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Community and Q&A

Monolithic Slab With Multiple Levels

gerrha | Posted in General Questions on

Hello, I am thinking about building a new house and the location (NE Wyoming) points best to a monolithic FPSF and slab. But, I have been struggling mightily to understand how to create different floor levels, such as the main floor, a covered patio and entry both 4-5 inches lower than the main floor, as well as the garage floor with no drop, but a 2% slope. I basically gave up and decided it would have to be done with two pours; the main floor followed by the garage, patio, and entry.

But, then I told myself to wait a minute. The house I am in right now, built in 2015, has a very complicated post-tension 7000-ft2 monolithic concrete slab foundation with 7 different levels. I was there as it was being poured and I never thought to watch what they did with the level changes. However, clearly it can be done, or does it require very specialized concrete people not commonly found outside of south Houston where this house is located?

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  1. Expert Member


    I don't think the difficulty will be forming up the slabs to pour all at once, it will be isolating them from each other to maintain the continuity of the exterior insulation for the house, and the differing demands of sub-grade insulation for the FPSF. You aren't going to want the house directly attached to the patio, or depending on whether you are conditioning it, the garage either.

    Before worrying about the sequence of construction, I would draw sections through the areas where these slabs come together, showing how the continuity of the exterior insulation is maintained.

  2. gerrha | | #2

    Thanks for the reply. I thought about your point plus the proposed location, NE Wyoming, with its relatively short number of frost-free days. I do not think it prudent to expect the heated areas of the house to be complete in one building season. Putting this together, I decided that all areas, main house, garage, patio, and entry, should be designed upfront as unheated spaces. With this view, I would like to do the entire slab with all these areas in one pour. With this outlook, the FPSF requirements for insulation are not insurmountable.

    As you say, it is the sequence and how-to-do-it that is unclear to me.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      The FPSF foam will keep the slabs safe from frost heave, but you will still have to isolate some of them from each other. The patio will be exposed to outside temperatures and act as a thermal bridge to the house slab. The garage will likely not be heated to the same temperature as the house either, so it will need isolating too.

  3. gerrha | | #4

    At the end of the day, that means the concrete will no longer be monolithic, which is OK, and what I have been planning. It was when I looked at the drawings for my house in Houston, which is 100% monolithic throughout, that I got this idea for doing the same for a new house. However, Wyoming is not Houston, which I guess, is why it works fine here, but will not work very well there.

    One thing remains though. I still do not understand how the concrete guys are able to make accurate level changes in a large concrete slab in one pour.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      Before our code required slab-edge insulation, builders here sometimes used to pour the garage and main floor slabs at the same time with no stem-wall in between. Because the drop between the two was typically only 4" to 6", it was formed with a 2"x staked with rebar and clips, much as the edge of driveways are. Once the slabs had been bull-floated and were firm enough to walk on, the form was removed. Power-troweling filled in the rebar holes.

  4. gerrha | | #6

    Thanks for the information.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Good luck with your build!

  5. gerrha | | #8

    Mr. Taylor, another question please.

    We have established that the house slab must be isolated from the exterior slab edges by insulation.

    My question has to do with the turned down edge footings. I can see two methods.

    Method-1, the house footing is completely separate from the garage, patio and entry footings despite having the house roof extend over each of them and in the case of the garage, walls that meet the house. This method would leave no thermal bridge through the footings.

    Method-2, the house footing extends through all three other slab areas creating a small thermal bridge through the footings. But, this method seems structurally superior to Method-1.

    I can see pluses and minuses to either method, but what is the best way to do this?

  6. Expert Member


    With a slab with thickened edges there is no distinction between the slab and footings. They are the same thing. If you are separating the slabs, you are separating the footings.

  7. gerrha | | #10

    Hello Mr. Taylor,
    I think my wording above was not too clear. I have attached a PDF that shows what I was trying to say. The model starts at the top with pouring the entire house and extending the concrete beam or footing(?) out for the garage slab. XPS is then added in the middle image to isolate the garage slab from the house. Finally, the garage slab is poured resting with a cold joint on the concrete beam or footing. This is what I was trying to describe above as Method-2. Is this a potential option?

    1. Expert Member
      MALCOLM TAYLOR | | #12


      So far the discussion has been based on monolithic slabs with thickened edges. If you are planning on pouring the footings separately, then why not use stem-walls as it typically done? It doesn't make any sense to pour footings (or grade-beams), and pour the slab on top of them.

      If you are using thickened slab, it doesn't help to just isolate the top 4", if it is continuous below.

  8. jollygreenshortguy | | #11

    I come to this with a designer rather than builder's experience. Before edge insulation was required it was a simple matter to do slabs with small level changes. And the economics of a single-pour slab on grade were hard to beat.

    Now it has become a lot more complicated and I'm coming to the conclusion that I should detail my slabs assuming they'll be done in 2 pours. It's probably more trouble and effort ($$$) for a crew to navigate around the hurdles of a single pour than to just break it into 2 easy steps.

    I like this website and forum a lot. But I wish more specialist contractors participated in it. I'd like to be able to pick a concrete contractor's brain on various approaches to doing insulated slabs.

    1. Expert Member
      MALCOLM TAYLOR | | #13


      gerrha has come up against the most challenging part of detailing foundations for houses that rely on continuous exterior insulation: How to deal with the transition from conditioned to unheated spaces - especially when the house uses a FPSF. When this has been discussed here in the past the solutions suggested were:
      - If using stem-walls, keep the footings continuous, but run the foam through the stem-wall above.
      - For both stem-walls and FPSF, keep the foam continuous, but join the two foundations together structurally with non-corrosive reinforcing.
      - For both stem-walls and FPSF, pour the two parts independently and hope there is no differential settlement.

      1. jollygreenshortguy | | #14

        Yes, I'm dealing with the same issue right now as well. I'm detailing a FPSF design with an attached garage and a large porch. The roof of the house extends over both and is supported by posts at the outer edge of the porch. So the support for those posts must be structurally tied to the main house foundation, to avoid differential movement.

        So your option 3 is out for me. Two is problematic. It looks like the answer is going to be 1, a continuous footing tying the pieces together, with the slabs poured separately. The detailing at places like french doors opening out onto the porch gets complicated. But then nothing ever seems to get simpler. That's life.

        1. Expert Member
          MALCOLM TAYLOR | | #16


          We should invent something like the Maine Bracket to tie the two together. Retire early and enjoy the royalties.

          Luckily here the frost depth is 18" so no need to FPSF - and the omnipresent carpenter ants make exterior insulation problematic.

          1. jollygreenshortguy | | #19

            Speaking of carpenter ants, I'm American but spend much of my time in France. Here autoclaved aerated concrete (AAC) is in common use. I've been wondering why we don't use it in the USA as exterior foundation insulation.
            It would need to be protected from groundwater, but that would be easy. It could be mortared right to the face of the concrete and provides no inviting material for insects to burrow in.

          2. Expert Member
            MALCOLM TAYLOR | | #20


            Allison Bailes shares your views on this:
            "Here in the Southeast... it can be difficult to insulate below-grade walls, especially on the exterior. Thank you, termites. It seems to me that anyone looking to do closed crawl spaces or even basements might want to check out the possibility of using AAC instead of CMU or poured concrete."

            Peter Yost is pessimistic about its prospects:
            "That said, this was not the first time AAC attempted to find market share in the U.S., and it was not the last, although each time it simply has not taken hold."
            "AAC attempts to be a stand-alone building envelope system for the U.S. market, but it simply has never convinced any industry leaders or bulk market interests to sustain a manufacturing presence in the U.S."

  9. gerrha | | #15

    Thanks for all of your comments.

    I have gone back and forth with the FPSF stem-wall vs monolithic option. My walls will be ICF, so either way there will be a lot of individual concrete deliveries and work going on. I even wonder if FPSF is the best option. Frost depth at this location is 42" to the top of the footing. Maybe it would be better to insulate only the slabs and drop down to 42" with the footing. In my case, I also must consider the for-certain reality that the heated area (main house) will not be heated through the first winter. So, with FPSF, the entire thing must be designed as unheated space, which is a lot of insulation.

    By the way, I too will have roof elements common from the house out to the patio, entry, and of course the garage. I asked on another forum if having 4 individual areas of concrete tied to each other through a common roof, but not structurally, would be a concern from potential differential movement. The consensus was it should not be a problem, but I still am not able to convince myself of that.

    1. Expert Member
      MALCOLM TAYLOR | | #17


      Stem-walls to frost depth with sub-slab and slab-edge insulation solves all the problems.

  10. gerrha | | #18

    Thanks for all the advice, and yes, I agree with you Mr. Taylor.

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