When I enlarged a bathroom closet to make room for a shower (this post) I thought it would be the perfect opportunity to use a new product called Kerdi-Board.  It takes the place of cementboard or drywall in a tile installation.  It’s not only a strong surface to tile over, but it is dimensionally stable, meaning that it doesn’t expand or contract or absorb water.  It’s also covered with the Kerdi waterproofing membrane so when you use it in shower it save a step to install the membrane.  You just have to do the corner pieces and the floor.

The large sheets come in various sizes.  You can use 1/2″ for walls with 16″ on-center studs.  The sheets that I used were 48″ x 96″.  It’s light, but because it’s extruded polystyrene foam with reinforcement on each side, it’s perfectly flat and straight and easily attached to framing or embedded in mortar, depending upon your project.  I love that it is covered with grid markings and it’s so easy to cut with a standard utility knife.  It’s easy to get perfectly cut corners and square openings.

The only downside to using Kerdi-Board is that it is expensive.  Each sheet is in the $80-90 range or so.  This is much more than cementboard, however, using it can easily save me at least a day of labor compared to installing cementboard and then adding the waterproofing.  You can install the Kerdi-Board and start tiling right away.

I did a sloped floor for the shower using sand topping mix and make sure it sloped at least 1/4″ per foot from the furthest point in the shower.  There are pre-formed shower floors that work fine, but this was an odd shape and the subfloor, though solid, wasn’t level.  I could fix that by making my own floor.

By the way, you can also use joint compound over the Kerdi-Board if you don’t tile an area.  For example, I used the panels for the entire shower wall, however, I only tile up to the showerhead or so.  Above this I mud and taped the corners and did a thin-coat of joint compound.  After a light sanding it’s ready to paint and prevents some sort of awkward transition between the tile and wall surfaces.

I enjoyed using the Kerdi-Board and will keep using it as appropriate projects arise.  It saved a lot of tile and mess and the client got a fully waterproofed and vapor-proof shower.