When the 12 Inch Footing Minimum Is Deeper Than the Frost

Two rules, the deeper one wins

The IRC puts exterior footings at least 12 in below undisturbed ground (R403.1.4), and below the frost line where the ground freezes (R403.1.4.1). The dig depth on every county page is the deeper of the two, rounded up to a whole inch.

How often the minimum decides it

In 1,050 of 3,144 counties the estimated frost depth is 12 in or less, so those pages say dig to 12 in. 173 of them do not freeze at all in a normal winter. The other 877 do freeze, just not past a foot. The largest of those: Harris County, TX, Dallas County, TX, King County, WA, Tarrant County, TX, Bexar County, TX.

Why 12 in even where it never freezes

The minimum is not only about frost. Topsoil is loose and full of roots, it shrinks and swells with rain, and burrowing animals and erosion work in the top few inches. A footing on undisturbed soil a foot down sits below most of that. So a frost-free county still digs.

A shallow estimate is not permission

An estimate of 9 in in a normal winter means a cold winter can go deeper. Your building department's adopted value may be deeper for exactly that reason. Check it before you treat 12 in as enough.

Where to go next

Compare across the range: Cass County, ND (64 in), Hennepin County, MN (48 in), Cook County, IL (28 in), New York County, NY (16 in), Miami-Dade County, FL (0 in).

Why Frost Goes Deeper Under a Plowed Driveway Than Under Snow · How Each County's Frost Depth Is Worked Out, and Where It Is Weakest