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Attachment of a stone fireplace surround — fasteners?

user-958947 | Posted in General Questions on

Is it a code requirement or recommended practice to install mechanical fasteners to fireplace surrounds, or is mortar alone satisfactory? I’m not talking about massive columns and mantles. Just thin granite slabs (not tile) without a mantle. More specifically:

I’m installing a wood-burning fireplace surround. It’s a very simple design—-2cm granite slabs (not tile, just whole one-piece slabs) about 14″ wide (1 top slab and 2 side slabs) without a mantle. My plan was to simply attach the slabs to Hardi with medium bed mortar and call it done. But in a general google search about fireplace construction, I ran across a “best practice guide” that highly recommends mechanical fasteners—see attached link from the Stone Federation (in the UK). I’m not in the UK, but physical science doesn’t have borders ……..well some of it apparently does, but that’s another topic 🙂,%20data%20sheet%20January%202015.pdf

I can’t find anything in the IRC code about this, but was told by one of the mortar manufacturers that it is a code requirement (they couldn’t cite where in code) to have fasteners on fireplace surrounds.
Note that the Stone Federation data sheet cites fatalities and serious injuries from fireplace surrounds collapsing. So, on page 2 of that document they have a general solution.

At first, I thought I would just use their drawing for my installation. However, it appears to me that their solution is more suited for thicker slabs. If I use their solution for my 2cm slab, that doesn’t leave much stone for the brackets and dowels to hang on to after cutting the grooves and drilling the dowel holes.
Note that my design is similar to the one in the UK drawing, but doesn’t have the bulky top and bottom pieces. I just have flat slabs with no mantle.

I would appreciate any comments on this.

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  1. GBA Editor
    Martin Holladay | | #1

    You wrote, "My plan was to simply attach the slabs to Hardi with medium bed mortar and call it done."

    You haven't really described your substrate very well. Is this a traditional masonry fireplace -- for example, one made of brick? Or is this a sheet-metal manufactured fireplace (a so-called zero-clearance fireplace)?

    If it is a manufactured fireplace, what type of Hardie product are you using for a substrate? And what is this Hardie product fastened to?

  2. user-958947 | | #2

    It's a modular masonry fireplace (Isokern).
    It's framed out with 2x's that are flush with the smoke dome and firebox face.
    The face of the smoke dome is rated for zero clearance.
    The framing will be sheathed with 1/2" Hardi 500. The Hardi will also lay flat on the smoke dome.
    The Hardi will be cut back to 8" clearance around the firebox per manufacturer's instructions (they consider Hardi to be a "combustible").
    The Hardi will be attached to framing with Backer-On screws made for cement board, spaced per Hardi recommendation (8" I think, maybe 6"--I'll have to check).
    The manufacturer says that 1" drywall screws may be used to attach to the smoke dome. I've requested clarification from the manufacturer---can't find 1" drywall screws and if I could, I have concerns about their performance in an alkaline environment. I'm a little worried about screwing into the smoke dome anyway, but if I don't, my attachment of the Hardi is limited to wood framing only.
    See attached photo and sketch.

  3. GBA Editor
    Martin Holladay | | #3

    OK, that helps. I guess the product you are talking about is HardieBacker 500 cement board.

    Concerning your code question: I think that you should contact your local building office and ask the code officials there if your plan is in compliance with local codes. My guess is that no one in the office will know the answer or care, but it's important to ask.

    Second, I would contact an experienced local mason to determine the best way to secure these vertical granite slabs. If you expect the mortar to be the only fastening, you had better be sure that you have chosen the right mastic or mortar for the job.

    When I learned stone masonry back in the early 1970s (I have built a stone basement and two 40-foot stone chimneys), I was taught by the old-timers, "Mortar is not glue. Stone masonry depends on gravity, and mortar simply provides a bed in which to lay your stones."

  4. user-958947 | | #4

    Our local codes are all based on IRC or IBC, so if it's not in there, they don't address it. I can't find anything in IRC under the "fireplace" category. But I think this should be considered whether it's currently in the code or not.

    Mapei has two high-end mortars that they think will work (low sag for large wall tile and stone), but also told me (on their tech help line) that "code" (which one and where they couldn't say) requires addition of mechanical fasteners above a certain size and weight (they didn't know the details).

    I don't think vertical loads are a problem for this installation. Gravity works for me if everything is perfect. The floor supports the hearth extension; the hearth extension supports the granite vertical legs; the vertical legs support the top horizontal granite slab. Unfortunately, things don't stay perfectly lined up forever. So at some point there could be lateral loads that kick it out of alignment or maybe even break the mortar bond (due to settling or framing movement).

    After reading the Stone Federation data sheet, its seems prudent to use a secondary (belt-and-suspenders) solution by adding mechanical fasteners. On the other hand, I suspect that very large wall tiles are commonly stuck to the wall with just thinset and without any problems.

    So, does anyone out there have experience with this? Should I do it? Any experience with how to attach with mechanical fasteners? Maybe this is more of a problem with much thicker or larger slabs?

  5. Chaubenee | | #5

    Some brick hangers might help. Is this going to be one inch thick granite veneer?

  6. user-958947 | | #6

    Approximately 3/4"

    2cm = .79"

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