selecting a tile backer board, make sure to get a thick-enough size—at least 1/2-inch thick—and always seal the tile backer board with a roll-on waterproofing membrane.
Best Cement Boards and Applications
There are several acceptable applications of cement board in the shower. All applications pair up the tile board with some kind of waterproofing material, whether a liquid membrane, plastic sheeting, an uncoupling membrane like Schluter Kerdi, or a board that is already faced with waterproofing.
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, and a roll-on waterproofing membrane is applied over the tile backer board. 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 rolled onto the cement board and allowed to cure. Then, tile is installed over the membrane.
Cement Board and a Sheet Membrane
In this popular application, a cement board is installed directly on the studs with no plastic sheeting behind it. Waterproof sheet membrane, also called uncoupling membrane, is applied to the cement board with thinset adhesive. One such brand of sheet membrane is Schluter Kerdi.
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.
Using cement board and a sheet membrane is one of the more expensive applications for waterproofing a shower. But it also offers the benefit of uncoupling the tile from the substrate, possibly mitigating tile or grout cracking.
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.
Worst Shower Backer Boards and Applications
Several traditional tile installation methods used materials that are no longer considered acceptable for shower applications. Other unacceptable applications—like drywall—were never used in the first place by professional tilers and contractors. They were mainly used improperly by do-it-yourselfers.
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. The solid gypsum core will then begin to crumble. Drywall has never been an acceptable backer board for shower tiles.
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.
While some local building codes do allow for greenboard as a tile substrate in showers, many other communities do not allow greenboard. But with many non-organic shower backer boards available as alternatives, there is little reason to use greenboard.
On top of it, greenboard is more difficult to find than ever since there are better, competing products available.
Plywood on its own cannot be used as a substrate under tile in showers. Some homeowners believe that painting or priming plywood with ordinary latex paint will render it suitable to use as shower or bathtub 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. With flooring in other areas of the home (other than bathrooms), moisture is less of an issue, if at all.