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Unread 05-31-2014, 11:45 AM   #1
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Thinset too thick for Kerdi?? Ditra Set??

Remodeling my shower and used the Schluter shower kit which I'm pleased with. Installaion was easy and the leak test passed. So now I'm onto tiling the walls. I have 1/2 inch Mold Tough Sheetrock (which Schluter said was OK to use) , glued and screwed over 16 inch OC studs. Over that is the Kerdi fabric set with Bostik Ditra Set (unmodified thinset). I'm using 12 x 24 inch porcelain tiles using 1/2 x 1/2 inch trowel and using the same Ditra Set thinset between the tile and Kerdi. Walls are flat and plumb. Per the tile manufacturer, I'm offsetting the tiles at 1/3 ratio to minimize lippage. Offsetting is a real pain and I don't think I'll use tiles this large in the future. So far so good ( I hope).

Only issue with the Kerdi is the build up of fabric and thinset at the interesections, especially where three surfaces meet.

Using the same method of setting kitchen cabinets where you start at the highest point on the slope of the floor, I unfortunately used that same logic and started tiling at the point on the wall which is most proud of the wall, at one of the three-way interesections.

As I move away from this corner to the middle of the wall, I now notice that the thinset behind the tile is about 1/2 thick, not the 1/4 thickness you'd expect using the 1/2 x 1/2 inch trowel. Most of this wall will now needs to be set using 1/2 inch thick thinset.

So my questions:
(1) is 1/2 inch thinset too thick for Bostik Ditra set?
(2) will Kerdi fabric support 1/2 thinset?
(3) if all OK, any suggestions on how the easily get enough thinset on the wall?

I'm halfway up one of three walls which has been set now for a week, and I see no issues. If I need to start over I'd obviously rather do it sooner rather than later.

Any advice is appreciated. This is an excellent forum.
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Unread 05-31-2014, 12:08 PM   #2
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For a normal thinset, yes, 1/2" would be too thick (typical thinset max is 1/4", with 3/16" nominal). BUT, if you read the spec sheet for DitraSet, it is also listed as a medium bed mortar, and yes, it will work at the depths you anticipate needing. If the walls were really flat (other than the corners), AND the tile are quite flat, you might get away with a smaller notch. Industry guidelines are based on a typical tile, and yours may or may not be. The Ditraset lists a range of water to use, and at the thicker application, you'd probably want to be using nearer the low end of the range of water. But, you still need it to be embedded into the fleece on the Kerdi.

IOW, you should be okay continuing with what you have.
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.
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Unread 05-31-2014, 12:27 PM   #3
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Thanks for the quick response and tips. I'll continue as-is with this wall but will also use smaller notched trowel for the other two walls. I'll let you know how it goes.
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Unread 05-31-2014, 12:42 PM   #4
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If you have some high or low spots in the other walls that are 1/4" or less, you may find it easier to skim over them with thinset the day before you install the tile.

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