Tile contractor Tom Bouchard shows how to make a tile-ready, watertight sloped shower pan with a linear drain using Schluter Kerdi-Board, Kerdi membrane, and thinset mortar.
The Kerdi-Line linear drain comes with Kerdi membrane already attached. Here, Boucher test-fits the drain into the hole in the floor.
The drain comes with a foam-plastic gasket tray that fits between the stainless steel drain and the plywood subfloor, giving even, firm support to the drain. Here, Bouchet checks the fit of the tray around the drain and next to the wall.
Boucher will use a rubber Fernco coupling to link the stainless steel drain to the PVC house drain installed by the plumbers on the job. Here, he fits the coupling to the Kerdi-Line drain.
To splice into the stubbed-up drain pipe, Boucher first has to mark and cut a short section of drain and attach it to a collar. Here, he marks the drain pipe for the cut.
Boucher uses an abrasive blade and a cordless grinder to cut his section of PVC drain pipe to length. It's not the perfect tool for the job, but it works and it saves a trip back to the truck.
After gluing a a repair coupling to the stub of drain pipe, Boucher uses a Fernco rubber coupling to attach the plastic drain assembly to the linear drain. Usually this step is less complicated, because the plumbers would ordinarily leave the stubbed-up drain pipe dry-fitted into the P trap for the drain, so that Boucher could simply cut that stubbed-up pipe to length, attach it to the linear drain, and then glue the plastic pipe into the P trap for a permanent installation.
After equipping the linear drain with its plastic drain pipe, Boucher tests the assembly's fit into the intended location in the shower floor.
"This is a one-shot deal," comments Boucher as he uses a small saw blade on a cordless drill to trim the end of the stubbed-up pipe from the P trap at the rough plumbing connection point. "Once you glue it, you’re done. So you have to make sure it’s right."
A tiny saw-blade bit chucked into the cordless drill is the right tool for trimming about an eighth of an inch off the end of the stubbed-up drain pipe to enable the linear drain to sit comfortably in place. This step isn't usually required — next time, Boucher says, the plumbers will know to leave this stubbed-up pipe dry-fit into the P trap so that Boucher can cut the pipe to the proper length before gluing that joint as a final step when he sets the drain assembly.
Bob Vedder holds the linear drain assembly as Tom Boucher does a little last-minute trimming on the stubbed-up drain pipe.
After making sure that the lengths will work, Boucher applies primer to the plastic pipe joint where the repair coupling splices into the linear drain.
Boucher sets the repair coupling in its place. The coupling will still need to be glued onto the stubbed-up drain under the floor when the linear drain is set in place. First, however, Boucher must prepare the shower floor.
Thoroughly washing the sub-floor is a key step that's required before the Kerdi-Board underlayment is mortared into place. This removes dust from the floor that would interfere with the mortar bond, and it also wets the subfloor a little so that the wood won't suck too much moisture out of the mortar before the mortar has a chance to set up and cure.
Mortaring the joint is a two-step process. Here, Boucher applies a thin coating of mortar to the plywood and works it thoroughly into the substrate to ensure a good bond between the mortar and the wood fibers.
Next, Boucher applies a measured thickness of mortar to the subfloor using a special Kerdi trowel with one-eighth-inch notches.
The Kerdi-Drain kit includes a polystyrene foam support that needs to be mortared to the floor before the drain is set in place. Here, Boucher sets the foam support into position after applying mortar to the floor.
Kerdi-Board comes in standard sizes, with a fold that allows the material to fit into its package. Here's a look at the underside of the material.
After placing the foam support piece for the drain, Boucher places his piece of underlayment onto the mortar bed. The waterproof polyethylene top face of the board is flocked to provide a bonding surface for the tile mortar bed that Bob Vedder will later place for the shower floor tile.
Boucher places a cut piece of the cardboard package onto the Kerdi-Board, then tromps down the piece to work it into the mortar bed.
Boucher pulls up the piece of Kerdi-Board underlayment to inspect the underside for proper mortar coverage. If he sees any thin areas, he'll place a little more mortar onto the floor at those locations before setting the Kerdi-Board back in place.
In this case, the Kerdi-Line linear drain and its foam support piece are a little shorter than the width of the shower. So Boucher cuts two small pieces of Kerdi-Board and mortars them onto the floor at the ends of the drain support to fill the gaps.
Boucher mortars the small piece of foam into the space at the end of the foam drain support.
In this example, the integrated Kerdi waterproofing flange for the linear drain will have to be folded into the corner of the shower wall. Here, Boucher cuts the flange with a scissors so that he can make that fold. Later, the cut location will be covered by a molded Kerdi inside corner piece.
Boucher applies mortar to the Kerdi-Board before setting the drain. This will bond the Kerdi flange of the linear drain to the underlayment board for a watertight joint. As always for mortar joints between two impervious surfaces, Boucher uses unmodified mortar for this joint.
Boucher applies PVC primer to the repair coupling, which will now be glued to the stubbed-up pipe under the floor as the drain is set into its final position.
Boucher applies PVC cement to the joint as he prepares to set the drain into place permanently.
"Moment of truth," says Boucher as he presses the drain into its final position, holding it firmly for 30 seconds to allow the PVC cement to set up.
Using a six-inch drywall finishing knife with the sharp corners ground off, Boucher gently presses the Kerdi flange of the drain into the fresh mortar, removing a small amount of squeeze-out. Once the mortar is fully set and cured, says Boucher, this bond will be water-tight and virtually impossible to separate.
Boucher applies mortar to the inside corner of the shower wall and floor. Here he will place a pre-molded piece of Kerdi membrane to cover the corner cut in the drain's integrated Kerdi where the drain piece was trimmed for folding up the wall.
With the corner pieces of Kerdi mortared into place, the shower drain is now waterproof. Boucher still needs to apply pieces of Kerdi to the rest of the joints between the shower floor and the surrounding walls. But once that's done, he says, "You could turn the shower on and this wall would be fine. This shower is now water-tight."