Frost line depth in St. Louis County, MO

How deep the ground freezes in St. Louis County, MO

20 in. Dig to 21 in, just below the frost. Nearest NOAA station: St Charles 7 Ssw, 5.2 mi.

Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.