Premixed thinset under Durock - dumb? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-06-2007, 09:38 AM
First off I'd like to say this is a great site with a wealth of valuable info and advice. I discovered this site a few months ago before I did a slate gas fireplace surround and hearth job which was also my first tile job. I'm glad I looked here first. It came out great.

Yesterday, I started another job - ceramic floor tile in the bathroom. I ripped up the vinyl floor and plywood underlayment and screwed down Durock over unmodified thinset which I had left over from the last job, over the OSB sub-floor. About 3/4 of the way through I ran out of thinset. I had a small bucket of Mapei premixed thinset that I bought before I learned that it was junk so I figured I could use it to finish up. The stuff was like glue and difficult to work with and I started wishing I would have made an emergency trip to the store for more good thinset but I ended up putting the second sheet of Durock down over the blend of unmodified thinset and premix. Now i'm stressed about it. Is there anything to be worried about or will it be ok since it is only under the backer board?



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John Bridge
03-06-2007, 05:40 PM
Hi Grant, Welcome aboard. :)

The so-called "pre-mixed" thin set is mastic, which needs air to harden. And then it doesn't really harden completely for years, at which time it becomes brittle. Are you okay? I don't know. Would I re-do the one sheet instead of worrying about it for the next few years? Yep. :)