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  • Another Minneapolis WFO

    Hello, my name is Sixto and I live in South Minneapolis. I'm about to start work on building a Pompeii-style oven and first wanted to thank FB and the Forum Moderators for creating such a useful source of information.

    I have 3 design/construction questions, but first a bit about what I'm planning to build. I'm planning on a 36" inside diameter oven. Initial mockups show this should be plenty big for our needs. I'm a retired architect, and my wife is a potter/teacher so we know enough to be dangerous, but not enough to do it right without help and community support.

    Living in Minnesota we know how cold it gets, and also what that can do to an unprotected masonry structure. Multiple freeze/thaw cycles can reduce a beautiful stone, brick or concrete building to an ugly pile of rubble in a few years. I would like to avoid that if possible, so I'm paying attention to drainage, flashing, weep holes etc. My wife is interested in cladding the dome with porcelain tile, which should help add a layer of protection.

    My first question is whether the community thinks that adding a layer made of thin aluminum flashing sheets between the insulation and expanded mesh would be good idea to direct any water that leaks through the render, around to the outside base of the dome where it can be flashed and drained out of the way before soaking bricks or insulation. These flashing sheets would overlap like shingles or fish scales, and have some play to take up any expansion/contraction. is there any downside to this? I'm hoping this plus a custom-made removable waterproof cover will let me fire up the oven in the winter without causing any brick damage due to rapidly expanding steam within the bricks.

    My second question is that I would like to minimize the depth of the vent area, but also have a good thermal break all around the dome arch - separating the vent floor, walls, and chimney from the dome. I'm thinking that I can help this by making the dome arch 3" shallower, so that the face of the arch at the vent centerline is 3" inside the outside face of the dome brick at the first course. I would still taper and chamfer the dome arch to make a smooth transition on the inside of the dome, I would just try to make it so the dome arch does not project beyond the base of the dome. Then I would add 2 or 3 ceramic rope seals between the dome arch flat face, and the vent/chimney bricks. Any downside to this approach? (see sketch attached)

    My last question is about the transition between the brick vent/chimney base and the 6" S.S. double-wall flue. I plan to have a 10" square base plate bolted or screwed to the brick chimney above the arch, but I'm wondering if a 4' flue section plus a flue cap above the plate is tall enough, and if having the anchors plus a cast cement cap would be enough to keep the flue from blowing over in a strong wind (even though it's pretty-much protected by two garages along the alley.)

    That's about it for starters... I have to keep digging (topsoil is out, hard clay and rocks below) bulld a 6" thick concrete floating slab with rebar top and bottom (I hope that a 7' square slab with all that weight on top won't get tippy from soil heaving) - Now that I think of it, I'll add some gravel/sand/plastic below the slab for drainage...

    Thanks again for all the information shared already, looking forward to the build and your feedback. - Sixto.
    Attached Files

  • #2
    Using aluminium flashing is often recommended by some builders. But remember that as well as preventing moisture from entering it also prevents moisture escaping. My own experience is explained here.
    https://community.fornobravo.com/for...573#post439573

    Minimizing the depth of the flue gallery makes working the oven far easier and is a good design feature IMHO. Unfortunately using brick units makes this more difficult. You can push it further towards the centre of the oven(which i notice you have done) as well as moving the pipe over the top of the oven mouth. Casting a flue gallery allows far easier design and building alterations as well as the ability to make the gallery far thinner, thereby also reducing its heat sink effect. Bolting or screwing an anchor plate introduces fixing problems like inviting cracking around the holes from thermal expansion. If you must drill use nothing but stainless fixings.

    A 4' tall flue pipe is about the limit of an unstayed pipe IMHO. I usually use a 900mm unstayed pip and if making it higher use stays.

    If your wife is a potter she could make some terracotta caps like mine. which work as a vent to allow moisture release as well as supporting the pipe.

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    Kindled with zeal and fired with passion.

    Comment


    • #3
      A simpler method would be to use brick units for the sides of the gallery with a cast top piece. In this case the anchor plate has been bothered and cast into the top of the casting.
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      Kindled with zeal and fired with passion.

      Comment


      • #4
        Thanks David , I value your expert advice! I do understand and share the concern about drying out the shell...I wonder if a loose shingle configuration for aluminum would allow both drainage of rainwater, and evaporation of absorbed water through the joints... I also think the moisture in the bricks can evaporate via the inside surface of the dome, and out the flue... I guess I'll have to try t now, and see if my theory succeeds by not holding too much water ;-)

        Regarding the vent depth, after talking to my wife, she thinks a 2' flue is tall enough, and I can support that at the base, and also at midpoint by raising the bricks around the flue part of the way. This could avoid anchors altogether, relying on gravity at the base and the cap a foot higher to keep it upright. I think the 3" shallower dome arch gives me the depth I was looking for... the cast vent idea is intriguing, but probably pushes our technical abilities further than I think we need to.

        Stay tuned!


        Comment


        • #5
          My oven has 4" of blanket insulation covered by a stucco render. The render has a waterproof coating over it. The coating is "water" proof, but vapor permeable. And that's a big thing in my opinion. Liquid moisture can not enter during a wet storm, but water vapor can escape during a fire. Over that, the dome is covered by a stone veneer.

          Since you'll be tiling over this, you can use a paint-on membrane, something along the likes of Hydroban Liquid which you might be familiar with if you spec out shower construction details. It's waterproof but vapor permeable. Unlike something like RedGard that is both water and vapor impermeable. You can tile directly upon the Hydroban. Or whichever membrane you use. Use a vapor-permeable grout.

          Vent pipes at the apex of the dome have been discussed. I added one almost as an afterthought, but after my experiences, I'm sold on them having value. A vent at the apex of the dome can provide another avenue for moisture to escape the dome insulation without it having to pass through the shell's vapor permeable coating and subsequently then having to escape through the exterior grout. that'll take a lot of pressure off of the thinset that bonds the porcelain tile to the stucco render.

          If I understand your vent arch construction, it's similar to mine. So I'll give you a thumb's up on that. lol

          Your chimney question, you might appreciate the added draw that a 4' tall pipe will provide. The added draw will bring more smoke up the chimney versus it wafting out the front of the oven. I have an anchor plate bolted to the firebrick at the top of the arch. A cast concrete cap sits on top of the plate. My 4' length of 8" pipe is unsupported. It's quite rigid. It's survived multiple storms in the 60-70mph wind range.

          I attached a few photos showing:
          1) the vent arch bricks wrapping around, yet structurally and thermally isolated (as best as I wanted to) from the dome arch bricks.
          2) The flared opening at the top of the vent arch.
          3) the simpe anchoring of the chimney plate
          4) the cast concrete cap that covers the anchor plate and the 4' flue in place.
          Attached Files
          Mongo

          My Build: Mongo's 42" CT Stone Dome Build

          Comment


          • #6
            Thanks Mongota, your chimney arrangement looks very similar to my plan. I might still reduce the unsupported length of the flue...in the audio world we call it WAF. My wife prefers a shorter flue profile, (and also small satellite speakers) while I like the sound of big woofers. She is not as concerned about draft.

            That's quite the nice job of grinding the curved surfaces at the base of the flue, did you do rough cuts on the saw and finish with a grinder?

            Also I am curious about the red beads of sealant holding the rope in place, will any high/temp caulkk work or do you have a specific recommendation?

            i also like the idea of a liquid moisture barrier that is also vapor permeable (Hydroban). i was not aware that such a substance existed. That may be a better solution than my aluminum shingle idea, will be researching that further. And of course the vent at the dome peak is part of my design also...

            This is is great, I can't wait till I can stop digging and start pouring concrete!

            Sixto

            Comment


            • #7
              Originally posted by Sixto View Post

              That's quite the nice job of grinding the curved surfaces at the base of the flue, did you do rough cuts on the saw and finish with a grinder?

              Also I am curious about the red beads of sealant holding the rope in place, will any high/temp caulkk work or do you have a specific recommendation?
              Sixto


              The curved surfaces on the firebrick were cut by hand. A hand-held angle grinder with a diamond blade. Quite fast, very easy. I'd recommend a continuous rim blade, a segmented blade could "catch an edge" when feathering the curves by hand.
              The red sealant is high temperature RTV silicone. I likely bought it at a box store. Likely rated in the 500-600 degree range. I used it to tack the ceramic fiber rope in place while building. Once built, the rope is "lightly pinched" between the bricks to prevent it from slumping should the silicone lose its bond over time.

              Best, Mongo
              Last edited by mongota; 07-17-2021, 07:44 AM.
              Mongo

              My Build: Mongo's 42" CT Stone Dome Build

              Comment


              • #8
                A cupped diamond wheel for an angle grinder also works well. I either case make sure you use a particle mask because the silica dust from the brick is not good for you or anyone around the grinding.
                Russell
                Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

                Comment


                • #9
                  Snow-belt question...Does anyone know if I make an 8" thick, reinforced concrete pad, with a grid of rebar top and bottom 2" in from edges, will that more or less stay level and not crack in half during the winter? Frost depth is 42" and I just don't wanna dig that far... I guess 12" thick would be better, but is it necessary? I'm expecting the oven will weigh 3,000 to 3,500 lbs when completed, the pad is 5' wide x 6" deep, so the load is about 100psf... soil is well drained, made up of average mix of sand and clay... I've removed about 12" of organic topsoil.

                  p.s. Thanks Russell, good reminder- I have several respirators with good particle cartridges, and will use them when mixing, cutting, grinding, and handling insulation.

                  Sixto
                  Last edited by Sixto; 07-20-2021, 08:30 PM.

                  Comment


                  • #10
                    Sixto
                    I don't recall just how far down I dug, but I'd venture it was in the 24" deep range. Frost is 42" in my area.

                    I built in a very well draining area. No surface ponding with good subsoil/gravel drainage. I might have gone to frost had the build location been different.

                    I dug out a perimeter trench (left a plug of undisrurbed subsoil in the middle) and then poured a perimeter footing. Then mortared a couple courses of CMU on top of the footing. I poured my "on grade" slab on top of the CMU. Used a 2x6 as the form, so about 5-1/2" thick. Reinforced with rebar. A couple of photos can be seen in this post: https://community.fornobravo.com/for...344#post253344

                    The oven has been through several winters, some with snow cover all winter, some winters well below freezing with minimal snow cover. No issues. Zero heaving.
                    Mongo

                    My Build: Mongo's 42" CT Stone Dome Build

                    Comment


                    • #11
                      Originally posted by Sixto View Post
                      Hello, my name is Sixto and I live in South Minneapolis. I'm about to start work on building a Pompeii-style oven and first wanted to thank FB and the Forum Moderators for creating such a useful source of information.

                      I have 3 design/construction questions, but first a bit about what I'm planning to build. I'm planning on a 36" inside diameter oven. Initial mockups show this should be plenty big for our needs. I'm a retired architect, and my wife is a potter/teacher so we know enough to be dangerous, but not enough to do it right without help and community support.

                      Living in Minnesota we know how cold it gets, and also what that can do to an unprotected masonry structure. Multiple freeze/thaw cycles can reduce a beautiful stone, brick or concrete building to an ugly pile of rubble in a few years. I would like to avoid that if possible, so I'm paying attention to drainage, flashing, weep holes etc. My wife is interested in cladding the dome with porcelain tile, which should help add a layer of protection.

                      My first question is whether the community thinks that adding a layer made of thin aluminum flashing sheets between the insulation and expanded mesh would be good idea to direct any water that leaks through the render, around to the outside base of the dome where it can be flashed and drained out of the way before soaking bricks or insulation. These flashing sheets would overlap like shingles or fish scales, and have some play to take up any expansion/contraction. is there any downside to this? I'm hoping this plus a custom-made removable waterproof cover will let me fire up the oven in the winter without causing any brick damage due to rapidly expanding steam within the bricks.

                      My second question is that I would like to minimize the depth of the vent area, but also have a good thermal break all around the dome arch - separating the vent floor, walls, and chimney from the dome. I'm thinking that I can help this by making the dome arch 3" shallower, so that the face of the arch at the vent centerline is 3" inside the outside face of the dome brick at the first course. I would still taper and chamfer the dome arch to make a smooth transition on the inside of the dome, I would just try to make it so the dome arch does not project beyond the base of the dome. Then I would add 2 or 3 ceramic rope seals between the dome arch flat face, and the vent/chimney bricks. Any downside to this approach? (see sketch attached)

                      My last question is about the transition between the brick vent/chimney base and the 6" S.S. double-wall flue. I plan to have a 10" square base plate bolted or screwed to the brick chimney above the arch, but I'm wondering if a 4' flue section plus a flue cap above the plate is tall enough, and if having the anchors plus a cast cement cap would be enough to keep the flue from blowing over in a strong wind (even though it's pretty-much protected by two garages along the alley.)

                      That's about it for starters... I have to keep digging (topsoil is out, hard clay and rocks below) bulld a 6" thick concrete floating slab with rebar top and bottom (I hope that a 7' square slab with all that weight on top won't get tippy from soil heaving) - Now that I think of it, I'll add some gravel/sand/plastic below the slab for drainage...

                      Thanks again for all the information shared already, looking forward to the build and your feedback. - Sixto.
                      Welcome Sixto. Nice drawings!
                      Kind regards,
                      Mark
                      My 42" build: https://community.fornobravo.com/for...ld-new-zealand
                      My oven drawings: My oven drawings - Forno Bravo Forum: The Wood-Fired Oven Community

                      Comment


                      • #12
                        While not quite as cold as MN here in Utah it can get sub zero here in the winter. I did a mono pour base about 6" with 8" welded wire mesh and #4 rebar around perimeter. The key for me was 8" of gravel underneath for drainage and to minimize frost heave. My soil is a sandy loam. I have not had any movement of the base. I would look at what you soil type is, if clay you will need to mitigate the soil expansion more than if it is a sandy loam. I think Randy J went both ways with his builds, you should PM him.

                        PS, even though I am an engineer and had access to all types of CAD programs, I prefer the old pencil and graph paper and "slide rule" LOL, just kidding on the slide rule.
                        Russell
                        Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

                        Comment


                        • #13
                          Sixto - great progress! And you are getting really good advice here from some amazing people (all the folks chiming in have helped me immeasurably!).
                          Mongo is right about the liquid barrier - just did my entire enclosure in same and it works really good.
                          As for the draw - I'm just completing my curing fires and the 8" diameter, 4' chimney is drawing like a vacuum! I know there is an aesthetic you are after - but in this case - I think function over form wins the day - just wonder how your wife will feel about the charring of the outside of the oven mouth from all the smoke coming out the front? I would heed this advice mate.

                          Dig deep - up here in Canada, and sitting in clay - I went down 36" until I hit bedrock and dug a similar base as Mongo. Framed and poured a massive amount of concrete in one pour (walls and final slab integrated). Wouldn't take chances here. Last slab I poured was a 40' x 25' one for a flagstone base and mortared the stones on top - even that one I went down 18" and was nervous.
                          Sweat equity coming your way mate - well begun is half done and encouraging you to have a solid foundation for the fun that is to come!

                          Best of luck with it all
                          Barry
                          You are welcome to visit my build HERE

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