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Who knows how to build a hearth???

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by ansehnlich1, Jan 16, 2007.

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  1. ansehnlich1

    ansehnlich1 Minister of Fire

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    Ok, I wanna build a hearth. (for under my hearthstone phoenix) I need an R-1 factor. I know my hearth pad dimensions so that ain't the issue. A couple things....

    1. The finished hearth will be surrounded and capped by mortared brick. (brick r = .2)

    2. If I use durock, it looks like I'll need at least (3) half inch sheets thick. ( 1/2 inch r = .26)

    So, .26 x 3 is . 78 plus the .2 brick gives me and r of .98.

    I would likely use 4 sheets of half inch durock as that brings me well above 1.0 r factor and also is the approximate thickness of a course of brick....so 2 inch of durock, capped by brick, and surrounded by 2 courses of brick. (r of this setup would be 1.24)

    My question is this, how to attach the durock to a 3/4 inch Advantech OSB sub floor?, and each durock layer to the next?

    Could I just lay it in there and mortar brick on top?


    Do I have to tape the seams, or can I just stagger them?

    Now, on another note, is it true an 8 inch hollow core block will give me a R-1? If I lay those block with the solid side facing up, the hollow core running horizontal, and brick the top and sides,

    I'd have a raised hearth yes?

    This would give me an r factor of 1.2 yes?


    Would I have to mortar the block or could I lay them in loosely side by side?

    Nobody, and I mean nobody, has been able or willing to tell me how to do these things, that is why I am posting these two possible hearth scenarios here :):):)

    thanks,
    Brad

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  2. kevinmoelk

    kevinmoelk New Member

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    There's an article here someplace that outlines how to calculate R-values. If you have not read that yet, do so. R-values and K-values are different, and have an inverse relationship, hence a little bit confusing when running the numbers. Make sure you are 100% here (using the right method), failure of the correct design could burn your house down.

    When building my hearth I simply staggered the layers of wonderboard (5 layers), no taped joints until the last 2 layers. But the fit between the panels were very tight also. If my cuts had come out worse, I would have filled the joints with thinset. It was more a time consideration than for any other reason. Each layer got screwed together with the proper screws, taking a level at each layer as well. I found the screws will sometimes dimple up the board just a hair, those dimples can be hammered down easily. I built on top of a wooden 2x4 frame which was screwed to the floor, just enough so that it wouldn't move, nothing excessive. I made certain that the last 2 layers of screws did not penetrate the framing, as the screws would act as a heat sink, and effectively ruin the insulation factor. The structure is so heavy, it's not going anywhere. That raises another concern about where the hearth will sit, and how much weight it will be... can the structure underneath take the load? I had to add a post and beam structure below.

    You should also consider Micore, which is a better insulative product than durorock or wonderboard, but is unavailable in some places. You could have 1/2 inch of Micore and the same insulative factor as multiple layers of concrete board. If you want a raised hearth that can certainly be done... just depends on what you want. A flush pad can also be constructed if desired. If you are using bricks or blocks for a raised hearth they should be mortared or otherwise attached to each other so they act as one structure. Also, a layer of sheetmetal should be used in such a design. See the hearth building page on Woodstock stoves for more details.

    To answer the question of how to attach to the subfloor, I'll leave that for someone more qualified than myself. Personally, I'd find a way. A custom hearth is custom for a reason. If you want a pre-made looking hearth pad you wouldn't go through all the trouble of building one yourself, right?

    -Kevin
  3. ansehnlich1

    ansehnlich1 Minister of Fire

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    Wrench,

    Thanks for the input. Our floor is engineered to handle the weight, we have planned for that in advance. I have read all about r and k value, and I understand what needs to be done. I really appreciate your telling me how you attached the wonderboard, as that was a main question of mine. I was not sure if I should use thinset in pancaking the layers of durock, or just screw them together. I took close note of the fact you did not run the final two layers of screws into the wood subfloor, makes sense to me regarding heat transfer. What I intend to do is 4 layers of durock, with a cap of mortared brick and running bond brick around the perimeter. I think it appears this plan will suffice.

    I'll run the whole plan past the inspector to make sure all of it is in accordance with his expectations.
  4. wg_bent

    wg_bent Minister of Fire

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    This sounds very similar to what I did. Topped mine with tile. What I did is put a band of air inlets around the base and placed decorative grates around the entire base. This was required to allow my 0 clearance fireplace to vent also, but it allows for airspace above the wood subfloor.
  5. elkimmeg

    elkimmeg Guest

    Here what I would do. Frame in the hearth and suro with metal studs. Attach some to the floor with drywall screws in a few places, then use un faced R11
    fiberglass insulation in between the 12" on center metal stud framing. Install a sheet of dura rock and brick or tile the top and permimeter.
    (done, inexpensive deal) I might figure where the legs will touch and add a metal cross stud under those areas as an extra margin of support

    The fiber glass insulation is non combustible and will never be exposed to temps where it would melt. plus it is protected by the covering and wonder board
  6. bcnu

    bcnu New Member

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    I'm also considering rebuilding my hearth and will post that question later. However, I don't understand this part so please explain. When building a hearth some have mentioned using metal studs or a piece of sheet metal. I assume they don't conduct or transfer the heat like the screws do. How are they different or is it in the way they are used?
  7. kevinmoelk

    kevinmoelk New Member

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    Bcnu, actually metal studs would be better. I had already started construction of my hearth prior to learning about Elks system, or I would have taken his suggestion. Metal will conduct heat quickly and avoid hot spots. Hence the placement of the screws to avoid the framing would not be as critical an issue.

    The piece of sheet metal is used to "even out" the temperatures. So let's say for example you have 2 layers of wonderboard, a piece of sheet metal, then another 2 wonderboard layers. If the heat were dissipating from the legs of the stove downward, when it hits that sheet metal it will conduct through the sheet and move heat away... again, avoiding any creation of hot spots. Read the article at Woodstock, as there is a better explanation there.

    -Kevin
  8. bcnu

    bcnu New Member

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    Thanks Wrench. Simple enough explanation that even I could understand it. :red: I'll reread Elks also. I have some materials I was going to use on another hearth and I will keep all this in mind. Time to go outside and make a snowman just for the fun of it!
  9. nshif

    nshif New Member

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    Im building a 14" high hearth and cant see how im going to be able to use metal studs. I dont want to hijack this thread so ill start a new one for mine soon.
  10. kevinmoelk

    kevinmoelk New Member

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    Ummmm... seriously? Just standard framing techniques. You can do anything with metal studs that you can do with wooden. Can you elaborate on your specific situation? Perhaps that would help.

    -Kevin
  11. ansehnlich1

    ansehnlich1 Minister of Fire

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    hey i don't care about hijacking this thread, i say lets talk about building a hearth, i want to know as much as i can before i start work on mine. no problem.
  12. nshif

    nshif New Member

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    Ok then.... The top height of my hearth needs to be ( or I want it to be 14".) I was thinking of 2x8 or 2x10 as a frame covered with durar, wonderboard et all. A 2 " Slab with 2"rock on top. I dont think ill find 2x8 0r 10" metal studs easily. On top Will sit a Quad 4300 Step top. The below floor frame has been beefed with 4x8 2' oc. so what do you guys ( or girls) suggest?
  13. kevinmoelk

    kevinmoelk New Member

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    Well, you can frame a tiny little perimeter wall, then frame across to build the platform. OR, you could build up a couple platforms and just stack them up. The hearth is a dead load, so some of the concerns you'd have with live loads do not apply. If you are against a pre existing wall, you could also attach a ledger and frame off of that... like building a deck off of your home. Lots of good books on framing, probably at your local library. Check a few out to gather some viable ideas.

    I also seem to recall seeing recently a member who had his stove on some I-beams. Those come in all sizes, and you could incorporate those into the platform.

    Sounds like your foundation will be plenty strong enough to build whatever you wish.

    -Kevin
  14. elkimmeg

    elkimmeg Guest

    Let me take a stab at this the hearth f outer frame can be combustiable providing it is beyong the clearance
    Say you needed 12" to the side 13" away could be wood

    How to lighten up the load Install metal stud channel for 2/4 or 2/6 to the outside wood perimeter

    Install metal joist in the tract and install center support legs from leftover metal studs. Fill the bottom cavity with unfaced R11 or R13 fiberglass insulation leave the air space.
    Install dura rock for the top layer and install tile or briclk stone slate finnish

    Hint plan for additional support where the 4 legs of the stove hits. Now you have a fairly light well insulated plus 10 " dead air space. You are done and it was easy
    I cut mteal studs with tin snipe and sreew then into place to the tract channell

    Keep it simple and easy
  15. colsmith

    colsmith New Member

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    We built a really raised hearth for our Hearthstone Phoenix. It allows for several days wood storage underneath, and is a nice level for access, plus viewing the fire. 2 x 4 framework with plywood on top, 1 sheet of Micore on that, and 1 sheet of Durock or similar on top, topped with tiles. Gives us slightly over R 1, we put the Durock on top just because Micore itself is squishy and somewhat fragile. We made the hearth larger than required, and only put screws into the areas outside the mandatory hearth size area, so that we don't have much concern about transmission of heat through the screws. The other places where we stuck the layers together we used construction adhesive or maybe it was the thinset mortar stuff, anyway something that does not penetrate through the layers to muck with the R 1 factor.

    Although we were going for a tall hearth, we didn't want the pain of connecting all the required layers of cement board together for the R 1, and then giving it some kind of attractive edging for the whole tall pile. We went to Mexico shortly before making the hearth. We got some lovely tiles, including the fancing curved edging ones, fairly cheaply. I should post a picture, as the hearth is really beautiful, but I am not at home right now. We bought the Micore directly from the place we bought our wood stove.

    I don't know much about construction, am generally the assistant, but I would think you'd want to mortar your blocks together if going that route. You want a very stable base.
  16. DonCT

    DonCT Minister of Fire

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  17. nshif

    nshif New Member

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    Good Ideas. Thanks. Im Now thinking what Ill do is frame a base layer of 2x6 with 1/2 cdx on top. cover all that with duroc then lay 2x4 metal studs with R19 unfaced between, then 2 layers of duroc Cover all that with 2" of rock and mortar and I should be good. I think.
  18. Gooserider

    Gooserider Mod Emeritus

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    Overall sounds good, but I'd probably go for 3/4" CDX for the decking given that it's getting a fairly hefty load put on it and that I'm a paranoid sort... :)

    Gooserider
  19. kevinmoelk

    kevinmoelk New Member

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    Good point Goose. I would also suggest that you build the second frame 90* to the first, again for greater strength.

    -Kevin
  20. nshif

    nshif New Member

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    Yea 3/4 is better Illl go that way Goose.
    Wrench, That was my plan, 1st deck 90 to floor joists and 2nd 90 to 1st.
  21. jjbaer

    jjbaer New Member

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    Elk,

    Unless the Dura rock alone meets the R-value required, how do you meet the R-value above each stud because at these points all you have below you is Dura rock and metal studs, not fiberglass (fiberglass is only between the studs)? What does code say about a situation like this?
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