A slab-on-grade foundation is a type of foundation consisting of a structural concrete slab poured onto the ground, but usually, there is a layer of gravel and a vapor retarder between the ground and concrete. View illustration of a slab-on-grade installation. No accessible space exists in slab-on-grade construction. Slab-on-grade foundations are popular in areas where there is a high water table. For example, there are very few full basements in Southern Florida. In general terms, a slab-on-grade foundation is poured on the grade or ground. complements of InterNachi @ www.nachi.org