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-   -   cement back board over concrete slab (https://www.johnbridge.com/vbulletin/showthread.php?t=123645)

fhueston 09-02-2017 08:52 AM

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.

eurob 09-02-2017 11:58 AM

Was the CBB full coverage bonded ?

What was the stone failure ?

fhueston 09-02-2017 03:19 PM

failure was lack of bond...there was still trowel marks in the thinset.

tilemanct 09-03-2017 01:38 PM

How many years did you install before starting to teach? Knees or back give out?

fhueston 09-03-2017 03:09 PM


Im 64 and started with my dad when I was 10.,,,so you do the math. LOL

Houston Remodeler 09-03-2017 04:54 PM

CBU is required to mechanically fastened to the substrate. That would be quite the challenge over SOG

eurob 09-03-2017 05:14 PM

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 ?

cx 09-03-2017 08:12 PM


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?

Davy 09-04-2017 07:56 AM

I would mud it up, myself but Wedi claims their board can be used.

Steve Taylor 09-05-2017 02:59 PM

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.

PetrH 09-05-2017 07:58 PM

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.

John Bridge 09-06-2017 03:44 PM


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. :)

Steve Taylor 09-06-2017 05:13 PM

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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