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Concrete-Free Slab Assembly: The Cons

Adam_F | Posted in Green Building Techniques on

We have been discussing the possibility of using a concrete free slab with our architect and builder.  It seems like a great way to reduce our carbon footprint and probably give us a more forgiving surface to stand on.

However, they have some trepidation about it.  I respect that there can be very good reasons not to do this kind of assembly, but I’m trying to understand what those might be.

For reference, this would be in an urban infill lot in Climate Zone 5. The soil conditions are a bit of a wildcard.  There was a house on the lot for about 60 years, but it had a fire, was condemned, and was eventually buried on the lot. Gentle slope down from back to front of the lot.

Are there obvious conditions or scenarios where going with a concrete free slab assembly becomes a bad idea or too much of an engineering challenge to be worth it, practically speaking?  What kinds of conditions would those be?

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  1. user-2310254 | | #1


    Off the top of my head (and as a non-expert). If there is fill debris under the new slab, you could have differential settling that would be an ongoing pain. To avoid that, you might have to dig out all the trash and then carefully backfill and compact the soil. At that point, it might make more sense to install a basement or crawlspace.

    1. Adam_F | | #4

      Thanks, Steve. What you say makes sense to me.

      It sounds like there is a certain cost to excavating and refilling that void. I'd guess that fill has to be engineered in some way to ensure stability going forward, which means probably trucking in a lot of stone. That's not exactly a low carbon exercise.

      1. Expert Member
        Deleted | | #5


  2. Expert Member
    Michael Maines | | #2

    The main argument against concrete-free slabs is that they are new and different, and in an industry where a lot of new-and-different approaches end up causing problems, practitioners are rightfully wary. But there is little difference between a concrete slab-on-grade and a concrete-free slab.

    The subslab prep should be identical, with one difference, and this is probably the strongest argument for using a concrete slab: the foam (or other sub-slab insulation) doesn't have to be perfectly leveled, because the weight of the concrete will push the insulation down. With a concrete-free slab, most of us are using a screed layer, either 3/8" peastone or sand, so the foam can be placed as close to level as possible. It's an extra step but not particularly challenging to implement.

    Just as with a concrete slab, point loads and interior bearing walls need their own footings. With concrete slabs, those footings are often just thickened areas, but with a concrete-free slab you need separate footings. Or, ideally, clear-span any loads to the exterior walls and you don't have to worry about interior footings.

    For what it's worth, while most concrete-free slabs use a double layer of sheet goods such as Advantech, they aren't saving as much embodied carbon emissions as using 2x4 sleepers and a single layer of subflooring.

    1. Adam_F | | #3

      Thanks, Michael, for your response.

      In your experience, are there situations where the concrete slab-free approach would be a problem given the site conditions? Or would those conditions make a poured slab equally problematic?

      I'm referring to scenarios such as disturbed soil or, in my case, a site where we have good evidence to suggest the previous building was demolished and buried into the on the lot about 40 years ago. In those cases, would it be wisest to just avoid the floor system relying on the earth for stability, and use some kind of joists to suspend the floor (crawlspace, or something like Elizabeth DiSalvo's detail w/ open web trusses hung on the foundation over a slabless 'slab')?

      1. Expert Member
        MALCOLM TAYLOR | | #6


        The requirements for fill compaction are the same for a slab or concrete-free floor system. So the choices are really as you say whether it makes sense to choose a floor which is supported or suspended.

        Complicating things somewhat is that depending on local conditions, it may still make sense to remove and replace the existing fill, rather than pour a basement or crawlspace. Unfortunately, rules of thumb aren't much help without exploring the particulars of both the site conditions, and the local economics of the various options.

      2. Expert Member
        Michael Maines | | #7

        I agree with Malcolm. Elizabeth's system or something similar could work well. In fact I'm leaning toward using a similar system on my own kitchen addition, mainly to minimize how much fill I have to bring in.

  3. Expert Member


    Might I suggest something in between?

    I applaud your efforts to reduce your carbon footprint, and you can do so by choosing a different type of cement, or a low cement type of interior slab. There are ways to make a concrete free interior slab, and FWIW I think it's scary and new and unfamiliar.

    There exists several low strength concrete options in the 2000 psi range which really doesn't have as much cement as you might think, it's more small and large aggregate, which you were going to end up bringing in anyways. Think of it as an all in one delivery of gravel, sand, and other stuff you need.

    In a sense, it makes it easier to pump your slab into place, and get it flat, compared to compacting a perfectly flat base of gravel. The environmental impact of 400 lb/yd^3 of concrete is somewhere around 2.5 tons of CO2, for a standard 2000 square ft slab, which in the grand scheme of things, is already lower than a standard house, and will hopefully be reclaimed by improvements made elsewhere.

    On top of that, concrete is cheaper than multiple layers of engineered wood, screws, and a lot less hassle.

    Here's a link to a good report all about controlled low strength materials from the ACI

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