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Unread 05-26-2016, 01:42 PM   #1
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Cement board over gypcrete?

I know it's not specified in the TCNA book but we all know very few jobs are prepped correctly for us.
The gypcrete with hydronic heat is poured over a concrete slab and I need the height up a 1/4. I've never actually installed cement board over concrete although installers I know have, without any issues. Probably will end up doing some self leveling cement but I'm just curious as to the potential problems of just thinsetting the cement board down?

Opinions please.


Last edited by TRAVIS (I.T.M.); 05-26-2016 at 04:16 PM.
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Unread 05-26-2016, 02:10 PM   #2
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I think without priming the Gypcrete with a 50% redguard/H2O ( or similar ) solution then a full strength coat of same, the gypcrete would pull the water out of the mortar too fast to achieve the needed bond and you'd wind up delaminating from or powdering the gypcrete.

Just read this after I posted earlier.....Ditra has an answer in the data sheet
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Unread 05-26-2016, 09:25 PM   #3
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What Greg said. Ditra XL would be a much better choice.
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Unread 05-26-2016, 09:53 PM   #4
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Portland cement and gypsum based products can cause a new mineral to grow over time between them as a reaction. This can affect the bond strength. It would be best if you sealed the gypsum prior to putting anything down on it with mortar. Something like this might be a good choice: http://www.mapei.com/public/US/produ...Grip_EN_lr.pdf
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Unread 05-26-2016, 10:41 PM   #5
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Absolutely not, Justin. That's a guaranteed failure.
Gypcrete means that it's probably a suspended wood-framed floor; which has much more deflection than concrete. Cement board is large enough and just stiff enough that when you step down on a spot in the mid section of the board it will act like it is on the fulcrum of a teeter totter, lifting the seam up away from it's adjoining seam; just a micro-amount, but enough to start the process rolling like a snowball. Fasteners are worthless in Gypcrete, and literally no sealer in the world is worth a darn on it (even though several claim to be), because it is so very often installed incorrectly (too wet) that the best bond in the word can only be to the sacrificial top layer that just flakes away.

Justin, go with Jim's Ditra XL idea. Quick, easy, and a much better job you can be proud of.
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Unread 05-27-2016, 08:01 AM   #6
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Just remember to follow the thin set manufactures prep recommendations for gypsum.

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Unread 05-27-2016, 08:15 PM   #7
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Thanks for all the input
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Unread 05-28-2016, 10:28 AM   #8
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Ditra will also get you the uncoupling that you absolutely need with hydronic heat anyway.
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