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Unread 04-23-2021, 09:38 PM   #12
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,381
Whenever I've seen the Schluter guys install Kerdi, when embedding the material, they end up squeegeeing some thinset out whenever they get to an edge. I noticed during the video, that the installer usually stopped at about the edge of the membrane, and if you look carefully when pulling an edge back, right at the edges there were some small areas not covered. That could easily have been filled in with subsequent activities, but any small gap over time could let moisture penetrate. Kerdi works when you have fleece to fleece contact, filled in with thinset. An excess of thinset without the fleece layers in contact will wick moisture.

Another thing that they stress in their classes is how important it is to wipe the surface with a wet sponge prior to spreading the thinset to ensure that it doesn't get moisture sucked out of it, making it stiffer than desired when then trying to embed the membrane. That would affect how fluid it is, and maybe end up with it being a bit thicker layer and preventing the fleece to fleece contact. That's only important when going over the drywall, as the fleece itself won't suck moisture when adding the banding, but could be an issue where there already was some thinset from the previous to have fresh stuff whenever you're embedding stuff. YOu also see them use a sponge to clean up excess thinset, but maybe that's just for esthetics rather than function.

From what I was told, Schluter in Plattsburg had a tower lined with Kerdi, then filled with water in their lobby for a never leaked and the water pressure from the tower was more than you'd ever see in a shower (about 0.43#/foot depth).

I've not used the Laticrete drain. The surface may specifically be treated or manufactured to accept thinset. Keep in mind that when thinset cures, the way it gets its bond is because the cement grows crystalline spikes that interlock into any microscopic imperfections in the surface. And, that process continues past the nominal 28-days the cement specs use for testing strength, but by then, their growth curve is almost flat. IOW, the bond will continue to get stronger and the density will increase, which increases the water tightness of the seams (and any tile attached later in the process).
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.

Last edited by jadnashua; 04-23-2021 at 10:11 PM.
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