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Hearth slab construction - Forno Bravo Forum: The Wood-Fired Oven Community


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Hearth slab construction

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  • Hearth slab construction

    Hi all this is a great site, Thank You James, I keep reading about the concerns of some of the members reguarding the the vermiculite-portland slab for the bottom part of the insulating hearth as not being strong enough I am ready to build my hearth,and would like a little more info should I pour a reinforced bottom layer of concrete and then pour a vermiculite layer and then a concrete layer for the top? Thanks

  • #2
    New hearth design

    Welcome aboard -- great name.

    I think the timing for your posting is good. We have been talking with our oven suppliers, and have decided to slightly modify our recommended hearth design.

    We have upped the amount of insulating concrete and moved the insulating layer to the top. The standard design calls for a 7 1/2" hearth, with 3 1/2 structural concrete underneath and 4" vermiculite on top, with the cooking floor set directly on the vermiculite layer.

    The reason for the change is primarily oven performance. We have found that putting the floor directly on the insulating layer makes it easier to hold high heat in the floor for higher #s of pizza and easier to refresh to bring it back up to heat when/if it falls. The floor still holds enough heat for roasting and baking.

    This definitely does not mean that there is a problem with ovens installed with the layers flipped. It's a matter of taste and there are some who still like that method, but we've concluded the insualtion on top method is more mainstream. Both methods are much better than the "sand" method in the new Sunset Outdoor Kitchen how-to book.

    Check the graphic in this thread for the layers.

    Vermiculite has very high compression strength, so that layer will never be a problem.

    As I noted elsewhere, it might be a good idea to download the Casa installation guide to get more details on the stand, dimensions, enclosures, etc.

    Pizza Ovens
    Outdoor Fireplaces


    • #3
      Any other alterations to the Pompei Oven Plans?

      As I'm gathering the last of the materials to build our first wood-fired oven, I noticed the change to the hearth construction. We will plan to alter our plans to the 7 1/2" thick hearth with the flip-flopped vermiculite/concrete layers.

      Are there any other changes to online plans that we should consider?

      As far as the oven dome construction is concerned, would you recommend using the same orientation for all half blocks or use a starter row on end as stated in the online directions?

      Thanks for your help. We are excited to begin our wood-fired adventure!


      • #4
        Foam forms

        Along with the new hearth, you might want to look at pictures of ovens in the forum built using styrofoam forms, rather than wedges, to get the right inward dome curve. That seems to work well.

        I think most ovens start with the first course upright, but it really is a matter of taste more than "right and wrong."

        Pizza Ovens
        Outdoor Fireplaces


        • #5
          finishishing perlcrete top layer

          we have the hearth completed with the perlcrete mixture on the top. 4" concrete on bottom, 3.5" perlcrete on top. the perlcrete is really bumpy and looks very rough.

          we plan on laying the cooking floor on sand/fireclay mix to even the floor. for the rest of the exposed perlcrete layer, how can you recommend us smoothing it out? we're planning on finishing the oven igloo style.


          • #6
            finish perlcrete layer

            If you want finish concrete look around your dome, you can trowel stucco mix over your insulating layer. If you want a more finished look, you can trowel tile adhesive (the concrete based, not the glue based) and lay terra cotta tiles. If you do this before you do your loose layer under the oven floor, you won't have to do fussy cutting of the tiles.
            My geodesic oven project: part 1, part 2


            • #7
              If you want a little more control over the organic Igloo shape, you can construct the outer shell from rebar and stucco lathe (wire mesh). You can bend and shape it to get the form you want, the cover it with a series of stucco coats.

              You can even fill the space between the castable vermiculite or perlite and the outer wall with loose insulation.

              Pizza Ovens
              Outdoor Fireplaces