Dimensions of a Concrete Foundation

Walls

• For a normal, full basement, the code for foundations is completely uniform. The one area of difference is the soil rating system. Soils need to be inspected before building can begin, and lighter soils have different requirements than heavier, wetter soils. Generally, however, there is no permissible basement foundation wall under 8 inches thick or permitted to stand higher than 8 feet. This is the minimum recommended by the National Institute of Building Sciences. All concrete used must have a load bearing capacity of 2,500 pounds per square inch.

Wall Variables

• The height of the ceiling, the weight of the soil and the height of the entire house are important variables. The heaviest house covered by regulations is the three story brick house. This requires a maximum wall thickness of 12 inches. These also include the #4 bar, which is a metal bar used in the concrete as reinforcement at 24 inches in length and 5/8 inch in diameter. These must be set at a maximum of 48 inches apart from one another. These must also penetrate at least 14 inches into the wall from below.

Footers

• Footers are the lowest part of the foundation, buried deep in the ground. They must be buried below the frost line, which is that part of the soil that does not freeze. This line will be different for different areas. The thickness of the concrete footer is dictated by the size of the house. The heaviest, three story brick house requires the maximum of 32 inches of footer width in concrete. Smaller houses or cabins can go as low as 12 inches, which is the minimum permissible. The minimum depth, regardless of the frost line, is 6 inches underground.

Floors

• Concrete floors do not resist as much pressure as the walls. The floors are almost always thinner than walls. The average floor thickness is about 4 inches of concrete sitting on a bed of gravel. If the home is just the slab on grade, or a slab sitting directly on the ground with no basement, the minimum thickness for this is 3 3/4 inches.