What is backer board and why is it used behind wall tile in wet areas?

• A. A decorative board used for styling.
• B. Cementitious backer board provides a stiff, water-resistant substrate that resists swelling and provides a solid base for tile; improves bonding in wet areas.
• C. A plywood panel used for underlayment.
• D. A foam board used to insulate.

Answer: (B)

Backer board is a cementitious panel placed behind wall tile to provide a stiff, water-resistant substrate. In wet areas, this surface stays dimensionally stable when moisture is present, so it won’t swell, warp, or deteriorate like wood or drywall can. That stability creates a solid base for the thinset and tile, improving bonding and reducing the chance of tile or grout cracks over time. It’s specifically designed to handle moisture exposure, which is essential in showers and tub surrounds where tiles endure frequent moisture.

Other options aren’t suitable for this purpose: decorative boards aren’t built to bear tile or resist moisture; plywood can absorb water, swell, and rot in wet areas; foam boards don’t provide the necessary rigidity or bonding surface for tile.