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danreg99 03-06-2012 02:23 PM

What did I do wrong?!?
I am in the process of rocking my fireplace and have a problem. My contractor told me I could use cement board and mortar hearth and stone directly to it (without using a scratch coat and wire mesh). I have a done 1 fireplace in my lifetime using the scractch coat method (over tar paper and sheetrock) and everything turned out great. Last night I got all of my cement board up and started with my hearth stone. I buttered the hearth stone with approx 1/2" of type s mortar and laid it directly on the cement board ( I wiggled it in there and put some pressure on it). Well I came back this morning and was able to pick up every piece of stone I laid :(. The mortar barely adhered to the stone or cement board (I was able to pry it off pretty easily)> Should I be using type N mortar or do I need to go the scratch coat direction. Thanks for any information you can lend to me.

Ironlight 03-06-2012 02:38 PM

I'm going to suggest that if you put it down last night and then tried prying it up this morning, that it came up for the simple reason that it had not yet completely set. I would not have touched it for at least another 24 hours.

As to the suitability of cement board for the backing without expanded metal and a scratch coat, I don't have the expertise to comment on that. But regardless of it's long term durability, the cement should bond adequately to the cement board.

danreg99 03-06-2012 02:53 PM

Actually I put it down at about 6 pm and lifted it at lunch time today. Wouldn't there be some adhesion by then? Cement board was basically clean of any mortar.

joed 03-06-2012 02:59 PM

How was the mortar? Hard? Soft? Crumbly?
Could be the board and stones were too dry and sucked all the moisture out of your mortar.

danreg99 03-06-2012 03:06 PM

When I mixed the mortar last night I mixed it to the consistency of mashed potatoes. When I pulled it off the rocks today it broke off into 2 " chunks and kind of reminded me of cookie dough.. Should I mix more water into the mortar? Or should I put a scratch coat with type s over the cement board ? I have read some other forums and they suggest wetting the cement board with a sponge. Do you think that would work?

jomama45 03-07-2012 08:06 AM

Are you using a pre-mix mortar? If so I'd suggest adding a few ounces of bonding agent to each bag and maybe even a little additional lime as well. And you definately want to get the backer damp first.

wewantutopia 03-07-2012 08:23 AM

Cement board is usually VERY dry. I usually mist it a few times using a spray bottle before tiling. I also spray each area again before I spread more mortar.

joed 03-07-2012 09:03 AM

The other question is are you premixed mortar or mortar cement that needs to be mixed with sand? What was your mix ratio if using mortar cement?

danreg99 03-09-2012 10:05 AM

Thanks for all the info guys. I ended up misting the cement board and using thinset on the rocks and that stuff really holds! The idea about the dry cement board was spot on!

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