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Community and Q&A

Crawl space floors

justinlacy | Posted in General Questions on

New home, currently near end of design phase, zone 2, highly expansive clay soil. The current plan is a closed crawl space with 4′ between crawl space floor and finished first floor. However, the crawl space floor will be about 12″ above natural grade, so first floor elevation will be about 5′ above natural grade. The height is a good thing for the aesthetics of the house (southern Louisiana style house with wide, deep covered porches at front and rear of house).

Since crawl space foundations haven’t been used in my local area for decades, and all the older ones are vented, this foundation will be very “unusual” for this area, and expertise for it is certainly lacking. I’ve been reading about these foundations for months to get a good understanding of them. Some of the designs I’ve seen that contain a concrete crawl space floor call for a thin slab, sometimes without steel. I’ve seen them called “rat slabs”. My foundation engineer has specified a 2″ thick slab that is referred to on the drawings as a 2″ concrete skim coat. The engineer confirmed that it doesn’t have any steel in it. The plan also calls for 90 drilled shafts for the home (3300 sqft plus 3-car attached garage).

I like the idea of a concrete crawl space floor (if I can afford it), but my concern is that it will quickly crack due to the expansive clay soil. Yes, about a foot of select fill will be brought in which will be between the crawl space floor and the clay soil, but it will compacted so I don’t think it will keep the upheaval forces from the clay soil from cracking the crawl space floor. The engineer agrees. I should note that even though he is experienced with crawl space foundation designs, most of the ones he has done are either vented and/or don’t have a concrete floor.

Is there a solution for this which includes a concrete crawl space floor, or should I give up on this detail and just put a water/vapor/air barrier on the ground and let that be the “floor”? It looks like the only solution for a concrete floor that won’t crack is to build something similar to a “regular” structural slab with void cartons, then build the crawl space on the slab.


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  1. Expert Member
    Dana Dorsett | | #1

    They're called "rat slabs", because a primary purpose/function of the non-structural slab is to prevent burrowing rodents from gaining entry. What intended function or purpose are YOU looking for the slab to provide?

    A chicken-wire reinforced 2" rat slab wouldn't keep it from cracking, but will limit how fast the crack propagates &/or separates.

    With a 1-2" leveled bed of sand over the vapor barrier a floor of 2" thick concrete patio pavers makes a relatively clean walkable/crawl-able rodent resistant floor that isn't much affected by the expansive soil. If it heaves and moves over time it can be re-leveled as-needed. That's usually more expensive than a 2" poured rat-slab though.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Rat slab was a bit of a misnomer for my neighbour Chuck. He had a bear tunnel into his crawlspace to hibernate.

      1. Expert Member
        Dana Dorsett | | #7

        >"He had a bear tunnel into his crawlspace to hibernate."

        DAMN! Some of those pesky critters can be PUSHY! In areas where the frost depths and footings would be deeper than yours it would be an enormous amount of digging.

        I imagine the local pest control companies didn't want to touch that one!? :-)

  2. Aedi | | #2

    There's nothing wrong with the "slab" developing cracks. The concrete is just meant to protect the water/vapor/air barrier beneath it, so the cracks are just aesthetic. If they bother you, you can go thicker, or omit the concrete entirely.

    If you are going to use the crawlspace for storage, or pests are a potential issue, then putting something over the poly is a probably good idea. Wood is also an option; some more information on that (and the function of slabs in basements) can be found here:

  3. moose_head27 | | #3

    As the other guys said the concrete is not structural. It's only there to protect your poly just below. Don't want to walk on the poly and puncture it with holes.

  4. Expert Member


    Another vote for what we call a Scratch Coat up here. There is a vast difference in usability of a crawlspace with a concrete floor compared to one with just poly.

  5. Jon_R | | #6

    Note that expansive soil becomes less differentially expansive when you put a house and plastic over it, reducing the moisture extremes. You might even extend the zone with underground roofs outside the foundation.

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