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Alternative to concrete block foundation - Forno Bravo Forum: The Wood-Fired Oven Community


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Alternative to concrete block foundation

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  • Alternative to concrete block foundation

    Living up north, frost is a concern. I am planning a lengthy countertop as an addition to the wood oven. The counter will be concrete. To get around having to dig an extensive 4ft deep trench and building up with cinder blocks from there, I have been working on the following plan and any thoughts you have are appreciated:

    1) dig a series of fence post holes and use 8 or 10 inch sauna tubes filled with reinforced concrete as the legs that hold up the countertop and hearth. Tubes will go down 4ft into the ground to get below the frost line.

    2) Build a form for the countertop / hearth that will sit on top of the sauna tubes. Cut holes in the bottom of the form to accomodate the sauna tubes so that they fit into the form by an inch or so. Support the form with wooden blocks to arrive at a 30 inch countertop height.

    3) drop sauna tubes into place through the top of the form.

    4) lay rebar inside the form to provide the countertop and hearth with strength.

    5) suspend rebar down into the sauna tubes and connect it to the rebar laying horizontally in the form for the countertop / hearth.

    6) Pour concrete to fill the form and sauna tubes at the same time. This will result in a single unit.

    This will be built in a rural area and I figure getting the fence posts dug will not be a problem. Once the tubes are in place and the form supported, it is a simple process of having a cement truck fill it up. This will greatly accelerate the building of the hearth and counterop - if it works...

    My questions outstanding:

    1) does this sound crazy?

    2) how wide a span can concrete reinforced with 1/2 inch rebar cover. This will determine how many sauna tubes I will need.

    3) The hearth is poured in 2 layers (1st - the basic concrete and 2nd - the insulated concrete as the top layer). Do you have to wait for the first layer to fully cure before putting on the top layer?

    Looking forward to any suggestions you have.

  • #2
    Re: Alternative to concrete block foundation

    I'm not convinced that sonotubes are a foolproof answer to frost heave.

    They are buried in un-drained soil, and they can create an ideal grabbing surface for the ice lenses that cause the problem. I can see the advantage of the single pour, but I think you're better off with the floating slab on well drained gravel, or the insulated shallow footings.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Alternative to concrete block foundation

      As long as it is done right you will have no problem using sona tubes. The pic with deck with the heave is how to do it wrong. He didn't go below the frost line. First you need to put in feet or footings for the sona tube legs@ approx 1.5x the dia of your tubes below the frost line. I would recomend 3 different pours, one for the feet , one for the legs and then one for the slab. Now as far as load goes if your doing a 5'6" slab you only need 4 legs, just bring them in a little they dont have to be on very outer corners. I would make the slab a little thicker than the standard 4" with plenty of rebar. As far a pouring the insulating concrete on top of wet cement, no problem, I would let the concret set up for a couple hours though. By the way I am also in the planing stages of building a 38" oven and it will be incorperated with the deck I'm building so I will be doing the same exact type of plan as you are. I live in michigan so the footings for my sona tubes will be down at least 36"


      • #4
        Re: Alternative to concrete block foundation

        Thanks for the feedback - wondering why the recommendation for 3 pours. while getting it all done at once might be a bit of a trick, it will certainly speed things up!

        Good luck with your project.


        • #5
          Re: Alternative to concrete block foundation

          RLB is spot on - sonotubes need footings to prevent frost heave. The reason for the multiple pours is to enable you to backfill the big cavities around the tubes before you move on to the slab pour.

          There is a way around this - There is a specialized tube with a flared bottom that avoids the need to pour separate footings. Here's the sonotube version of same:
          Plastic Concrete Form Footing
          You could engineer this so that your forms were completely backfilled, and the slab forms fit to the top of the sonotubes, to do your unitary pour. You might even make the sonotubes come up to the height of the oven, and then do decorative brick or stone work to enclose the whole thing. I just have a pressure treated deck like floor to my wood storage area, which is over a cavity formed by full footings. It seems to work fine.
          My geodesic oven project: part 1, part 2


          • #6
            Re: Alternative to concrete block foundation

            I am no concrete expert, but I took a similar approach, with a single pour for the footer and walls and then poured the hearth slab. It was too complicated to form it all at once for me. Here is a thread with some pictures of my stand.


            My Oven Thread:


            • #7
              Re: Alternative to concrete block foundation

              If you have well drained soils you can just use a monolithic slab. I did this with many buildings when I lived in NY, about an hour south of Montreal. I had a sawmill, and a stone quarry. I had a total of six buildings, all done with monolithic slabs, the largest being 3000 square feet. I never had frost problems. This was on gravel soils. I would not do it on clay soils.