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finchsaunders 12-24-2009 10:56 AM

granite hearth cracked twice
Hi--we have a big problem and don't know what to do. Our granite hearth cracked after our first fire, we had it replaced and it cracked again. It's a 1 and 1/4" slab of honed black absolute granite, placed on top of our wood floor. Our fireplace is normal-sized and we burn average pieces of firewood. We think the problem comes from the unfinished edge of the granite heating up (the edge that extends the width of the fireplace), expanding the granite, then creating cracks since the rest of the granite is cold and doesn't expand. But--are we the only ones with this problem? I thought people used granite slabs frequently for hearths with no problems!

We're not sure if the next slab should be lowered into the wood floor so it's level with the floor, thus keeping the edge from being exposed to the heat, or what. Help!

Scuba_Dave 12-24-2009 11:37 AM

I bought a used piece of granite & had the same thing happen
I know its there, most people will not notice it
My hearth is raised, so direct heat hitting it

Bob Mariani 12-24-2009 12:15 PM

A thicker slab will help. But setting on a wood subfloor is the biggest issue creating the problem. Now the granite and the subfloor expand and contract at greatly different coefficients. Set it on concrete as it should be and you will have no problems.

finchsaunders 12-29-2009 11:30 AM

Thanks for the input. I was researching this further and saw that there are also flexible underlayment materials like Micore board. It looks like cement board. Do you think this would work?

Bob Mariani 12-29-2009 11:31 AM

no. it will most likely cause more cracking. What you want is to install over Ditra. This membrane works as a decoupler to help movement in the sub base being tranferred to the slab. It is this movement that is causing your problems.

WFC 04-24-2011 04:49 AM

So glad to find this thread. Having the same issue.

Ron6519 04-24-2011 07:47 AM

When you say it's laying on the wood floor, do you mean it was just brought into the house and put in front of the fireplace on the finished floor?
This stone needs to be "installed" on a proper setting bed.

Daniel Holzman 04-24-2011 09:30 AM

I believe Mr. Mariani hit the nail square on. There is no way that differential heating of the granite would be causing this problem. If the granite is not locked in position, it would not crack. The slab must be restrained on all sides, preventing movement, so when it heats the slab cracks rather than moves.

Installation using thinset or conventional mortar over ditra should solve this issue, just make sure the sides of the granite are not restrained. You may need some elastic caulk along the edges, when we did our stone kitchen floor we purchased colored caulk (I think it was polybutyl) that formed the 1/2 inch edge, allowing movement of the stone. Worked really nicely.

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