How Thick Must a Concrete Foundation be?

Walls

• The wall thickness for a foundation is the key factor. While floors are also important, they do not have nearly as much work to do as the walls. The walls must not only withstand the entire weight of the house, but they must withstand the pressures of water and soil as well. The minimum thickness for a concrete foundation wall is 8 inches. For multistory homes, or homes sitting on moist and rich soil, the standard is between 10 and 12 inches of poured concrete.

Floors

• The floor is also withstanding pressure. Normally, this becomes significant in areas where the water table is high, and the floor is then working against hydrostatic pressure. If the home is built in a moist area, then the floor is built at between 4 and 5 inches. The floor is never as thick as the walls. In moist areas, the hydrostatic pressure can surround the foundation, making it like a boat, as the water comes up from underground and is squeezed to either side of the house. The floor thickness is then about 5 inches maximum. Floors normally require about 2 inches of gravel to serve as a base beneath the concrete. This gravel stabilizes the soil — it acts like hard, stable soil — and permits quick drainage of any water accumulation.

Footers

• The footer is the lowest portion of the foundation. These are usually very thick, even as thick as 20 inches, buried under the frost line. The "frost line" refers to that part of the soil that does not freeze. A frozen ground only goes down into the earth so far. That part of the soil that never freezes is the "frost line," and, in much of the United States, except the Deep South, that is between 12 and 14 inches. Footers are thick because they bear the weight of the whole structure in the soil. Depending on the size of the house and the nature of the soil, the average footer is between 10 and 20 inches. This also includes an inch or so of gravel to balance the weight on the soil. A three-story house built on unstable soil often needs a footer that is 18 inches thick and about 24 inches in diameter. This stabilizes the weight of the house as it presses on the ground under the earth.

Reinforcement

• Foundation walls must use some kind of steel or iron rods to reinforce the concrete. The size of the rods depends on the thickness of the concrete. The thicker the concrete, the thicker the rod. For example, it is common for an 8-inch-thick foundation wall to use 3/8-inch vertical rods, spaced about 9 inches apart. For the larger 12 inch wall, the rods go as large as 5/8 inch, and are spaced about 10 inches apart. This adds strength to a thick wall without necessitating more concrete.