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Granite hearth on cement board over rug

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by hydestone, Jan 11, 2006.

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  1. hydestone

    hydestone New Member

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    I recently installed a wood stove using the chimney from my existing fireplace. I wanted to put the wood stove as far into the room as possible to gain the most heat so I built a granite hearth to rest the stove on. The problem is that I have a really old and really large area rug that I do not want to cut so the stove will fit. I placed Durarock cement board over the rug and 2" thick slabs of granite on top of that. The joints between the 4 slabs of granite are filled with a silicone based fireplace mortar. Does anyone think this is going to be a problem? I will schedule the building inspector to come look at the installation but I want to be sure it will pass before he comes over to look at it.

    Thanks,
    John

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  2. Sandor

    Sandor Minister of Fire

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    Durarock and granite are both poor insulators and I would NOT do that in my home whether an inspector passed it or not.
  3. Rhone

    Rhone Minister of Fire

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    Sorry, that's woefully underinsulated. Stoves want a very high R-value hearth, I checked a medium sized one and it specs a hearth with R6.6 with a stove that has 4" legs, and R2.5 for a stove with 6" legs AND a bottom heat shield (Yikes!). The radiant heat pools on the surfaces below and if they pass enough heat they can spontaneously combust the flammables underneath. Stone in particular is a bad insulator. Granite, of stones happens to be one of the worst even worse than marble. The 1/2" durock has an R-Value of .26, the 2" of granite is contriburing R0.08 combined so basically, you have around R-Value of 0.34, that is a very far off from what stoves like particularly if you see the same medium sized one that wants an R6.6 hearth.

    Glad you asked, looks like you've got yourself a time bomb. Instead of the cement board, use many pieces of 1/2" thick Micore 300 board which has an R-Value of 1 per 1/2" thickness and somewhat solid, it's sort of like a cork tack board. Not particularly hard, but great R-Value and fire retardant.
  4. Rhone

    Rhone Minister of Fire

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    I edited my post above if anyone saw it previously.
  5. babalu87

    babalu87 New Member

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    Get a sheet of micore300 and put a layer or two down

    I have one if someone wants it
    Boght two sheets and only needed one :(
  6. rmcfall

    rmcfall Feeling the Heat

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    where are you? I might be interested.


  7. KarlP

    KarlP Feeling the Heat

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    What does the stove install guide say? That would be perfectly fine with some stoves and a guaranteed fire with others.

    For example, my VC Madison only needs a piece of sheetmetal under it if the factory heat shields are installed. Without the heat shields the requirements for floor protection are far more than you have.
  8. hydestone

    hydestone New Member

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    Although your replies created a lot more work and some more heavy lifting I am glad I asked! I am going to pick up the granite and durock and put down (2) layers of Micore 300 with the granite back on top. The only problem is that the extra 1/2" thickness is going to make it tough for me to achieve the 1/4" per foot pitch required for the stovepipe heading back into my old masontry firebox.

    I have a Jotul 602 stove. The bottom of the stove is 12" above the hearth.

    What are your thoughts about leaving the oriental rug below the micore? The footprint of the stove is 13 wide by 22 long. The hearth extends 19" in front 12" in back, and 23" on either side.
  9. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think you'll ruin the rug if you put that much weight and heat on it for a long period of time. If nothing else, it won't fade/patina the same way. So I guess, what's the point in sticking with the in that location?
  10. babalu87

    babalu87 New Member

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    SE Massachusetts, Middleborough

    My hearthpad has K and R values that far exceed what was required for my stove
    It is plywood/micore300/durock/porcelin tile
  11. Rhone

    Rhone Minister of Fire

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    Sorry, the carpeting has to go. The manual for the Jotul F 602 USA says on page 6

    The floor protection must have a minimum R Value of D.45. Warning! Never put any type of floor protection on top of carpeting.

    Though I've never heard of R Value of D.45, maybe they meant a 0 instead of the D and you only need 0.45. Take out your durock, the carpeting underneath, put down a piece of Micore 300, your granite on top, and you'll be over R1.0. The problem is, if you later want to expand your stove with a more powerful unit, you'd have wished you used more Micore 300. So, if you think there's a chance in the future for a bigger stove, better beef up the hearth now and sorry, the carpeting has to go.
  12. hydestone

    hydestone New Member

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    Yeah it seems inevitable that I have to remove the rug. Thanks for your input!
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