Shower waterproofing is the single most important part of a tile shower build. Get it right and the shower lasts for decades. Get it wrong and you are dealing with mold, rot, and a major repair within a few years. Schluter KERDI is one of the systems we use. Here is how it works.
What Is Schluter KERDI?
KERDI is a bonded waterproofing membrane by Schluter Systems. It is a fabric-reinforced polyethylene sheet that goes over the substrate and creates a continuous waterproof barrier behind the tile. The tile bonds directly to the membrane using Schluter-specified adhesive. Moisture that gets behind the tile hits the membrane and stops there.
KERDI is more expensive and takes more time to install than Go Board. On the right application it is a solid system. For most residential showers in Central Florida, Go Board is our first choice because it installs faster and is waterproof by design. But KERDI has its place, especially on complex applications where a sheet membrane is the better fit.
Step 1: Substrate Prep
KERDI goes over cement board, KERDI-Board, or other Schluter-approved substrates. The substrate needs to be flat, fastened correctly, and free of gaps. Fastener heads need to be flush. Gaps between panels get filled with adhesive before the membrane goes on.
Step 2: Treat Corners with KERDI-BAND
Every inside corner and transition in the shower gets pre-treated with KERDI-BAND before the field membrane goes up. Apply adhesive to the corner, embed the band centered on the joint, and skim adhesive over the top. This is where most DIY installs fail. The corners are the vulnerable points and they need to be addressed correctly before the field membrane covers them.
Step 3: Apply KERDI to Walls
Spread Schluter-specified adhesive with a V-notch trowel. Embed the KERDI sheet with a minimum 2-inch overlap at all seams. Press it flat with a grout float, working out any voids. The membrane needs full contact with the adhesive to work. Bubbles or unbonded areas are failure points.
Step 4: Shower Pan and KERDI-DRAIN
The shower pan needs to slope 1/4 inch per foot toward the drain. The KERDI membrane on the floor bonds up the walls a minimum of 2 inches and seals over the KERDI-DRAIN flange. The Schluter drain is designed to clamp the membrane between the drain body and the clamping ring, which creates a waterproof connection at the most critical point in the shower floor.
We also build mud pans with KERDI membrane on top and Schluter drains. A mud pan gives you more flexibility on the slope and floor shape. The KERDI goes on after the mud dries.
Step 5: Flood Test Before Tiling
Plug the drain, fill the pan to the curb height, and leave it for 24 hours. Check the ceiling below, the walls adjacent to the shower, and the floor around the base. No dampness anywhere means the system is holding and you can tile.
Never skip this step. Finding a leak before tile is a small problem. Finding it after costs significantly more to fix.
After the Flood Test: Tile
Once the system passes, tile goes over the KERDI using Schluter-approved adhesive. The tile bonds to the membrane, not to the substrate behind it. The grout lines are the last line of defense, but the membrane does the real waterproofing work.
For natural stone, make sure the adhesive is fully curing before grouting. For mitered corners on natural stone tile, epoxy the mitered corner for strength before grouting. It keeps the joint from opening up over time.