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Slab thickness - Forno Bravo Forum: The Wood-Fired Oven Community

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Slab thickness

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  • Slab thickness

    I spoke with a vendor at the Arlington Texas Garden Show today about building a WFO who claimed to be a master mason and he told me that I should make my foundation slab atleast 12 inches thick. Is he all wet or just trying to discourage me from building my own WFO? He was trying to sell me a $3500 prefab oven that he claimed I would be making pizza in 2/3 hours after he started the build.

    Phil

  • #2
    Re: Slab thickness

    Phil - that seems a little thick to me. Mine is only 3.5 inches except for the perimeter footings. Maybe he builds a lot of airport runways.

    Les...
    Check out my pictures here:
    http://www.fornobravo.com/forum/f8/les-build-4207.html

    If at first you don't succeed... Skydiving isn't for you.

    Comment


    • #3
      Re: Slab thickness

      A layer of gravel so it doesn't get contaminated by fines and 3 1/2 to 4 inches should be fine. Use lots of rebar: 3/8 on 8 inch centers both ways bent at the ends.

      Comment


      • #4
        Re: Slab thickness

        Mine is 4" with one #3x20' rebar cut into 5 peices laid in around the perimeter and across the center. I mixed the mud from scratch with some free sample masonry cement, gravel, and sand. By my calculations, I needed the slab to be at least 300psi (approximately equivalent to packed Dirt), and I figure I hit at least 2000. My advice is to overbuild as much as you can afford, but in Plano, 12" sounds pretty extreme.

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        • #5
          Re: Slab thickness

          Just for the record, a twelve inch thick footing on undisturbed soil, below the frost line, protruding six inches beyond the perimeter of the structure, is code for masonry fireplaces. That said, almost no one builds freestanding ovens this way. If it's part of your house, and you need to design to building code, then fine. Otherwise, a 3.5 inch slab on well drained crushed rock is all you need.
          My geodesic oven project: part 1, part 2

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          • #6
            Re: Slab thickness

            Phil - dmun is right. You don't need the 12'' slab but I would put piers under it down as far as you can dig (check out Fairview WFO on this). Our soil is unstable and very susceptible to movement. I'm in the final planning phase and will be putting in piers soon. - Scott
            Scott -

            My projects: http://www.facebook.com/#!/scott.kerr.794

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            • #7
              Re: Slab thickness

              Scott - By piers do you mean concrete using those round forms available at HD? What diameter, and how many? Or do you mean a footing (trench) around the perimeter that is 12 inches or so deep?

              Phil

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              • #8
                Re: Slab thickness

                >>>personal opinion alert<<<
                I think putting random sonotubes under your slab is the worst of both worlds from a frost heave perspective. Without flared footings at the bottom of your pillars, it just gives the ice lenses something to grab on to.



                I say, either build proper footings below the frost line, or pour a reinforced slab on well drained crushed rock. Don't do a bit of both.
                My geodesic oven project: part 1, part 2

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                • #9
                  Re: Slab thickness

                  Plano does not have a frost line, just black gumbo mud.

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                  • #10
                    Re: Slab thickness

                    Frost line is not an issue but the black gumbo is a pain. Dig with a post hole digger or a bucket auger (Elliot's Hardware) down at least 3 feet on the corners. Put a layer of sand about 3 1/2 to 4 inches thick under the 3 1/2'' slab to take care of the shrink and heave gumbo issue. Think of it as building a table with legs where the table top is your slab and the piers are the legs and the sand allows our soil to move without affecting your slab. I just bought a bucket auger I can loan you. Contact me at scottk22@mac.com.
                    Scott -

                    My projects: http://www.facebook.com/#!/scott.kerr.794

                    Comment


                    • #11
                      Re: Slab thickness

                      "Just for the record, a twelve inch thick footing on undisturbed soil, below the frost line, protruding six inches beyond the perimeter of the structure, is code for masonry fireplaces."

                      Yes. That is generally a smaller footprint and supporting the twenty foot masonry chimney.

                      Comment


                      • #12
                        Re: Slab thickness

                        "Don't do a bit of both."

                        Or.

                        Don't build a slab at all !

                        Comment


                        • #13
                          Re: Slab thickness

                          When my footing slab was done....it was about 15 cm (5 inches) thick with re-enforcing bar through the cement. I think this was needed as the size of my workbench and weigh of the WFO was about the 700 kgs mark. The workbench/support slab that was made is alittle different. The slab is around the 12 - 13 inches thick mark (yes, very thick!!!) with enough reo bar in it the build a house on. But the brickie who helped me did his calcuations wrong and made the concrete double strength!!! But it has worked out good as it is a free standing slab and I know there is no way its going anywhere!!!

                          Scott

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                          • #14
                            Re: Slab thickness

                            PlanoPhil,

                            I'm down the road from you here in Austin. I put in a 6 inch slab with rebar and some used wire shelving for reinforcement and it has worked fine so far. From all that I read on the Portland Cement Association website one key to concrete strength was in the curing. I poured, kept it damp for 3-4 four days and then let it cure for 4 weeks before building.

                            Comment


                            • #15
                              Re: Slab thickness

                              If you have reasonable soil conditions, you don't need footings or piers.

                              Many ovens are built on slabs alone.

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