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A few questions about laying the hearth. - Forno Bravo Forum: The Wood-Fired Oven Community

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  • A few questions about laying the hearth.

    I finished the base stand last week, and am getting ready to lay the hearth.

    2 standard peices of plywood will fit almost perfectly on top of the hearth, covering about half of the cinder-blocks below.
    In the Forno Bravo PDF, it says to remove the ply-wood, but do you really need to? I know in my last house they just left it under the concrete where they laid the front steps, and it just eventually rots away.

    Also, why not use v-crete or p-crete for the first layer of the hearth? Im guessing its not as strong, but was not sure.
    Thanks for any advice.
    --->My Build<---
    --->My Album<---

  • #2
    If I understand your first question, are you considering "trapping" plywood between the hearth and cinder-blocks? If yes, I think it is a bad idea as the hearth could settle (or at least have some compromised bearing area) when the plywood deteriorates. I used cement board, which also covered about half the thickness of my stand blocks - I picked it because I could leave it in place without worrying about having it rot out and cause any instability. The FB plans recommend removing plywood because they anticipate that it is not trapped and only used to keep the concrete from spilling into the center of your stand.
    If your second question is why isn't p/v crete used for the first layer of the hearth, I don't think they have the strength to span your stand bricks and support the weight of the oven. They seem to work well if they laid upon a good structural concrete hearth for insulation purposes.
    My build thread
    http://www.fornobravo.com/community/...h-corner-build

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    • #3
      Originally posted by danjmath View Post
      I finished the base stand last week, and am getting ready to lay the hearth.

      2 standard peices of plywood will fit almost perfectly on top of the hearth, covering about half of the cinder-blocks below.
      In the Forno Bravo PDF, it says to remove the ply-wood, but do you really need to? I know in my last house they just left it under the concrete where they laid the front steps, and it just eventually rots away.

      Also, why not use v-crete or p-crete for the first layer of the hearth? Im guessing its not as strong, but was not sure.
      Thanks for any advice.
      It is such a simple job to cut the ply to fit within the blocks and support it while you pour I cannot see why you would do otherwise. Just make sure the ply has good clearance from the block otherwise you will trap the ply you can seal the gape with tape , mastic or such

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      • #4
        Sorry no advice here, just questions about the hearth! Just want to be sure before we spend time/money. We're building the base/wall with concrete blocks in a U-shape, with support in the middle as well. Our foundation isn't as large as we'd like (due to the ground/space restrictions). Can we make a frame for the hearth that extends the sides of the base/wall by a few inches? The weight of the oven itself would be resting on the cement blocks, but are those blocks with some center support (not solid) enough to support the weight of the hearth and oven, and will the hearth be strong enough to support the extended opening if hanging over the end of the cement blocks by a few inches? How do we remove the frame once the Portland Cement/Vermiculite is poured?

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        • #5
          An overhang for the hearth concrete does work, I had about a 3" overhang. I suspect others could chime in about the maximum recommended overhang.

          To release the concrete, (both regular and vermiculite+Portland cement) I used vegetable oil so that the wood frame would release from the concrete.
          David in Calgary
          My Build Thread

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          • #6
            First of all, you do not want a vcrete mix as a support hearth only as an insulation layer on top of a concrete structural heath. VCrete alone is not strong enough to an oven. A small overhang of a few inches should not be an issue since the bulk of the weight will be under the dome walls which are over the CMUs. You do not say how thick you hearth is or how large the oven is though so can't comment on span.
            Russell
            Build Link............... Picassa Photo Album Link

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            • #7
              To clarify to Iventi, my hearth overhang was ~3" using proper concrete for the hearth, the thickness was about 3.5" (i.e. a 2x4 board) and contained wire and rebar re-inforcing. The V-crete was on top of the insulating heart.

              I tracked down a couple of my photos from my build to show what i'd done.
              David in Calgary
              My Build Thread

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              • #8
                Thank you all very much for your help. I'm definitely missing something....our intent is to build a u-shaped concrete base (on top of the poured concrete foundation) out of 4" thick blocks, then lay 4X6" beams crosshatched to support a plyboard/frame where we'd pour a 5:1 Portland Concrete:vermiculite 4" hearth on which we'd lay the firebricks. I believe there's supposed to be rebar in that mix as well.
                1. Sounds like my layers aren't correct.
                2. How do we go about removing the plyboard from under the concrete hearth? I can remove the sides, but under the concrete?
                3. Will the crossed, treated beams (in theory under the concrete hearth) be ok given the heat from the oven?

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                • #9
                  I am still confused on the layout. The 4 X 6 beams wood or a masonry lintel? If wood, IMHO you are heading in the wrong direction. You need to pour a concrete hearth that DOES NOT have perlite or vermiculite in it then you pour the p or vcrete (5 to 1) on top of it then the firebrick. You still have not given any info on oven size or base size. I suggest you down load the e-plans from Forno Bravo and review the construction process, they are only 2-3 bucks and although dated will give you a good baseline to help you understand the building process.
                  Last edited by UtahBeehiver; 07-11-2018, 07:55 AM.
                  Russell
                  Build Link............... Picassa Photo Album Link

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                  • #10
                    Your description of layers sounds like it replaces the concrete hearth with 4x6", then putting the vcrete on to of that. I don't know if that would work.

                    To clarify, the hearth I'm describing (as in the pompeii plans) is the 3.5" of standard concrete on top of the u shaped cinder/concrete block base.

                    The vcrete on top of the hearth is instead a layer for insulation, not a hearth.

                    To remove the plywood under the concrete, you would build a temporary support structure underneath to support the plywood and keep the concrete from collapsing. Then after concrete is cured, you would remove the temp structure and plywood.
                    In my case I replaced the plywood with cement backer board, and then left it in place under the concrete. I also used an H shape base structure so I di not need a temp support structure.
                    David in Calgary
                    My Build Thread

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                    • #11
                      Aha it's beginning to make sense - sorry a bit dense; definitely a newbie. I was missing the concrete hearth under the insulation layer...and that's where the rebar goes! Thank you both! And using concrete backer as the base of the frame is perfect - nothing to remove but the outer boards. Thank you much - this is proving to be a slower process than I'd expected, but at some point in the future hope to post a picture of a finished product producing awesome breads, pizzas, and roasts!

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                      • #12
                        If you have a big span between your U in your base, you may still need a temporary support under the middle of the concrete backer board until the hearth cures. Others may have to chime in here about acceptable span. I designed my spans to be smaller, to make multiple smaller storage compartments, so the temp support wasn't needed.

                        As mentioned by Russell, providing the planned size of your oven and dimensions of your base will help others to provide answers. (pictures help too)
                        David in Calgary
                        My Build Thread

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                        • #13
                          Our foundation is 52"X71" - it's poured concrete ranging from 4" to ~12" in depth. The plan is to place concrete blocks along the back 68" and sides 44" (so they'll be 52" when you add the 8" from the back wall). We'd also stack blocks about 1/3 and 2/3 of the way across in the middle to support the hearth, insulation, and oven. Then we'd pour concrete into the blocks. Our plan is to place concrete backer on the block wall that extends out past the wall in order to gain more surface area, then build a wood frame on top. We'd add rebar to the hearth layer so it could extend out a full foot (hopefully?), thus supporting our arch/door. We'd also stack more blocks under the arch/12" overhang. Our goal is to build as large an oven as our sadly small foundation will allow - we have more space, but the ground underneath isn't easily leveled to allow for a larger foundation. Thus hoping to keep the actual oven on the new foundation but the arch extending onto the supported hearth (that overhangs the wall).

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                          • #14
                            Ugh. Make that 64" across the back for the wall. We'd definitely want the oven extend a bit past the block wall if a rebar-enforced hearth would bear that weight. Otherwise this oven will be puny.

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