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We are adding a fireplace outside on our porch, and my wife fell in love with the thought of a limestone mantel. So, we have a beautiful 90.5"x14"x4" slab of limestone sitting here (just a hair under 500lbs). There's a 5" ledge built into the framing, and we have 2 limestone corbels already mounted.

So far, the plan is to put custom-made iron angles bolted to the framing at the top of the slab to hold the wall side down, and let the rest of it sit on the ledge/corbels. Is this enough (the weight supported by the ledge and corbels, the "twist" held down by the L brackets)?

I should also mention that the brick façade is only 1.5-2" thick, restricting the bottom of the L brackets to being that long so they are hidden. And the corbels are probably 6" on a side at least, so MAYBE enough?

ELEVATION (SIDE) VIEW

|
||
||____ <-- iron angle
|------------------------
|    stone slab
|________________________
|___________       /
  framing   |   __/ <-- corbel
   ledge    |  /
            |_/
            |
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  • I think I follow your plan, but a sketch would help. I'll add one, and you can revise.
    – isherwood
    Nov 4, 2022 at 16:20
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    The answer lies in the integrity of the corbels, I think, and how they're mounted.
    – isherwood
    Nov 4, 2022 at 16:30
  • What's custom about the iron angles?
    – isherwood
    Nov 4, 2022 at 16:31
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    Your profile says Illinois. Anywhere near the Wabash Valley seismic zone? How will the limestone slab be retained if earthquake movement wants to shake it past the lip of the iron angles?
    – MTA
    Nov 4, 2022 at 16:52
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    I don't know how to do the calculations but the thing that stands out to me is the possibility of the stone cracking/breaking where it is unsupported beyond the ledge and between and outside the corbels. I think your ledge/corbels can easily be made strong enough to support 500 pounds if the weight was made from imaginary unbreakable material. It's the stone itself I'd look at. Imagine it cantilevered with all the upward force provided only by the tiny front edges of the corbels.
    – jay613
    Nov 4, 2022 at 16:57

1 Answer 1

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Solved (installed)! Thanks Isherwood for your sketch, that's exactly what I was trying to describe. Turns out, however, that the corbels were also held down with "L" brackets, then mortar used to level where those were, and the slab then put on top of it all with angle iron on top (but short enough projection out that it is covered by the brick façade applied last above it). My contractor seemed more confident than I was originally, but his final solution seems very sturdy and permanent. (and to answer the other question, no, we're quite a ways off of the Wabash earthquake zone. Probably a little more worried about New Madrid, but not close to that, either.) Again, thanks to you all for your help!

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