I finally started doing the Kerdi waterproofing membrane on the shower. I’m always talking about it with people, so I tried to document the process a bit.
Above left, I’m test fitting the piece that will be used in the base of the shower. I’ve cut it so that it turns up 2″ around all the edges, and laps over the curb. It’s a simple circular cutout at the drain, because the special drain flange bonds directly to the membrane. Above right, I’m wetting the cement board and mortar bed so that all the moisture doesn’t get sucked out of the thinset. I trowelled the thinset mortar onto the cement board and base, then laid the membrane on top.
The membrane is carefully pressed into the thinset mortar (the waterproofing membrane has a fleece-like surface on both sides, so the thinset bonds to it). I use a wide putty knife (above left). An important point is you must use unmodified thinset mortar: The modified thinset mortars do not set up or cure between two layers of waterproofing membrane – that would be total disaster.
Above right I’ve installed the membrane onto the left wall also. At the base, I run the membrane over the top of the 2″ that laps up the sides. You can see a patch I had to put on the wall: I accidentally poked a hole into it while smoothing it (corner of the trowel dug in and tore a notch). The membrane is quite strong, but you have to be careful if the surface under the membrane is uneven.
I like to work from the base up, because I like the idea of the membrane layering over the piece below, but apparently it’s not necessary. The difficulty with the way I do it, is the bottom piece has to be carefully protected while I’m working on the sides.
Here’s a bit more of the process. I’ve troweled the thinset on the wall above left, then laid the kerdi membrane on and flattened it (above right).
Above left I’ve put the membrane on the right wall, and above right shows the waterproofing at the window. I’ve lapped it a bit onto the window frame, so that I can seal that joint. I don’t want water seeping between the edge of the membrane and the window frame. I’ll also add silicone caulk when I tile it, but apparently the thinset mortar will adhere to the vinyl window ok.
Totally unrelated, but I liked this Dilbert cartoon: Reminds me of someone I know.