Types of Foundation Slabs

T-shaped

• The T-shaped foundation slab features a cast-in-place foundation beam, or wall, that extends beneath the earth's surface to meet with a buried footing. Because the footing is wider than the foundation beam, a cross section of this type of slab has the appearance of an upside-down T. The beam runs along the perimeter of the structure and the space inside is filled with dirt. Then a slab is poured across the interior at surface level. Because the foundation wall extends below the frost line, this type of slab is suitable for freezing climates.

Slab-On-Grade

• Slab-on-grade foundations are monolithic, reinforced concrete slabs poured thick around their perimeters. The extra thickness at the edges acts as footing. The slab is reinforced with rebar and wire mesh and typically rests upon a thin layer of percolating material, such as gravel. Slab-on-grade foundations don't extend beneath the earth's frost line, making them generally unsuitable for freezing climate. The freeze/thaw cycle expands and contracts the earth beneath the concrete slab, sometimes causing it to heave and crack. Builders can add below-grade insulation and drainage ditches to make the slab more resistant to frost heaves.

Floating

• A floating slab consists of cast-in-place beams, or footings, that rest atop a deep layer of percolating material such as gravel. Because the layers of drainage material extend deep beneath the concrete, the floating slab is not subject to the heaving caused by freeze/thaw cycles. The "beams" are usually less than a foot thick and can be up to two feet wide. Floating slab foundations are an alternative to T-shaped slabs in areas where extensive excavation is too costly or impossible, such as rough, rocky terrain. According to "Rob Roy's Earthwood Home" in "Mother Earth News" magazine, floating slabs were architect Frank Lloyd Wright's favorite foundations.