Ask the Architect: Frost Protected Shallow Foundation

"My home is about 20 years old, with a poured concrete foundation and a full

The exterior basement wall is exposed for about four foot. I
would like to excavate (approx four feet) and build a "sunken patio" (below
the basement floor level) but I am concerned about exposing the footing.

I live in northern Virginia were we have to consider freeze/thaw cycles.

Is a footing normally installed a fixed distance below the basement floor level,
or is it dependent on the "back fill" to create the insulation from the cold?
Can I excavate without jeopardizing my footings?

Robert T."

Robert, I'm glad you asked the question before you started digging!

Photo courtesy of NAHB Research Center.

Typically, the footings under basement walls are directly beneath the basement floor or slab, and they are usually less than 1 ft. thick. If your local frost depth is greater than 1 foot (and I'm guessing that it is), you will have to protect your footings from frost damage.

If your local code allows it and your footings are at least 1 ft. below your proposed grade, this can be done by excavating to the bottom of the footing and installing rigid foam insulation down the side of the footing and out horizontally for several feet. This system is called a "frost protected shallow foundation" and is allowed by most building codes.

Good luck with your project,
DavidDavid Moore, AIA
Original Home Plans

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