Cultured Stone on Cement Board

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Old 12-28-10, 10:21 AM
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Cultured Stone on Cement Board

Hi Everyone

I have a bit of a problem. I spent the whole weekend putting up cultured stone around my fireplace. I am only going up to the mantel. I am using Durock cement board as the backer and using Type S Quickcrete as the mortor. i did not do a scratch coat on diamond lathe.

However i found that this morning the cultured stone blocks did not "stick" very well to the backer. i could pull most of them off with my hands.

Any idea what went wrong and how to fix it.

thanks

Moe
 
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Old 12-28-10, 11:31 AM
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Maybe your mortar was not of the correct consistency. Did it seem dry and when applied and did it seem as though you had to pack it on the backer or was it smooth and went on with a sweep of the trowel? How long before applying did the mud sit around after you mixed it? Was the mortar fresh before you used it from the bag or could that sack have been chunky and old? How long after placing the stone did you give it before attempting to pick the stone off the backer board? Sometimes, the first course needs something to rest on or it could slip and weaken the bond. Did you not only plaster the surface but also the back of each rock? Just a few possibilities to throw at you.

bs5
 
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Old 12-28-10, 11:49 AM
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I've seen pros need to mix a few different batches to get the right consistency in the mortar
 
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Old 12-28-10, 02:00 PM
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It appears you know what went wrong. No lath. No scratch coat. No butter on back of the rocks. It all contributed to the failure. It can't be corrected without pulling the rock off and starting over.
 
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Old 12-28-10, 09:31 PM
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Type S directly to cement board is not an approved method of application. Modified thinset can work, but is also not an approved method.
 
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Old 12-29-10, 08:43 AM
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Remove what you have done and clean it up as best you can. Use 15lb felt over the cbu, then 2.5lb expanded lathe, then type N mortar. I don't think you'll get what you are trying to do to work. Type S mortar doesnt bond real well to cement board.

It appears you know what went wrong. No lath. No scratch coat. No butter on back of the rocks. It all contributed to the failure. It can't be corrected without pulling the rock off and starting over.
Yep

Type S directly to cement board is not an approved method of application. Modified thinset can work, but is also not an approved method.
Modified thinset will work. The weak link, is that thinset really shouldn't be used in excess of 1/4" in thickness as it tends to shrink too much when it cures. Typicallly, you'll want to apply the mortar at least a 1/2" to 3/4". For the record, Ive used modified thinset, and a modified medium bed mortar for this a couple of times on small interior jobs and it does work. Best to do it the right way though.
 
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