Granite over metal fireplace...adhesive?

I have a metal zero clearance fireplace, surrounded by drywall. I had granite cut to cover the metal and about three inches over the drywall (all the metal will be covered except for the lower vent). What technique and adhesive should I use? Modified thinset, silicone adhesive, etc? The metal does get somewhat hot if I have a good fire going, so I wonder about thermal expansion differences. This is a vertical installation, so the granite should be as flat to the metal as possible; a substrate like a metal grid under the granite would look terrible. I'm hoping to find the right adhesive that will allow me to simply put the granite right up against the metal. The granite pieces are heavy. Also, how much does granite expand when warm...should I leave a small gap between the pieces? Hope to hear answers from people that have actually done this type of installation and used their fireplaces successfully!
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I'm doing the same sort of install, and talked to a pro who suggested: cover the metal with thin backer board & attach with thinset. He said the metal would move around way too much to attach it directly. He also said "it is no great trick to get them to stick, the trick is getting them to stick for 20 years."
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On 7/4/2008 10:51 AM Eric in North TX spake thus:
>

So how to attach backer board to the metal? Drill and use sheet-metal screws? (Is it OK to penetrate the metal?) Adhesive? Both?
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I'm going to use sheet metal screws. There shouldn't be a problem with that causing leaks or anything, not with backerboard, thinset and granite on the outside. I don't know about your insert, but mine already has screws, that hold it together, intruding into the fire box.
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Is there an ultra thin backer board out there? Otherwise, I'm not sure what to do with the edges...the granite will end up sitting the thickness of the backer board away from the fire place.

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ConnetiCat wrote:

Gotta be a way. You see granite, marble, bronze, etc. plaques attached to all sorts of things.
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Try Rutland furnace cement. Years ago it was use to cement firebrick to combustion chambers. Might be other formulations at a ceramic kiln shop that could work.
Joe
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OK, just emailed Rutland and asked which cement to use. They have high heat silicones, too, and all kinds of weird cements that bond metal.
Most logical advice yet...thanks, Daddy
(yeah, he's my Dad)....
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