Why the Frost Depth Here Is Not the Number Your Inspector Uses

Two different numbers with the same name

"Frost line" means two things. One is physical: how far down the ground actually freezes in a normal winter. The other is legal: the depth a jurisdiction writes into IRC Table R301.2(1) when it adopts the code, which footings must go below (R403.1.4.1). FrostDepthCheck estimates the first one for 3,144 counties. The second one is set locally, often decades ago, and there is no national list of it.

Why the legal number is usually deeper

A jurisdiction picks a value that covers its worst case: the coldest part of town, the soils that freeze deepest, a hard winter rather than a normal one, sometimes with margin added. A physics estimate for one soil in a normal winter will often come out shallower. When they disagree, the adopted value applies, whichever way round it is.

Where an estimate still helps

It tells you roughly what you are dealing with before you call: a few inches, or most of five feet. It shows how your county compares with places you know, for example Denver County, CO at 22 in and New York County, NY at 16 in. And it flags when a quoted figure looks off. If a contractor tells you 12 in where the estimate here is 107 in, as in North Slope Borough, AK, ask where their number came from.

How to get the legal number

Call or search the building department that issues permits for your address: city, or county if you are unincorporated. Ask for the frost depth in their adopted Table R301.2(1), and whether it differs for decks or accessory buildings. Write down the answer and who gave it.

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).

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