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Insulated Floating Slab for Porch

Joe O | Posted in General Questions on

Hello.

I’m in southwest Wisconsin, and built a log house on top of a 20-by-40-foot insulated floating slab.

I’m now going to build a wrap-around porch, with the posts on top of buried concrete piers. The porch rafters will be attached to the posts and also to the log house — or at least that’s my plan.

My question: Does an insulated floating slab move at all?

I understand that if the house floats, I’d want the porch posts to float. But I’m not sure if the insulated slab actually moves, and if it doesn’t, obviously I would want to bury the piers deep enough so they and the posts wouldn’t heave.

Would large footers make any difference? Is there another design possibility I’m not thinking of? I realize I could put more posts up against the house and attach the rafters to those so as to make the porch and house separate structures, but for many reasons I don’t want to do that.

Probably better planning would have been a good idea, but as it’s too late for that, any other advice?

I asked a version of this question a while back, but am still scratching my head and wondering if other solutions are out there.

Thanks,
Joe

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Replies

  1. Mark Nagel | | #1

    Everything moves.

    I'd run into the same dilemma when pondering adding a porch to a deck off of my manufactured home. For manufactured homes you cannot attach an external structure to them, so the only solution is the one you're not wanting to do: float the entire porch structure separately, which means posts next to the house (this is where things turned non-workable for me- I couldn't make the aesthetics work, in which case I abandoned the entire idea/project).

    And from the frying pan to the fire I went... I decided I'd build a new house. I've spec'd the new build on a floating raft slab and it's to have a porch! I think that folks can see where this plot is turning! (Why do I do these things to myself?) I have what I believe a solution for me. The porch will be an extension of the roof and will be fully supported by the house (the deck, ground level, floats on its own). No need for outer (or inner) support posts. It's not a solution that would work for you as it's totally dependent on raised heel trusses (with returns back to ledger boards at/on the outer walls): I'm figuring that it's workable with a target porch depth of 6'. I doubt that anything like this is possible for you, though perhaps it might trigger some other solution.

    Bottom line is that unless everything can move together it's going to move independently. How much is the question... A structural engineer might be required to provide the solution.

  2. Expert Member
    Malcolm Taylor | | #2

    Joe,

    You will get a lot more differential movement between the porch roof and the house due t0 the settlement and seasonal change in the log walls than any slight movement of the two foundations. Posts for roofs attached to log h0uses usually have adjustable steel bases.

    1. Joe O | | #3

      Thanks, Mark and Malcolm.

      Well I definitely don't want to start over, Mark! But I appreciate as many perspectives as I can get on this one. As an amateur, solutions often seem to come from learning about a diversity of approaches. I hope your own project goes well.

      Malcolm, it's an old log house we moved and rebuilt. The logs were cut 150 years ago. There's still some shrinkage and expansion, but I'm thinking not enough to make too much of a difference. Do you have any more information on those adjustable steel bases? Is there a brand name or would the local hardware store generally be familiar with them? They sound like maybe a specialty product.

      Thanks for your insights.

      ~Joe

      1. Expert Member
        Malcolm Taylor | | #4

        Joe,

        I've used them for the post supporting stairs in Panabodes (a squared log home). They are pretty common. I think Simpson makes them, and some versions are available at the big box stores.

        To return to your original question: As long as they are properly built, neither of the two foundations should move enough for it to be a problem.

  3. Expert Member
    Michael Maines | | #5

    I believe you are asking about movement at the soil level. If the insulation meets the requirements for a frost-protected shallow foundation, the house won't move, because the ground can't freeze and expand under the house. So it's safe to put posts on piers that extend below the frost line.

    The term, "floating slab" usually means an uninsulated or under-insulated slab on drainable fill, usually for garages and other outbuildings under a certain size. That kind of slab CAN move in certain situations so you should not have posts that won't move next to it.

    1. Joe O | | #6

      It is an insulated slab, like you describe Michael. It has rigid foam beneath the slab. And then on the outer walls, the rigid foam goes down one foot and horizontally beneath the soil 3 feet. The one issue is that the guys who put the foundation in did not put the insulation under the footers. I saw this only as the concrete was being poured. But I'm hoping that wouldn't change the dynamics of whether it moves or not.

      Thanks for the clarification. This forum is a great support.

      1. Expert Member
        Michael Maines | | #7

        Joe, I believe that in Wisconsin you have your own code, but the IRC provides guidance on what constitutes an effective frost-protected shallow foundation: https://codes.iccsafe.org/content/IRC2015/chapter-4-foundations#IRC2015_Pt03_Ch04_SecR403.3. The details depend on your Air Freezing Index.

        At the outer walls, does your foam extend to the top of the foundation and is the 3' horizontal foam on the exterior as well? If so, having foam under the footings is not important (from a frost-movement viewpoint; there are energy concerns) as long as you meet the requirements for dimensions and R-values.

        If you want a deeper understanding, this is a good document.

  4. Joe O | | #8

    Thanks again, Michael. Yes, the 3 feet of horizontal foam is on the exterior, running away from the house. The foot of vertical foam is flush with the top of the foundation. I'll check out that document, and get to building. ~Joe

  5. plumb_bob | | #9

    For log house applications I have had custom screw-jacks made up by a fabricating shop. As the log walls shrink and compress the beams that are resting on these walls will drop vertically, so the posts that carry the beams need to be able to drop vertically as well.
    The post foundations should either be below frost or insulated against frost.
    The horizontal foam butted against the thickened portion of the slab is a standard detail, but I have seen footings built with or without the insulation underneath. I understand the energy penalty of not insulating under the footing, but have recently seen foam completely eaten away by bugs so this would make me nervous as your structure could fail if the the foam under your footings disappears. Also, if anybody ever spilled gasoline close to your foundation the foam could melt leaving your footing unsupported.

    1. Joe O | | #10

      I'm curious to know more about the design of the screw-jacks. Do you have any photos, or know any on-line sources that might have pictures or descriptions?

      I'm not super worried about the logs shrinking and expanding, as the house is 150 years old. But I know there will still be some minor shrinking and expanding, so the screw-jacks might be a good system to pursue.

      Thanks,
      Joe

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