Frost line depth in Marathon County, WI
How deep the ground freezes in Marathon County, WI
51 in. Dig to 52 in, just below the frost. Nearest NOAA station: Wausau Dwtn Ap, 6.7 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.