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Unread 09-02-2017, 08:52 AM   #1
fhueston
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cement back board over concrete slab

Just looked a stone installation failure where they used a cement backer board over a concrete slab. I cant find any guidelines for this. Would you ever use CBB over a concrete slab. If so why? Going to use this failure in my next stone inspection class...classic failure.
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Unread 09-02-2017, 11:58 AM   #2
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Was the CBB full coverage bonded ?

What was the stone failure ?
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Unread 09-02-2017, 03:19 PM   #3
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failure was lack of bond...there was still trowel marks in the thinset.
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Unread 09-03-2017, 01:38 PM   #4
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Fred
How many years did you install before starting to teach? Knees or back give out?
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Unread 09-03-2017, 03:09 PM   #5
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Im 64 and started with my dad when I was 10.,,,so you do the math. LOL
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Unread 09-03-2017, 04:54 PM   #6
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CBU is required to mechanically fastened to the substrate. That would be quite the challenge over SOG
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Unread 09-03-2017, 05:14 PM   #7
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I think the requirement for mechanical attachment to the substrate -- plywood -- is for the deflection , where the concrete slab does not deflect. I don't see any potential problem with the CBB properly adhered with mortar , except for compression . And that may only be problematic if left uncovered .

Why would someone go to the trouble of using CBB over the SOG , where mud or SLC could be used ?
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Unread 09-03-2017, 08:12 PM   #8
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Quote:
Originally Posted by Roberto
I think the requirement for mechanical attachment to the substrate -- plywood -- is for the deflection , where the concrete slab does not deflect
And I believe it has nothing at all to do with the deflection of the substrate, which is quantified as a requirement by the CBU manufacturer. It has to do with fastening the panel to the substrate.

While it might be possible to set the CBU on a concrete substrate, it would be much like setting thin tile panels and that's not an easy thing to do. It also makes no sense to me at all to even attempt. What might be the purpose?
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Unread 09-04-2017, 07:56 AM   #9
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I would mud it up, myself but Wedi claims their board can be used.
https://www.wedi.de/en-us/areas-of-a...rete-subfloor/
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Unread 09-05-2017, 02:59 PM   #10
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Cement backer unit must be mechanically fastened to the substrate. The structure of the CBU is not designed to be adhered to a substrate like a tile. The product is not structurally sound like plywood. We have had people try this and they nearly always fail. I agree they should have done a bonded mortar bed.
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Unread 09-05-2017, 07:58 PM   #11
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makes completely good sense to me, " need to raise the floor 1/2", but the minimum mortar bed thickness is 3/4, let's just glue down some Hardi" Too bad it doesn't work out in the real world. Though, I can't really think of a good reason why it would not work. I mean like Steve said it doesn't work, but I really don't understand why, provided you get a good bond over and under.
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Unread 09-06-2017, 03:44 PM   #12
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Petr,

Hardi-backer can actually delaminate. I've seen it more than once. And cement backer can pull apart also. That's what Steve means when he talks about the structure of the board. The stuff really needs mechanical fasteners.
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Unread 09-06-2017, 05:13 PM   #13
Steve Taylor
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If you need a 1/2" or less adjustment, you may want to consider a self leveling underlayment. We are seeing more and more of this used to get a perfectly flat floor.
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