How Deep Is the Foundation of a House?

Basement Walls

• Standard basement walls are 8 feet high, and most concrete forms are also 8 feet high. Your foundation contractor can form the basement walls with taller forms, however, if desired. The standard 8-foot wall is 8 to 10 inches thick. Because soil pressure on a foundation wall increases as the wall's height increases, if you choose a taller basement wall, it may also have to be thicker to maintain structural integrity.

Inside the Basement

• The exterior height of basement walls is not identical to the interior distance between the basement floor and the joists above. In a standard 8-foot, or 96-inch, foundation, the contractor pours a 4-inch basement floor on top of the footings. The floor reduces the interior basement height by 4 inches, leaving you with 92 inches from the floor to the top of the foundation. Before the contractor installs the joists, however, she attaches a sill plate on top of the foundation walls. The standard sill plate is a treated 2 by 6, anchored to the top of the foundation wall. A 2 by 6 is actually 1.5 inches thick, which raises the interior height of the unfinished basement to 93.5 inches. The depth, however, depends on the thickness of the installed ceiling.

Crawl Spaces and Frost Line

• Some houses do not have full basements but only crawl spaces that provide room to run plumbing, ducting or wiring. The general rule when building a crawl space is to pour the footers on which the foundation walls will sit, below the frost line. Because the frost line is deeper in cold climates than it is in warm ones, local building codes determine the minimum depth of crawl space footers. Above the line, the soil is less stable and subject to expansion and contraction as the moisture within the soil freezes and thaws. Pouring the footers below the line helps stabilize the foundation walls.

Exterior Foundation Elevation

• While local codes determine how deep your foundation footers must be, subdivision covenants and HOAs usually decide how much of the foundation can extend out of the ground. To reduce the risk of insect infestation, the top of the foundation should be a minimum of 6 inches above the soil, but entire foundations that sit higher than neighboring home foundations, can create a drainage problem in the neighborhood. For this reason, no matter how much of the foundation appears above ground level, it’s likely that the tops of all the foundations in a subdivision sit at the same level. Only the depth of soil grade around the foundations differ.