How deep below the frost line do footings need to go?

Footings must extend below the frost line set by your local jurisdiction in Table R301.2(1), or use an approved frost-protection method (IRC R403.1.4.1).

The IRC does not publish a single national frost depth. Instead, R403.1.4.1 requires footings and other permanent supports to extend below the frost line specified by the local building department in Table R301.2(1). Frost depths range from 0 inches in mild climates to more than 60 inches in the far north.

There are three accepted alternatives to digging below frost: a frost-protected shallow foundation built per Section R403.3, a design conforming to ASCE 32, or footings bearing on solid rock.

Because the number is local, always confirm the required frost depth with your jurisdiction before excavating.