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Unread 01-08-2020, 11:24 PM   #1
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Modified thinset with Schluter mistake

So, tiling my bathroom floor using schluter ditra, and made the mistake of accidentally starting to use modified thinset over the ditra. I realized it about halfway through the floor, maybe 1/2 hour in.

I pulled up the tile, scraped and wiped the tiles and scraped up all the modified thinset from the floor down to level with the pockets.

My plan is to let the modified in the schluter pockets dry until tomorrow night and then re-set the tile over it with unmodified thinset.

Will this pose an issue, or will the unmodified stick ok to the dried modified?

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Unread 01-08-2020, 11:51 PM   #2
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Welcome, Kris.

That should work just fine. Actually, depending upon exactly what thinset mortar you used it would most likely have been just fine with the modified mortar. If your tiles were large, it might have taken a bit longer for the mortar to fully dry, but it would likely have cured just fine. But what you did will allow you to comply with the Ditra manufacturer's recommendations. Tile manufacturer may disagree.

My opinio; worth price charged.

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Unread 01-09-2020, 12:39 AM   #3
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Thank you for the response. Yes, I have just read about 7-10 day cure times for using modified over ditra, and really didn't want to hsve to wait that long. I am glad to hear that I wont have to pull up the ditra though.
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Unread 01-09-2020, 07:38 AM   #4
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Porcelain tile seems to have more of an issue with that, due to their low absorption rate, but I've never had an issue using modified mortar over Ditra.

I often fill the cavities with mortar the day before setting tile, making installation a little easier in some respects. But with only the mortar above Ditra to dry, the drying time doesn't seem to be excessive.

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Unread 01-09-2020, 07:26 PM   #5
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As CX mentioned, it really depends on the exact formulation of the modified mortar you used. There are at least four different modifiers. The ones that are most problematic aren't used as much today, and they contain latex.

Schluter did a little experiment with a large glass plate set over a latex based mortar. It literally took nearly 80-days for the latex additive to dry in the middle of the tile. Some modified thinsets work just fine under a porcelain tile over a waterproof substrate. SOme of the modifiers solidify in a similar manner to the cement. The whole mess needs to be solid. If they don't, then, you can end up crushing the cement crystals, and like Velcro that got old, things don't hold together well. NOw, a typical tile is rigid, and depending on what weight and how it is applied onto it, may not have an issue.

FWIW, Schluter does sell some modified thinsets that they know will work, as they were engineered for use in this situation. Other manufacturers have as well, but sometimes, you need to ask them specifically. Schluter's issue is they are not willing to test and keep track of all of them out there, since, what may work today, may not with that other company's new, improved version. As a result, they recommend a thinset they know will work, assuming it is installed properly.
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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