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foundation depth confusion

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  • foundation depth confusion

    Hi everyone. This is my first post. I'm still in the early planning stages but my target for cooking is July 4th. I did a search on the forum and am still a bit confused. Thanks in advance for short easy answers.

    I checked with my local building inspector today to see if I needed a permit for an oven build. He said no.

    I described what I wanted to build, and asked him how deep he thought my foundation should be. He gave me the stock answer of 3 feet. This is the frost depth for house foundations in my area of the lower Hudson Valley in NY.

    This seems like way overkill to me. Do I really need to be below the frost line for such a small structure? Small compared to my house that is.

    Since superstorm Sandy everyone around here is installing Generacs on small concrete slabs. They probably aren't quite as heavy as a pizza oven, but the slabs they are pouring for the generators aren't anywhere close to three feet deep.

    Anyone have good advice on foundation depth for the Northeast? (including the depth of the gravel below the concrete? Many thanks again in advance.

    Also, am I being overly optimistic for a Fourth of July pizza party?
    Last edited by ronwass; 01-14-2013, 01:00 PM. Reason: changing the thread title.

  • #2
    Re: foundation depth confusion

    This frost line issue, the first us newbies have to deal with upon embarking on a build seems to cause a lot of disagreement around here and much hand wringing before we can even get started.

    I can't figure out how footings are going to protect such a structure in any case.

    Here's my logic:

    Footings are below frost line, and slab rests on footings AND the ground in between the footings. If the ground could freeze enough to push up the slab, (which is in dispute) what is to stop the ground from pushing the slab up right off the footings if there were footings? This would seem to me to affect the stand and oven the same as if there were no footings.

    In a house, the structure is supported right on the footings. through the walls and floor joists attached to it. there is either a crawl space or a basement between the floor of the house and the ground below so freezing ground has nothing to push up. Is anyone who advocates for footings assuming that the base slab will be built as such? Suspended over the ground? I don't think so. Of course in a finished basement the floor is on a slab, but if that gets pushed up in a frost, it isn't going to affect the walls of the house or the rest of the structure which have enough mass and weight to stay still when the basement floor shifts or bulges or cracks.

    In an oven build, the weight of the stand, hearth and oven is on the slab, and we are talking about a MUCH smaller mass, and most of that weight is not really on the footings, but on the ground below the slab.

    If the slab moves it is going to stay structurally in one piece, and not crack if reinforced properly. Even if it cracks, and the sides of the concrete block stand built upon it move around, the hearth probably won't crack which means the oven won't crack because of this. Worst case is an oven not level. Now this isn't something particularly ideal, but my point is that the footings aren't going to protect from this.

    Tying the foundation slabs to the footing is probably worse, because then you will get freezing expansion in the middle of the slab pulling against the tied down sides and get a crack.

    A question: Is anyone advocating building a slab on footings that has space between it and the ground between the footings below it?

    And the most important question:

    Has anyone ever had substantial enough slab movement that affected your oven in a negative way, whether or not there were footings?

    Ron Wasserman
    Last edited by ronwass; 01-14-2013, 01:06 PM.


    • #3
      Re: foundation depth confusion

      This seems to be the only guy I have found on this forum who is building the stand directly over the footings like a house.



      • #4
        Re: foundation depth confusion

        There is no dispute that frost will cause heaving. What you have to decide is whether it is worth the risk. I am lucky enough that the only frost we get around here is on our beer bottles, but if I was in an area where the ground froze over 2' deep I would make sure I went below it for my footings.


        • #5
          Re: foundation depth confusion

          but how do you protect the oven assuming that the stand walls are not built directly over the footings, but on the foundation slab which is built directly over the freezing ground, whether or not there are footings? That is my philosophical question.


          • #6
            Re: foundation depth confusion

            I am sorry, I don't understand your question.

            For certain types of slab construction, they will form a space under the slab where there are no footings. This is done in areas with highly elastic types of soil (clay), so I would assume something similar is done for frost areas.

            If you have to get a permit, then you will have to build to code for your area, which I would also assume means a footing below frost.

            If no permit is required, I would be tempted to build a floating slab on a gravel bed. At the worst, you may have a tilted oven.


            • #7
              Re: foundation depth confusion

              These pictures describe what I am talking about, and illustrate the difference between a structure built right on the footings, and a structure built on the slab on the footings, like an oven.

              How do these oven footings keep the oven slab from bulging if there is pressure at the center of the slab?

              It is clear that a house structure is unaffected by slab bulge because the structure weight is directly over the footings.
              Attached Files
              Last edited by ronwass; 01-14-2013, 05:14 PM.


              • #8
                Re: foundation depth confusion

                The slab won't bulge, it will either resist the force, crack, or lift. At the PSI we are working with on an normal oven and reinforced slab, it won't crack, it will lift. The thrust forces are the same no matter if the weight is directly above the stem walls or closer to the center unless the cantilever is pretty extreme; there is a given amount of weight and it is spread thoughout the foot print of the stand, and then through the slab. If the slab were thin enough, and the offset (cantilever) were large enough, then it would matter.

                I would assume that the gravel in your drawing has a design thickness deep enough to allow for what heave may take place.


                • #9
                  Re: foundation depth confusion

                  Agree with floating slab on crushed rock base will be fine. What will get you in trouble quickest is building over newly back filled site especially if the virgin soil was not level and you have more depth of fiil on one end than the other. If your site is not level back fill with crushed shale and whacker pack wet shale.
                  In the midwest it is common to support decks, room additions, etc. with poured concrete column footings eight foot on center and four feet deep. Bring rebar out of the hole and into the slab. This is much cheaper than a continuos footing and might be worth if for peace of mind. Reinforcing the slab is much more important.


                  • #10
                    Re: foundation depth confusion

                    No , I don't think July 4th is too soon..I started April 1st and finished middle September.
                    Good Luck! Research The Plans!
                    “Never regret. If it's good, it's wonderful. If it's bad, it's experience.”


                    Home Sweet Home~ Colorado Springs ~



                    • #11
                      Re: foundation depth confusion

                      So Tscarborough, then you are saying that it doesn't matter if there are footings below the slab? Both will rise or crack.
                      This is how I think it should be done, like the guy I linked to above: http://www.fornobravo.com/forum/6/6/...rted-3701.html (New Oven started)

                      Silvfox, I don't think you could do this with tubes. You need continuous, but of course it's not a large footprint. The most fuss is in the digging, not the cost of the concrete for the footings.

                      Only difference from this gentleman, is that I have to foot down three feet. Ug. Less concrete than a slab though.

                      Edited the illustration to show the frost line.
                      Attached Files
                      Last edited by ronwass; 01-14-2013, 07:51 PM.


                      • #12
                        Re: foundation depth confusion

                        If you are going to build a floating slab, you do not need footings. Piers would also work fine, you do not need continuous footings.


                        • #13
                          Re: foundation depth confusion

                          I can only figure out one way how to use piers and still avoid frost movement. In the first two drawings below, you have to span the space between the piers, and that span, either a girder or a slab will be on the ground.

                          The third way, with a suspended girder seems much worse and uglier than my no-slab/continuous-footings idea in a post above. Might as well build a deck and put the oven on a stand on the deck.
                          Attached Files


                          • #14
                            Re: foundation depth confusion

                            If you use continuous or column footings under your stand you should still pour a slab on grade to support your stand. Rebar will tie the slab to the footings. Hope this helps clarify my ealier post.


                            • #15
                              Re: foundation depth confusion

                              Dear elder forum statesman.

                              Though I am a babe in the woods as far as this WFO thing is concerned, I still get the feeling that I am not making myself clear and that no-one is understanding the gist of my question.

                              Oh, that I were a better writer, I shouldst be better able to make my meaning truly ring out clear like a bell tone. Let me try one more time to calm this tempest which may prove to exist solely in a teapot.

                              1. If you have a floating slab on grade, it is going to be affected by frost (to one extent or another. (probably to only a tiny extent, or probably to no extent). The oven (the "structure") is directly on the slab, so the oven will be affected as the slab moves, or cracks, probably, at the worst, simply going out of level a tiny bit.

                              2. In a house or a garage or shed, the structure is slightly DIFFERENT because it is NOT on the slab. It is directly on top of the footings. The slab in the basement floats on grade between the footings. The slab is affected similarly, but since there is nothing structural on top of it, it is not a big deal. Besides that, there is ambient heat from the house that keeps the frost a bit at bay.

                              3. The Pompeii plans dictate the structure of the oven NOT over the footings, but four inches towards the center away from the edges of the slab. How are footings under a WFO slab going to prevent the slab itself from moving? Most of the slab is on grade between the footings or piers, so frost will push up on it, and push it right OFF THE FOOTINGS.

                              4. It seems to me that it doesn't matter if you have footings or not. Frost will be an issue (perhaps a minimal or non-existent one) UNLESS you build the walls of the oven stand directly over the footings and have essentially no slab, (or a non-structural one) which is NOT what the Pompeii plans proscribe.

                              5. Where is my logic wrong here?

                              Sorry to be such a dope about this.
                              Last edited by ronwass; 01-15-2013, 09:37 PM.