Frost Line Depth by State: How Deep the Ground Freezes

Q: What is the air freezing index?

A count of how much cold a winter delivers: degree days accumulated below 32 degrees over one freezing season, from the daily high and low. It is the quantity the frost provisions of the model codes are written on, so it is the honest way to compare two places. The figures on this site are computed from NOAA's daily record for 141 reference cities, back to 1951.

Q: Can I work out my frost depth from the freezing index?

Not from the index alone. How far frost drives into the ground also turns on the soil, how wet it is, how much snow sat on top of it and what is built over it, which is exactly why the model code leaves the depth to a local official who knows the ground. The index tells you how hard the winter pushed, and lets you see which states with a comparable winter settled on which figure in their codes.

Q: Do the states that publish a number agree with each other?

No, and it is worth seeing. Wisconsin has the hardest winters of the 6 and requires 4 feet; North Carolina requires 12 inches. Michigan averages a harder winter than Massachusetts, at 1,120 degree days against 422, and asks for the shallower footing of the two. A code figure is a policy choice built on climate, not a reading of it.

Q: Which state has the deepest frost line?

Of the depths written into a state code, Minnesota's is the deepest at 42 to 60 inches. The hardest winters measured at any reference city on this site are at Fairbanks, Alaska, averaging 5,310 degree days a season.

Q: Who do I ask for the number where I am building?

The building department for the town, city or county where the work is. They hold the figure whether or not the state publishes one, they are who a plans examiner will quote back, and in the states that do publish one they will also know which edition of the code is in force this year. Each state page names the agency and links its document.

Q: Does the frost line matter for anything besides footings?

Buried water service is the other one, and the two rules are written separately: 14 states set a burial depth for water piping in the plumbing side of the code, sometimes tied to the local frost depth and sometimes to a flat number of inches. 16 states write a footing depth sentence of their own. Both are quoted on the state pages where they exist.

11

States with a depth in the code

6 one number statewide, 5 by county or zone

5 feet

Deepest figure in any state code

Minnesota

5,310

Coldest winter measured

degree days a winter at Fairbanks

141

Cities on the record

daily readings since 1951

What it is

What the frost line depth actually is

Water expands when it freezes. Ground with water in it lifts as the freezing front works down through it, and drops again in spring, and anything resting in that moving layer rides up and down with it. A footing set below the moving layer sits in ground that never freezes, so it stays where it was put. That is the whole idea, and the frost line depth is the number a code fixes it at.

The number matters because it is enforced. It sets how deep a deck footing, a porch pier, a garage slab edge and a house foundation must go, and it is usually the figure a buried water line is measured against as well. It is not a forecast and it is not a reading of this winter: it is a design figure chosen once and written into law, and the plainest codes say so in a single sentence.

R403.1.4. Minimum depth. All exterior footings and foundation systems shall extend 42 inches below actual grade. Where applicable, the depth of the footings shall also conform to section R403.1.4.1 of the code.

R 408.30522 Minimum depth, Construction Code, Part 5. Residential Code, Michigan Administrative Rules, read September 23, 2026. Read it at the source

14 states also write a separate depth for buried water piping. It is often expressed as a clearance below the frost line rather than a figure of its own, which is one reason the frost line depth keeps turning up in plumbing work that has nothing to do with a foundation.

Frost protection for a building sewer shall not be required where the predicted depth of frost as determined from Figure 382.30-1 and Table 382.30-6 does not extend below the top of the building sewer.

Chapter SPS 382, Design, Construction, Installation, Supervision and Inspection of Plumbing, Table 382.30-6, Wisconsin Administrative Code. Read it at the source

An open footing trench cut around three sides of a rectangle of undisturbed earth beside a brick house, wet concrete lying in the bottom of the trench — A footing trench cut for a house extension, with the first concrete in the bottom. The depth of that cut is the whole question: it has to reach below the level the ground freezes to, because a footing resting in soil that freezes will be lifted by it. “2020-12-08 Concrete poured in the footings (1)”, Kolforn, CC BY-SA 4.0

Who sets it

Where the number for an address comes from

States answer this question in five different shapes, and knowing which shape a state uses is most of the work. 6 states write one depth into the residential code and it applies from the border to the border. 5 publish a table instead, a depth per county or per zone, 155 rows between them.

The rest hand the figure downward. 37 states adopt a code that asks for the frost line depth and leave the value itself to the local building department, which is the office that issues the permit and the office whose number is binding. For an address in one of those states, the call to the building department is the answer, and the state page below has the agency, the code in force and what the state does say about footings and water lines.

In the absence of a determination by an engineer competent in soil mechanics, the minimum allowable footing depth in feet due to freezing is five feet in Zone I and 3-1/2 feet in Zone II.

1303.1600 Footing Depth for Frost Protection, Minnesota Office of the Revisor of Statutes. Read it at the source

State	What the state publishes	Depth in the code	Freezing index
Alabama	Local	From the building department	23
Alaska	Local	From the building department	2,588
Arizona	Local	From the building department	98
Arkansas	Local	From the building department	73
California	Local	From the building department	84
Colorado	Local	From the building department	811
Connecticut	Statewide	42 inches	521
Delaware	Local	From the building department	126
Florida	None set	From the building department	1
Georgia	Local	From the building department	35
Hawaii	Local	From the building department	No station
Idaho	Local	From the building department	429
Illinois	Local	From the building department	435
Indiana	By county or zone	24 to 36 inches	379
Iowa	Local	From the building department	1,044
Kansas	Local	From the building department	300
Kentucky	By county or zone	24 to 33 inches	153
Louisiana	Local	From the building department	6
Maine	Local	From the building department	1,270
Maryland	Local	From the building department	263
Massachusetts	Statewide	4 feet	422
Michigan	Statewide	42 inches	1,120
Minnesota	By county or zone	42 to 60 inches	2,150
Mississippi	Local	From the building department	17
Missouri	Local	From the building department	250
Montana	Local	From the building department	734
Nebraska	Local	From the building department	639
Nevada	Local	From the building department	225
New Hampshire	Local	From the building department	978
New Jersey	By county or zone	30 to 36 inches	287
New Mexico	Local	From the building department	83
New York	Local	From the building department	781
North Carolina	Statewide	12 inches	34
North Dakota	Local	From the building department	2,169
Ohio	Local	From the building department	428
Oklahoma	Local	From the building department	91
Oregon	Per address	From the building department	76
Pennsylvania	Local	From the building department	325
Rhode Island	By county or zone	30 to 40 inches	320
South Carolina	Local	From the building department	13
South Dakota	Local	From the building department	1,295
Tennessee	Local	From the building department	68
Texas	Local	From the building department	58
Utah	Local	From the building department	348
Vermont	Statewide	5 feet	1,148
Virginia	Local	From the building department	57
Washington	Local	From the building department	225
West Virginia	Local	From the building department	237
Wisconsin	Statewide	4 feet	1,162
Wyoming	Local	From the building department	942

One row per state. The depth column shows what the state publishes; a state that sets it locally shows where to ask instead. The freezing index column is the average of that state’s reference cities, in degree days, and is a measurement of winter rather than a depth.

The published depths

How deep the state codes go

The 11 states that publish a depth, deepest first. A bar shows the span for a state whose code carries a table, because those states do not have one number at all: Minnesota asks five feet across its northern third and three and a half feet everywhere else, and that difference is the table doing its job.

Minnesota42 to 60 inches
Vermont5 feet
Massachusetts4 feet
Wisconsin4 feet
Connecticut42 inches
Michigan42 inches
Rhode Island30 to 40 inches
Indiana24 to 36 inches
New Jersey30 to 36 inches
Kentucky24 to 33 inches
North Carolina12 inches

Each figure is quoted on that state’s page with the section it comes from and the date the code was read.

How hard it freezes

The freezing index, city by city

Codes do not pick a depth out of the air. The quantity behind frost protection design is the air freezing index: for one winter, add up how far each day’s mean temperature fell below freezing, in degree days, and take the deepest run the winter managed. A winter that sits at 22 degrees for a month scores 300; a winter that dips below freezing for a week and recovers scores almost nothing. It is the closest thing to a measurement of how much cold the ground had to absorb.

Below is that figure for 141 cities, computed from the NOAA daily temperature record for each station since 1951, coldest first. The once a century column is the freezing index a Gumbel fit puts at a one in a hundred winter, which is the return period the frost protection standards themselves work to.

City	State	Freezing index	Once a century	Nights below freezing
Fairbanks	Alaska	5,310	7,689	222
International Falls	Minnesota	2,783	4,569	198
Minot	North Dakota	2,330	4,210	187
Fargo	North Dakota	2,251	4,104	176
Duluth	Minnesota	2,174	3,649	182
Caribou	Maine	1,956	3,079	182
Bismarck	North Dakota	1,926	3,744	185
Anchorage	Alaska	1,902	3,754	190
Aberdeen	South Dakota	1,843	3,629	179
Leadville	Colorado	1,797	2,790	266
Sault Ste. Marie	Michigan	1,623	3,012	173
Wausau	Wisconsin	1,591	2,960	166
Jackson	Wyoming	1,566	2,874	241
Lake Placid	New York	1,547	2,619	187
Minneapolis	Minnesota	1,492	2,889	152
Berlin	New Hampshire	1,407	2,438	176
Sioux Falls	South Dakota	1,391	2,820	164
Decorah	Iowa	1,211	2,557	154
St. Johnsbury	Vermont	1,175	2,184	166
Marquette	Michigan	1,172	2,192	157
Burlington	Vermont	1,120	2,314	150
Dubuque	Iowa	1,116	2,328	147
Bangor	Maine	1,115	2,040	156
Madison	Wisconsin	1,099	2,304	155

Showing the 24 coldest of 141. Every state page carries its own cities in full. Source: NOAA National Centers for Environmental Information, Global Historical Climatology Network daily.

The comparison

What the states with one number made of the same winter

The 6 states that answer with a single statewide depth are the only ones whose codes can be set beside each other directly. Ranking them by the freezing index at their own reference cities shows the coldest at 1,162 degree days and the mildest at 34, a fivefold spread of winter, against depths running 12 inches to 5 feet.

North Carolina3412 inches
Massachusetts4224 feet
Connecticut52142 inches
Michigan1,12042 inches
Vermont1,1485 feet
Wisconsin1,1624 feet

Left to right, the mildest winters to the hardest. The figure on the right is what that state wrote into its code.

They do not line up, and the exceptions are the interesting part. Massachusetts has the milder winter of the two and asks for the deeper footing: 4 feet against Michigan’s 42 inches, at a freezing index of 422 against 1,120. 3 such pairs turn up among the 6. A code depth carries soil, snow cover, drainage, a margin for the hard winter nobody has seen yet, and the judgement of whoever wrote the amendment, so the freezing index explains a lot of the ordering and never all of it.

What the scale is good for is reading a code, not choosing a depth. A depth belongs to a jurisdiction, and the one that counts is the one the building department issuing the permit enforces. Only Minnesota publishes how deep frost actually drove in a given winter, and it does so because the state measures roadbeds all winter for load restrictions.

Curling white blades of ice growing out of a bare soil bank in a forest, each blade carrying a cap of soil on its end — Needle ice in the Adirondacks. Water is drawn up to the freezing front and grows there as blades, and each blade lifts the soil on top of it. The same process a few feet down, under a footing, is what building codes set a frost line depth against. “Ground Ice Curls”, Theedster123, Public domain

By state

Frost line depth, state by state

Each state page has the depth the state publishes in its own words, the code it sits in and the agency that adopts it, the separate footing and water line rules where the state writes them, and the freezing index for that state’s reference cities.

Frost line depth questions

How deep is the frost line in my state?

That depends on which of five answers your state gives. 6 states write one depth into their residential code. 5 publish a table by county or zone. Oregon returns the figure for one address at a time through a state tool. Florida's code carries the row and enters NA in it. The other 37 adopt the model code's local determination and name the official who makes it. Every state's answer is in the table below, quoted from the state's own document.

What is the air freezing index?

Can I work out my frost depth from the freezing index?

Do the states that publish a number agree with each other?

Which state has the deepest frost line?

Who do I ask for the number where I am building?

Does the frost line matter for anything besides footings?

Frozen pipesThe nights cold enough to burst a pipe, and when they usually arrive.Ice damsThe winter that puts meltwater back up under the shingles.First snowWhen the first measurable snow usually falls, city by city.

Codes read September 23, 2026 from each state’s own published code or the agency that adopts it, with the page and the date on every state page. Freezing index computed from NOAA daily temperature records for the 2026 record. Codes are amended, and a local amendment can be deeper than the state figure, so the building department issuing the permit has the number that binds. Start with your state in the table above.