Superior Tile Substrate
Updated: Mar 21, 2018
For more than 40 years, cement backer board has been the go-to substrate for setting tile, and it’s easy to see why: Backer board is strong, impact-resistant, and less susceptible to water damage than moisture-resistant drywall. And its cementitious constitution provides an excellent bonding surface for thin-set mortars.
However, you may be surprised to learn that cement backerboard is not waterproof. In fact, when installed in wet environments—i.e.: shower stalls and tub surrounds—it’s necessary to apply a waterproof coating to the backer board prior to setting the tiles. So, when I was preparing to tile our bathroom, I eschewed cement backer board and installed a new product from Schluter Systems called, Kerdi-Board.
Kerdi-Board is made from rigid extruded polystyrene, which makes it extremely lightweight, easy to cut and, most importantly, 100% waterproof. Each panel is covered with a cement-free reinforcement layer that’s topped with an anchoring fleece that provides a gripping surface for the mortar. The panels come in thicknesses ranging from 3/16 in. to 2 in. For my bath remodel, I used ½-in.-thick x 4-ft. x 8-ft. panels. The 1/2-in. Kerdi-Board is also available in two other sizes: 32 in. x 48 in. and 48 in. x 64 in., which are designed for standard tub surround installations.
I loved working with Kerdi-Board and found it extremely easy to install. Besides being lightweight, it cuts quickly with a utility knife and produces no dust or mess. (Plus, I love its bright-orange color!) Here’s a brief description of the installation process. For more detailed information, contact Schluter Systems.
Measure the wall space and cut a piece of Kerdi-Board about ¼ in. smaller, so it’ll fit without binding. Set the panel into place and use a cordless drill/driver, or better yet, an impact driver to secure the panel to the wall studs using metal washers and 1 5/8-in. galvanized screws (shown below, right). Space the screws 16 in. apart, and drive each one just deep enough to compress the panel’s surface and create a shallow circular dimple. Install the remaining Kerdi-Board panels in a similar manner.
Use scissors to cut one 5-in.-sq. piece of Kerdi-Band fabric for each exposed screw head. Next, use a notched trowel to spread thin-set mortar over the screw head. Press the fabric square into the mortar (below), then embed it with the 6-in. putty knife (below, right). Finish by smoothing the mortar patch with a damp sponge.
After securing the Kerdi-Board panels, Kerdi-Band fabric is used to seal all corners and seams between the polystyrene panels, creating a waterproof installation.
At this point you might be thinking that Kerdi-Board is just too good to be true, where's the catch? Well, there's no catch, per se, but it does cost considerably more than standard cement backerboard. To be precise, 1/2-in. Kerdi-Board costs about three times more: $3 per sq. ft. as compared to $1 per sq. ft. for cement backer board. Is it worth it? That's for you to decide, but for the higher upfront cost you get a product that's much easier and neater to work with, and it creates a totally waterproof, longer-lasting installation.