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Old 12-04-2015, 10:21 AM   #1
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Cement board cracking above ventless fireplace


I had a 48" linear vent-free fireplace installed with a paintable clean finish with drywall surround and cement board above(16"X52"). It was installed per manufacturer instructions which said not to attach the cement board to the fireplace unit but only to the wood studs. Within a month of use, there was cracking above in the paint. After further investigation, cement board had cracked deep enough to slide the point of a knife in it Now it needs to be taken off and replaced. I am looking for suggestions as to what to use.
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Old 12-04-2015, 02:51 PM   #2
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It would be helpful if you told us what you used. Cement board is pretty generic. Pics would be helpful too. Do you think the framing was done right? Sure sounds like a framing problem, not concrete backer or anything else.

Was the concrete backer also taped with the correct fiberglass mesh and thinset mortar?

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Old 12-05-2015, 08:45 AM   #3
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picture of framing


Thank you for your response. I don't know the manufacture name of the cement board because it was supplied with the fireplace unit and the contractor installed it. He assured me that the c-board(16"X52") was attached only to the wood studs and not to the fireplace unit and that the studs around the unit were placed properly per manufacturers instructions. He used mesh tape with joint compound because I want a smooth finish to be painted the same as the rest of the room. The cracks in the paint and the joint compound stem from the cracks in the c-board. Any suggestions are appreciated.
Cement board cracking above ventless fireplace-image1.jpg
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Old 12-11-2015, 01:12 PM   #4
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Are you saying that the finished surface is cement board, finished with joint compound and paint? Is the crack at a joint that has failed, or has the cement board broken?
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Old 12-11-2015, 03:28 PM   #5
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The cement board is 53x16" with sheet rock around that and taped with joint compound. There are two cracks either side of center running up the board. They are about 16" apart. No cracks at joints. The cement board was attached with screws along the sides in the wooden studs and across the top. Just recently found that two screws were placed through the board into two of the three metal standoffs which are bolted to the top of the fireplace.
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Old 12-12-2015, 03:51 PM   #6
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From what I see, and you're describing, I'd wonder whether the cement board was broken or cracked when the contractor put it up. I don't see anything about that situation that would create 2 vertical cracks. But maybe the standoffs move around with the heat and caused the crack.

If thickness allows, replace the CB and put 1/4" drywall over it.
If not, you could try priming and plastering it with hot mud, or real plaster, or a tougher plaster like Hydrocal, but if the thickness is 1/4" or less, you're always going to have the risk of it cracking.

All that is subject to what the manufacturer's instructions say, and to what edge you plan to put between the fireplace and the surround.
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Old 12-14-2015, 08:30 AM   #7
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I agree that there are cracks because the c-board was screwed in to the metal standoffs which were bolted to the top of the fireplace.
Thank you for your help and ideas.
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