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We are putting slate on our main floor, and have radiant heat. We used a second layer of plywood as guides/spacers between the pex loops, rather than concrete over the whole thing to help cut down on overall weight, as the main floor is over a full basement. We plan to fill in the gaps with thinset as we go to help with heat transfer. I'm wondering if it would be better to put a layer of Ditra down as well before we do the tile install.

Thank you!

  • I would have used 3 inches of gypcrete and poured it right over the top. By the time you're said and done, I think you'll have as much weight with the extra layer of subfloor, thinset and ditra anyway. – maplemale May 6 '15 at 21:58
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Gypcrete is about 27% lighter than concrete (http://en.wikipedia.org/wiki/Gypsum_concrete). If it's not too late and you don't mind tearing up the second layer of plywood, the gypcrete will be a LOT more favorable in every way. Better for heat transfer. You can tile right over it.

That said, if you're already set on the path you're going, yes, I would use Ditra. You have to use something (a lateral anti crack membrane like Ditra, or hardieboard etc.) Ditra comes highly recommended as apposed to hardieboard. There are also Ditra alternatives that are the same basic thing (lateral anti-cracking underlayment.)

  • Thanks - all our entry doors were framed too low to allow the 3" of gypcrete, which I did look at. That was our mistake not communicating it to the framers. So, this seemed the next best option. I liked what I read about Ditra, and was pretty sure that was the direction to go. I appreciate the quick answer! – Cheryl W. May 7 '15 at 0:50
  • I had the same issue (with the doors) in a house once. We were adding a good 2-3" of flooring (for radiant) and the General Contractor just trimmed all the bottoms off the doors. lol Of course, our doors were already high enough that losing 2 inches didn't matter much: dailymotion.com/video/… – maplemale May 7 '15 at 14:42
  • Great video - thanks. We know we have to trim the pantry door. That was framed WAY low. We were originally going to go with hardwood floors, which was also part of the thought process of the plywood. But came across a great deal on the slate, so here we are. I ordered the Ditra today - fingers crossed! – Cheryl W. May 8 '15 at 0:29
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I recently completed pex installation on the main floor of my new house. I elected to use 14" I-joists on 16" spacing to carry the extra load of 1 ½" of lightweight concrete and ceramic tile on top of that.

If you can add floor joist reinforcement, I would just remove the plywood and pour lightweight concrete thick enought to cover your pex. Then tile over that.

Concrete actually has better heat transfer characteristics than Gypcrete and you don't need 3" of it. Gypcrete will also get soft if it gets wet so a leaking pipe could cause more problems. You also don't have the problem of your doorways being so much shorter. A 1 ½" loss of head height in a doorway is bad enough but 3" lower doorways and doorknobs is quite a lot.

Good Luck!

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