How to Install Moisture-Resistant Backing Board in a Shower

Backer board acts as the structural foundation for your shower tile, serving as a moisture-resistant barrier that keeps your wall studs dry. Unlike standard drywall, which will disintegrate when exposed to constant humidity, cement-based boards provide a rigid, inorganic surface that withstands the wet environment of a bathroom enclosure without warping or growing mold. Success in this project depends entirely on managing the gaps and seams. If you leave moisture a way to reach the studs, your tile work will eventually fail. Done well, this substrate will last for decades, providing a rock-solid, waterproof backbone for whatever tile or stone you choose to finish your shower wall with.

1. Measure and Cut Precisely. Measure the distance from the top of your tub flange or shower base to your desired wall height. Use a carbide-tipped scoring tool to cut the board to size, then snap it over a hard edge.
2. Position Boards Flush. Place the board against the studs, ensuring the factory-taped edges face each other for smoother seams. Leave a 1/8-inch gap between individual sheets to allow for minor structural movement.
3. Anchor Boards Securely. Drive cement board screws into the studs every 8 inches along the perimeter and in the field. Ensure the screw heads are flush with the surface of the board rather than countersunk, which weakens the bond.
4. Tape Every Joint. Lay alkali-resistant fiberglass mesh tape over every joint between the boards. Ensure the tape is centered over the seam so that it bridges both boards equally.
5. Fill Joints Flush. Apply a thin layer of modified thin-set mortar over the taped joints using a flat trowel. Push the mortar through the mesh so that it fully embeds the tape into the seam.
6. Seal Corners Smoothly. Fill the inside corners where the walls meet with a generous bead of high-quality silicone caulk instead of mortar. This allows the joint to flex slightly without cracking.