A foundation assembly for use with expansive soils. The assembly includes a slab form, such as metal pan, for receiving poured concrete, a structural slab cast into the form, and structural members, e.g., piers, footing pads, and the like, contacting and/or extending into the soil. An elongate support member, e.g., a rectangular concrete beam, is positioned on an upper support surface of the structural members so as to contact a lower surface of the slab form. In this manner, the support member supports the slab form and structural slab and defines a void space between the soil and the slab form without the use of biodegradable materials.