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Old 10-07-2008, 02:31 PM   #16
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Hardibacker Cementboard Strength


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If these manufacturers are selling this cementboard for this purpose of mounting rock, then they should publish their weight capability.
They're not. Hardiebacker, wonderboard, and durarock are sold and marketed as tilebacker. That doesn't mean that they can't be used for applications involving thin rock or cultured stone though. An application with rocks of this size should be conventionally laid like a brick wall in my opinion (ties to wall structure, brick ledge or angle iron, rocks bear the weight of other rocks). Relying on the tensile/shear strength of a cement-based product for a wall veneer of that weight is just asking for trouble.

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Old 10-07-2008, 02:57 PM   #17
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Hardibacker Cementboard Strength


Even if you are laying tile, thin rock, stone veneer, etc they should let you know how much weight they are capable of holding. But I think you are right that the bottom line is that this is partially a "misuse" of the cementboard product.
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Old 10-07-2008, 04:21 PM   #18
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Hardibacker Cementboard Strength


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With DIY'ers over building is far better.
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Old 10-07-2008, 06:51 PM   #19
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Hardibacker Cementboard Strength


First, the primary concern is not compressive, flexural, or shear strength. Cement board exceeds the requirements in all three categories by a considerable amount. The reason the company does not test for shear is because it is a useless number for any application it is recommended for.

The only property that should concern you for this application is bond strength. Using a modified thinset, you will achieve bond strength numbers far in excess of what is required.

Any failed adhered veneer wall that you have seen failed for other reasons, most likely a freeze-thaw (or thermal expansion, like in the EMS's story above) issue on a wall that was stuck using type S mortar.

If you plan on using lath and stucco, then do not waste the money on the concrete backer board. It is an either/or, not both.

There is no real standard at this time for adhered veneers as to pounds per square foot, but most faux stone manufacturers specifiy a weight of around 11# PSF, with thin cut natural stone running 12-15#.
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Old 10-14-2008, 01:38 PM   #20
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Hardibacker Cementboard Strength


I see you have commented on several threads regarding cementboard, stone veneers, and mortars. What are the advantages/disadvantages between modified thinset and type s mortar?

One of the stone veneer manufacturers recommend cementboard stuck direct with mortar, the other says plyboard with lath and scratchcoat. Some say thinset, others type s, and another type s + sand. For interior applications, aren't all of these techniques/products more than sufficient for 15# psf stone?

Thanks for your help.

Last edited by kinglerch; 10-14-2008 at 01:42 PM.
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Old 10-14-2008, 02:21 PM   #21
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Hardibacker Cementboard Strength


I prefer lath and stucco for exterior wall applications and board for interior walls and exteriour "shapes", like BBQ islands, mainly for speed. I do not use mortar on board, only only on lath and stucco. I do not feel the surface of the board is rough enough to allow for sufficent bond using mortar. Thinset only on board, mortar on stucco.

The only issue wth thinset is short board life and a long "grab" time. You have to mix it drier than normal, and in small amounts. There are, however, some thinset manufacturers producing thinsets tailored for the adhered veneer market. Ardex X9 is one I have used that gave a 2+ hour potlife and very good grab (and will peel the cement board before it breaks loose).
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Old 10-14-2008, 02:35 PM   #22
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Hardibacker Cementboard Strength


Outdoor worries me because of unknown heat, cold, moisture, shifting, wind, etc. But for interior my guess is that many different methods are sufficient.

I also noted as you did that the cement board seemed too smooth for direct application, although one of the stone veneer manus said exactly this. So I put lath on top of my board, not because it was necessary but because the cement board was already bought and installed.

But you bring up another good point about "cement board strength". Bond strength (or at least bondability) is important, but so is (for lack of a better term) breakaway strength. What good is a perfect bond if the backer board rips off...rock, mortar and all. Of course I think this won't happen until 200# psf or more, but it would be a useful spec nonetheless.
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Old 10-14-2008, 03:26 PM   #23
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Hardibacker Cementboard Strength


Do this simple test:

Thinset a faux rock to cement board. Get a hammer, a big one, and beat on the rock. Half of the time (80% of the time for real rock), the rock will shatter, leaving the thinset on the board. The other 50% of the time, the thinset will delaminate the concrete board. Unless you plan on using the wall as a climbing wall for cows, you won't need to worry about the cement board shear strength. As stated originally, that is why it is not tested, because it is not even remotely close to critical for any recommended use.
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Old 10-14-2008, 03:36 PM   #24
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Hardibacker Cementboard Strength


I agree 100%, except that "you won't need to worry about cement board shear strength...it is not even remotely close to critical for any recommended use" would be a great and useful tagline for these products.
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Old 10-14-2008, 05:26 PM   #25
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Hardibacker Cementboard Strength


Because then some moron would use the product in an application where it DID matter, and sue them. The product has a narrow range of use and is perfectly suited for those uses only.
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Old 01-23-2009, 11:35 PM   #26
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Hardibacker Cementboard Strength


Has anyone tried applying faux stone veneer directly to cement board in an interior application (i.e. fire place surround, indoor chimney, etc.) I am wondering why you couldn't skim coat the cement board and then apply the faux stone directly to the skim coat. This would eliminate the need for the wire mesh.

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I agree 100%, except that "you won't need to worry about cement board shear strength...it is not even remotely close to critical for any recommended use" would be a great and useful tagline for these products.
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Old 01-24-2009, 07:09 AM   #27
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Hardibacker Cementboard Strength


You don't have to skim coat it, it can be applied directly to the cement board, preferably with a latex modified thinset.
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Old 01-25-2009, 06:54 AM   #28
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Hardibacker Cementboard Strength


My specific application will have the stone veneer going around the fireplace and the sides of the fireplace. Its height from the floor will only be 5 feet.

Have you used the application process that you posted on any jobs you have done and how long have they been done?
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Old 01-25-2009, 08:13 AM   #29
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Hardibacker Cementboard Strength


The one one the left last year the one on the right 2 years ago:

http://www.5125851496.com/gallery-pa...temporary.html

Trust me, you can not beat the stone off with a hammer without destroying the board. Cement does not normally degrade over time, it gets stronger.
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Old 01-26-2009, 10:52 AM   #30
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Hardibacker Cementboard Strength


Thanks for the reply; the two jobs look great. How did you apply the cement board to the studs? I have heard of some people using galvanized washers with galvanized screws plus gluing with construction adhesive. I have also heard of just nailing it on with roofing nails. I'd rather be safe than sorry.

Do you have a preference for the cement board you use? Durarock, Hardibacker etc. and what thickness?

I'm planning on using a veneer stone with mortar joints in between the stones. I'll use mortar to grout between the stones. Will the mortar adhere to the thinset without trouble?

Thanks,

Charlie

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