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Planning to use a slab of limestone, 4" high by 8" deep by 5' long, as a floating mantel shelf for a fireplace remodel. The existing fireplace is red (Chicago) brick.

I'm resurfacing with ledgestone overlay. I removed the cheap floating mantel (wood) and plan to hang this limestone. My question is how I should hang it.

I originally planned to use some size of rebar as dowels epoxied into predrilled holes in the brick and the back of the limestone slab. While rebar is strong it tension, I fear pullout or bending that would allow the slab to eventually slip off the supports.

Now I am considering using lag bolts into the brick, cutting off the heads and epoxying into predrilled holes in the slab. Any thoughts on this, or the size of lags, or completely different method for attachment?

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That is a pretty big piece of rock.

It would probably be better to distribute the load by using brackets.

enter image description here

In the photo above, the mantle is wood, but the principle for stone is the same.

What you want to do is build the brackets into the chimney as deeply as possible. So, you actually remove entire bricks and insert the brackets deep into the chimney superstructure. Then you just rest the mantle on the brackets. You can drill 1/2-inch holes in the bottom of the mantle and the top of the brackets and insert close-fitting steel pins to join the the mantle with the brackets, so it will not slide off.

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    Yeah, brick corbels are usually integrated into the wall. Surface-mounting something that heavy seems a little sketchy. – isherwood May 20 '17 at 21:26
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Agree that you'll never be able to 'float' a stone mantle this size. Stone corbels are usually built in as the wall goes up. I have seen steel brackets used, but they were bolted to the wall behind, and then built into and through the brick facade. Difficult to retro-fit.

You could have some wrought iron brackets made up and then drill deep holes through the bricks and into the chimney itself. Resin fix the bolts in place. Bolt the brackets up and saw off the excess threads and use a decorative cap to hide the nut etc. Still risky though, either the weight of the slab or the tightening of the bolts could crack the brick facade if it's not fully mortared at the back...

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