Water intrusion through the shower pan liner at sidewalls is the movement of water behind the shower pan liner and into the wall cavity.
The bathroom is one of the harshest environments in the home. Water battles with building materials that don't like water. The shower has leaks, splashing water, and large amounts of water vapor, all of which can easily cause wood to rot and deteriorate. In the presence of so much water, the backer board can't be made from materials that will rot and deteriorate. Neither standard paper-faced wallboard nor greenboard should be used as a backer board for tile in shower areas. They don't block moisture from entering the wall cavity; once they get wet, these types of backer boards will fail.
In contrast, water-resistant backer board resists moisture, protecting the wall from water intrusion. When combined with good installation and construction practices, water-resistant backer board is also more likely to resist mold growth. Installing building paper behind the backer board provides an extra layer of protection. Building paper acts as a drainage plane, creating a last line of defense against water intrusion and keeping the framing behind it dry.
Layering the liner with the backer board and building paper
Properly layering the shower pan liner with the backer board and building paper is important in preventing water intrusion at the sidewalls. If the backer board and building paper aren't correctly layered with the shower pan liner, water can enter the wall cavity. In addition, the shower pan liner has to extend far enough up the wall, so the backer board and building paper can be layered the correct amount over the liner. Too little or no layering may allow water to enter the wall cavity. When water intrudes behind the shower pan liner, it can wet framing materials and cause mold growth on the grout joints between the wall tiles. When wood framing gets wet and is unable to dry, it can rot and cause structural damage.
Cause 1:
Inadequate extension of liner up sidewalls
If the shower pan liner doesn't extend far enough up the sidewall, the backer board and building paper can't be layered the correct amount over the liner; without sufficient layering, water can enter the wall cavity at that location.
Solution
Extend the shower pan liner up the sidewalls of the shower a minimum of 9". Code typically requires the liner to extend 3" above the shower curb. Water that reaches the backer board or building paper will drain onto the liner and into the drain.
Cause 2:
Not using a water-resistant backer board
If standard wallboard or greenboard is used as the backer board, the paper facing can absorb moisture, deteriorating the board and allowing water to enter the wall cavity.
ProTip!When applying the tile against the backer board, make sure the thin-set mortar is troweled vertically, so water that gets behind the tile has a drainage path leading downward.
Solution
When water penetrates the grout between the wall tiles, it should meet a water barrier that stops moisture at the surface. Select a water-resistant backer board. The backer board should be made of a water-resistant material or have a fiberglass facing. Don't use wallboard or greenboard as the backer board.
Cause 3:
Improper layering of building paper and backer board over liner
If the building paper and backer board aren't layered over the shower pan liner by the correct amount, water that drains down the wall can reach the framing or floor sheathing, instead of being directed toward the drain. If the backer board touches a saturated mortar bed, it will wick water over time.
Solution
Install the building paper and backer board so that they overlap the liner by the recommended amount. The building paper should overlap the liner by approximately 2", and the backer board should extend down to within ½" of the mortar bed. The ½" gap minimizes the chance of water wicking up the backer board.
ProTip!When installing the backer board, the manufactured, uncut edge should be at the bottom.
Cause 4:
Poor corner folding and fastening techniques
If the corners of the shower pan liner aren't properly fastened to the framing, they're susceptible to water intrusion. Punctures that are too close to the bottom of the pan and improper folding techniques can create easy paths for water to reach the framing and subfloor.
Solution
Fold the corners according to the manufacturer's instructions. Don't cut the liner to create corners. Once a corner is folded properly, secure the fold with a roofing nail within ½" of the top.
ProTip!To easily attach the liner at the sidewalls, make sure there's 2x10 blocking between the studs.
