# The thinset under CBU



## TileWizard (Jan 14, 2011)

I understand why we put thinset under the CBU. But is it actually supposed to stick to the substrate or uncouple? I say uncouple, This has been on ongoing argument with a fellow tile man.


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## cleveman (Dec 28, 2007)

If it is supposed to uncouple, good luck.

I once had a small piece of durock stuck to 3/4" osb which I had removed after the fact. It wouldn't be any fun to get the two separated.

I think it can flex enough that it doesn't need to uncouple.

Also, are the fasteners supposed to uncouple as well?


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## tenon0774 (Feb 7, 2013)

Uncoupling membranes, as I understand them, are set with a modified thinset.

I've heard all this stuff about leaving space between underlayment and debates about which iso-membrane is the best.

If the subfloor doesn't fluctuate wildly in temperature and humidity, iso membranes are superfluous.

They are "more flexible" than CBU's, but if the subfloor isn't moving to begin with as a result of a controlled environment, then why?

I still thinset and mastic CBU's down as per manufactures specs.

If the tile cracks, i don't want any of the manufacturers to to be able to point in a circle, and me be left stuck holding the bag.

I want to be able to testify in court, if I have to, that I followed the MANS SPECS to the letter.

You're original question of thinset under an uncoupling membrane:

Does it make senses?

Yes and no.

To glue it down, yes.

To act as another layer of uncoupling on a troubled floor, No.

But to me, schluter and wedi shower systems don't make sense either.

...but that's another thread, thoroughly debated here already.


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## Metro M & L (Jun 3, 2009)

Just wanted to clarify on the modified unmodified thing. Check the ditra install guide and it specd latex pc cement for the bedding layer and unmodified for ceramic porcelain and natural stone.


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## Inner10 (Mar 12, 2009)

TileWizard said:


> I understand why we put thinset under the CBU. But is it actually supposed to stick to the substrate or uncouple? I say uncouple, This has been on ongoing argument with a fellow tile man.


According to JB:

"• Cement Backer Boards

Cement backer boards, such as PermaBase, Wonder Board, Durock, Hardi-backer, and others are used to “uncouple” a tile installation from the subfloor below. Before they are fastened, CBUs, as they are called, are bedded in thin set mortar, which is the usual adhesive used in setting floor tiles. The panels are then nailed or screwed to the subfloor following manufacturers’ specific directions. CBUs do NOT improve the stiffness or structural value of the floor."


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## MIKE ANTONETTI (Jul 17, 2012)

Inner10 said:


> According to JB: "&#149; Cement Backer Boards Cement backer boards, such as PermaBase, Wonder Board, Durock, Hardi-backer, and others are used to &#147;uncouple&#148; a tile installation from the subfloor below. Before they are fastened, CBUs, as they are called, are bedded in thin set mortar, which is the usual adhesive used in setting floor tiles. The panels are then nailed or screwed to the subfloor following manufacturers&#146; specific directions. CBUs do NOT improve the stiffness or structural value of the floor."


 JB ? John Bridge is that a Mans Name, company, I know of the website, maybe 10 minutes I've been on it.
Anyway from what I've learned CBU installed properly screwed down, nailed, thinsetted is not an "uncoupling" system. When you fasten it to substrate it becomes "one", 
If you lay CBU on top unfastened the two may move independently of one another, not that it will work but the example is the explanation.
A flat sheet membrane also is not an. "Uncoupling" membrane , the uncoupling takes place above the flat portion of schluter where the bridging above works independently to some extent to what's going on below.
I would say something like the way buildings in quake zones are built with a flex system between two rigid materials.
It's all in the numbers, the amounts of flexibility they need to achieve objective.


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## jarvis design (May 6, 2008)

TileWizard said:


> I understand why we put thinset under the CBU. But is it actually supposed to stick to the substrate or uncouple? I say uncouple, This has been on ongoing argument with a fellow tile man.


Neither!, although a good thinset will certainly "stick". 
Thinset under CBU, IMO, is there to ensure that their are no voids between the subfloor and CBU.

CBU is used because it creates a better bond to tile vs plywood and is dimensionally stable, meaning it won't swell or shrink due to moisture.

On a funny note, I watched part of a Holmes on homes episode where they laid CBU over a plywood subfloor...then installed ditra. Took me a few minutes to realize why..... Durock and Schluter sponsor the show!

Finally, if you like ditra, which I do, you'll love Laticrete stratamat!!


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## TileWizard (Jan 14, 2011)

I know its not uncoupling like a ditra, I wasn't sure what word to use. This came about because of the advantech subfloor. Thinset doesnt stick to it. I said it doesn't have to permanently stick.


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## MIKE ANTONETTI (Jul 17, 2012)

I see so many discrepancies on TV, does he still have a show?
I watched Lizard Lick and thought it was funny so one sat or sun I watched 3 back to back and figured out it was dramatized and kinda dumb.
I think I'm over all those BS shows, flip em!


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## jarvis design (May 6, 2008)

MIKE ANTONETTI said:


> I see so many discrepancies on TV, does he still have a show? I watched Lizard Lick and thought it was funny so one sat or sun I watched 3 back to back and figured out it was dramatized and kinda dumb. I think I'm over all those BS shows, flip em!


Pretty sure he's still on the air hawking whatever products that give him $$ to do so


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## MIKE ANTONETTI (Jul 17, 2012)

Yeah, an endless supply of money and assistance after the fact, "easy button"


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## Davy Stephens (Aug 3, 2014)

I've heard the same as Jarvis, the thinset is only there to fill any voids between the CBU and plywood, not to hold down the CBU, the fasteners do that. I've also heard that cheap thinset can be used for this, doesn't have to be modified.


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## TNTRenovate (Aug 19, 2010)

Ditra is a great way to build up a floor to thickness on top of any substrate. I'm not sure what point is trying to be made about putting it over CBU.


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