Crawl space floors
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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part