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Old 03-22-2011, 12:14 AM   #1
AJMac
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CBU over Ply, then Ditra - should CBU be thinsetted down.

Hello,

I've picked up a lot of information from this community - so first off a big thanks to all the contributors.

My current dilemma - I've used 1/4" CBU board over a 5/8 plywood subfloor (2x10 on 16s framing) - without bedding in thinset - My rationale, this was only to build height to match an adjoining hardwood install. The CBU is screwed (4" perimeter, 6" through field) but no thinset underneath. I intend on levelling the floor with SLC (up to 1/2 thick), followed by DITRA and then 12x24 porcelain tiles.

Seems most posts point out to thinsetting CBU to plywood - but my thoughts are this is irrelevant given I'm using DITRA in this detail such that it will decouple any movement.

What are your thoughts - is it worth taken the CBU up again to thinset it down, or does the rest of this assembly make this a wasted step. For context we are talking a small washroom - 5x8ft.

Thanks.
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Old 03-22-2011, 06:15 AM   #2
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AJ, you're doing a pretty decent job of talking yourself out of doing something you know you really should do. Ditra and SLC are not used to correct an improperly prepared subfloor.

My question, though is why are you using Ditra? Once you have the backerboard down (properly), you have your underlayment. The SLC is filler/leveler, but not your underlayment. Ditra buys you nothing.
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Old 03-22-2011, 09:05 AM   #3
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The purpose of thinset under CBU is to fill in any voids that might exist between the subfloor and CBU layers. Any void that exists will translate to movement, which is something you don't want. It doesn't matter what you put on top of that void, whether it be SLC or Ditra, or both, the movement will still be there.

The only way to correct that is to undo what you've done, then reinstall it properly. And like Bob said, there's really no benefit to installing two different layers of uncoupling, i.e. CBU and Ditra. If you need to level the floor, do it after the CBU is properly installed. You can use SLC if you need to fill in areas that are deeper than 1/4", or just use thinset if it's 1/4" or less.
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Old 03-22-2011, 09:16 AM   #4
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ah - i suspected I was in for a redo - that's ok, just wanted to be sure it was necessary. I guess I was viewing the CBU as just 'shimstock' to buy some elevation so I didn't need to do such a large SLC pour.

In my mind the CBU was just like adding plywood (hence the screws at 4 and 6 centers). Out of interest, would you have agreed with the layup if I had substituted say 1/4 ply in place of the CBU, then ditra, then tile? (I'm not trying to go sidewise to find the answer I want - just want to understand this inside out. My suspicion was yes, which is why I posed this get out of jail question)

I guess on the flipside you are saying I don't need to go with Ditra as the CBU serves that purpose. One step forward, one step back - I can live with that leave me more or less on budget time wise
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Old 03-22-2011, 10:56 AM   #5
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Quote:
Out of interest, would you have agreed with the layup if I had substituted say 1/4 ply in place of the CBU, then ditra, then tile?
No, but only because there is no suitable grade 1/4" plywood for tile subfloors. Had you said 3/8" plywood, then the answer woud be yes, because that's been tested by Schluter and is an approved installation method.

Quote:
I guess on the flipside you are saying I don't need to go with Ditra as the CBU serves that purpose. One step forward, one step back -
I consider this 1 step back, 2 steps forward.
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Old 03-22-2011, 05:10 PM   #6
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Make sure the cbu you used is compatible with SLC on top of it, not all are!

The advantage of using cbu then slc is that you can use less slc, and feather the edge. SLC over ply nomally needs to be both thicker and can't be feathered out - it must have a minimum thickness everywhere.

With cbu down, Ditra is not entirely redundent, but won't gain you as much as you would have had with the Ditra bonded directly to the ply. With only a 5/8" subfloor, you'd be better off with 3/8" ply and the slc, as the ply would improve the subfloor above the bare minimum...cbu does not add any useful stiffness to the floor.
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Old 03-22-2011, 08:09 PM   #7
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Guess it does not matter since you have the hardi already but Ardex says this slc can go over ply with primer. no lathe, feather to 1/16th. Ready to install tile in 48hrs. I use over heat cable. I use the Ardex 8+9 in conjuntion with it just for little somthin extra.

http://www.ardex.com/productDetail.a...5&ProductID=21
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