What Is the Typical Thickness of a Concrete Slab Foundation?

Slab Design

• Slab design is based on calculations for the heaviest load the slab has to carry, with soil-bearing capacity, slab thickness and concrete mix strength factored into the calculations. In the most basic form, slabs are laid on native soil that has adequate bearing capacity for the structure. The concrete is reinforced with wire mesh, and the concrete mix is designed to reach the bearing strength, typically 2,500 to 3,000 pounds per square inch, or psi.

Slab and Bending

• Concrete has great compressive strength -- it resists pressure bearing on it -- but it is weak in tensile strength, or the forces pulling it apart. When soil support beneath the slab isn't uniform, uneven settling will cause the slab to bend. Concrete can flex to some degree, but it doesn't have the tensile strength to resist pressure from bending. Eventually the concrete will crack all the way through, leading to structural failure. Thickening the concrete helps compensate for a poor subgrade.

Concrete Thickness

• Thickness increases the stiffness of the slab and its ability to distribute the load over the supporting soil, and it strengthens the slab's resistance to cracking. The thickness is based on the stiffness of the subgrade and the weight of the load. The typical thickness of a concrete slab is 4 inches, which is suitable for most residential construction, with edges thickened by 50 percent, or to 8 inches, to form an integral footing. For developments with heavier loads, the thickness of the slab is increased.

Shrinkage and Thickness

• Concrete shrinks nonuniformly during the curing period. Although concrete is formed and poured to the specified thickness, it's common that a concrete slab doesn't conform to the thickness after it cures, and there are variations throughout the slab. Concrete continues to strengthen for years, although most of its strength is gained in the first 28 days. The thickness of the concrete after curing is less important than the bearing capacity of the slab when evaluating its performance.