Q:
In Tom Meehan’s article “A Sloping Floor for a Barrier-Free Bath” (FHB #185, or www.finehomebuilding.com), why does he use cement backerboard on the walls? With the Kerdi waterproof membrane, why not use plywood or gypsum board?
Deric Knowles, Deep River, None
A:
Tom Meehan, owner of Cape Cod Tile Works, replies: I don’t use plywood for a couple of reasons. It can move too much, and if it somehow gets wet, the wet wood can attract bugs, mold, and rot. I also don’t like drywall (even moisture-resistant drywall) in moisture-prone areas because if it gets wet, it becomes crumbly and moldy. The Kerdi membrane is used to protect the walls from moisture from the shower, but what if the moisture source is inside the walls, such as pipes that become damp with condensation? Then the drywall or plywood can soak it up and possibly rot.
For these reasons, I prefer to use cementboard, which doesn’t move, can absorb some moisture without becoming a food source for ants or mold, and provides a solid, stable base to keep the tile from moving and the grout from cracking. Finally, the extra cost for cementboard is minimal considering the extra quality.
