Frost Depth by State: How Deep Footings Must Go
Applies to footings for houses, attached garages, attached decks and other permanent supports under the IRC and the state residential codes that amend it. Your local building department has the final word on the number.
There is no national frost depth. The International Residential Code sets two rules for every exterior footing: it must sit at least 12 inches below undisturbed ground (R403.1.4), and it must extend below the frost line (R403.1.4.1). The code then leaves the frost line itself blank. Table R301.2 tells each jurisdiction to "fill in the frost line depth column with the minimum depth of footing below finish grade." Whichever depth is greater is the one you dig to.
Most states pass that blank straight down to cities and counties. A handful fill it in for the whole state, and four (Minnesota, Indiana, Oregon and Kentucky) publish a county-by-county table. This guide lists only values we read in the adopted code itself, with the edition and section for each. States that leave the number to local building departments are listed as such, with no guessed value, because a wrong frost depth is an expensive mistake to find out about in March.
The familiar "US frost line map" images that circulate online are not adopted by the IRC or by any state code we checked. They are rough reference contours, and they cannot tell you what your inspector will accept. Use the tables below, then confirm with your building department before you dig.
Footing width and thickness come from IRC Table R403.1(1); the depth to the bottom of the footing comes from your frost line. Free, no signup.
The Rule: 12 Inches or Below the Frost Line, Whichever Is Deeper
Three IRC sections work together. R403.1.4 sets a 12-inch minimum below undisturbed ground for every exterior footing, in every climate. R403.1.4.1 adds frost protection: extend below the frost line, build a frost-protected shallow foundation (R403.3 or ASCE 32), or bear on solid rock. Table R301.2 note b hands the frost-line number to the local jurisdiction. Depth is measured from finish grade to the bottom of the footing, and the footing must bear on undisturbed soil (or properly compacted fill), never on frozen ground.
| Requirement | What it says | Code section |
|---|---|---|
| Absolute minimum | Exterior footings at least 12" below undisturbed ground | 2021 IRC R403.1.4 |
| Frost protection | Below the frost line, FPSF per R403.3 / ASCE 32, or on solid rock | 2021 IRC R403.1.4.1 |
| Who sets the frost line | The jurisdiction fills in the frost-line column of Table R301.2 | 2021 IRC Table R301.2, note b |
| Small detached buildings | No frost protection for freestanding accessory structures ≤600 sq ft light-frame (≤400 sq ft other), eave ≤10 ft | 2021 IRC R403.1.4.1, Exceptions 1–2 |
| Attached decks | Footings below the frost line where the deck is attached to a frost-protected structure | 2021 IRC R507.3.3 |
| All decks | Footings at least 12" below undisturbed ground | 2021 IRC R507.3.2 |
States That Publish Frost Depth in Their Residential Code
These eleven states fill in the frost depth themselves, statewide or by county. The value is the minimum depth from finish grade to the bottom of the footing unless noted. A local building department can still require more. Going shallower takes one of the code’s alternatives, such as a frost-protected shallow foundation, footings on solid rock, or an engineered design (Michigan and Minnesota also name evidence-based exceptions in the rule itself). Minnesota, Indiana, Oregon and Kentucky have county tables, shown in full below.
| State | Frost depth | How it is set | Adopted code |
|---|---|---|---|
| Connecticut | 42" | Statewide | 2022 CT State Building Code (2021 IRC portion), Table R301.2 |
| Florida | None (NA); 12" minimum applies | Statewide | 2023 Florida Building Code, Residential, Table R301.2(1) |
| Indiana | 24", 30" or 36" | By county (92 counties) | 2020 Indiana Residential Code, Table R301.2(1), note c |
| Kentucky | 24" to 33" | By county; 24" unless listed | 2018 Kentucky Residential Code, Table R403.1.4 |
| Massachusetts | 48" | Statewide | 2021 IRC with MA amendments (780 CMR), Table R301.2(1) |
| Michigan | 42" below actual grade | Statewide; building official may modify on evidence | Michigan Residential Code R403.1.4 (R 408.30522), 2015 edition |
| Minnesota | 42" or 60" | By county (two zones) | Minn. R. 1303.1600 |
| New Jersey | 30" south / 36" north | Two zones by county | 2024 IRC New Jersey Edition, Table R301.2, note b |
| North Carolina | 12" minimum; check local | Statewide floor, local may go deeper | 2024 NC Residential Code, Table R301.2, note d |
| Oregon | 12", 18" or 24" | By county (36 counties) | 2023 Oregon Residential Specialty Code, Table R301.2 |
| Wisconsin | 48", or deeper frost penetration | Statewide floor | Wis. Admin. Code SPS 321.16(1)(a) |
New Jersey: North and South Zones
New Jersey splits the state along county lines. Note b of the 2024 New Jersey Edition: "SNJ consists of Monmouth and Burlington Counties and all counties to the south; NNJ consists of Mercer and Middlesex Counties and all counties to the north." That edition took effect August 17, 2026; projects permitted under the prior edition follow it.
| Zone | Frost depth | Counties |
|---|---|---|
| South Jersey (SNJ) | 30" (2'6") | Monmouth, Burlington, Ocean, Camden, Gloucester, Salem, Cumberland, Atlantic, Cape May |
| North Jersey (NNJ) | 36" (3'0") | Mercer, Middlesex, Somerset, Hunterdon, Union, Essex, Hudson, Bergen, Passaic, Morris, Warren, Sussex |
Minnesota Frost Depth by County
Minnesota Rule 1303.1600 sets the minimum footing depth for frost at 5 feet in Zone I and 3½ feet in Zone II, "in the absence of a determination by an engineer competent in soil mechanics." Together the two lists cover all 87 counties. Minnesota also allows a slab on grade on any soil except peat or muck for detached one-story garages, carports and sheds up to 1,000 sq ft (Subp. 2).
| Zone | Frost depth | Counties |
|---|---|---|
| Zone I (north) | 60" (5 ft) | Aitkin, Becker, Beltrami, Carlton, Cass, Clay, Clearwater, Cook, Crow Wing, Douglas, Grant, Hubbard, Itasca, Kanabec, Kittson, Koochiching, Lake, Lake of the Woods, Mahnomen, Marshall, Mille Lacs, Morrison, Norman, Otter Tail, Pennington, Pine, Polk, Red Lake, Roseau, St. Louis, Todd, Traverse, Wadena, Wilkin |
| Zone II (south) | 42" (3½ ft) | Anoka, Benton, Big Stone, Blue Earth, Brown, Carver, Chippewa, Chisago, Cottonwood, Dakota, Dodge, Faribault, Fillmore, Freeborn, Goodhue, Hennepin, Houston, Isanti, Jackson, Kandiyohi, Lac qui Parle, Le Sueur, Lincoln, Lyon, McLeod, Martin, Meeker, Mower, Murray, Nicollet, Nobles, Olmsted, Pipestone, Pope, Ramsey, Redwood, Renville, Rice, Rock, Scott, Sherburne, Sibley, Stearns, Steele, Stevens, Swift, Wabasha, Waseca, Washington, Watonwan, Winona, Wright, Yellow Medicine |
Indiana Frost Depth by County
Indiana puts a "Foundation" column in its statewide Table R301.2(1). Note c defines it: "Foundation is the minimum depth, in inches, from the finish grade to the bottom of the footing." Depth steps up from the Ohio River counties (24") through central Indiana (30", including Marion County / Indianapolis) to the northern counties along Lake Michigan and the Michigan line (36").
| Frost depth | Counties |
|---|---|
| 24" | Bartholomew, Brown, Clark, Clay, Crawford, Daviess, Dearborn, Decatur, Dubois, Floyd, Franklin, Gibson, Greene, Harrison, Jackson, Jefferson, Jennings, Knox, Lawrence, Martin, Monroe, Ohio, Orange, Owen, Perry, Pike, Posey, Ripley, Scott, Spencer, Sullivan, Switzerland, Vanderburgh, Vigo, Warrick, Washington |
| 30" | Blackford, Boone, Clinton, Delaware, Fayette, Fountain, Grant, Hamilton, Hancock, Hendricks, Henry, Howard, Jay, Johnson, Madison, Marion, Montgomery, Morgan, Parke, Putnam, Randolph, Rush, Shelby, Tippecanoe, Tipton, Union, Vermillion, Warren, Wayne |
| 36" | Adams, Allen, Benton, Carroll, Cass, DeKalb, Elkhart, Fulton, Huntington, Jasper, Kosciusko, LaGrange, Lake, LaPorte, Marshall, Miami, Newton, Noble, Porter, Pulaski, St. Joseph, Starke, Steuben, Wabash, Wells, White, Whitley |
Oregon Frost Depth by County
Oregon's Residential Specialty Code lists a frost line depth for every county. West of the Cascades most counties are 12". Central Oregon is 18", and the east side is 24". Note e lowers Jackson, Josephine and Multnomah (Portland) to 12" for sites below 2,500 feet elevation, so most homes in Medford, Grants Pass and Portland build to 12".
| Frost depth | Counties |
|---|---|
| 12" | Benton, Clackamas, Clatsop, Columbia, Coos, Curry, Lane, Lincoln, Linn, Marion, Polk, Tillamook, Washington, Yamhill |
| 18" | Crook, Deschutes, Douglas, Jefferson |
| 18" (12" below 2,500 ft elevation) | Jackson, Josephine, Multnomah |
| 24" | Baker, Gilliam, Grant, Harney, Hood River, Klamath, Lake, Malheur, Morrow, Sherman, Umatilla, Union, Wallowa, Wasco, Wheeler |
Kentucky Frost Depth by County
Kentucky moved its frost rule into its own Table R403.1.4, "Minimum Frost Protection Depth," measured from the proposed finished grade to the bottom of the footing. Every county not listed is 24". The deeper counties are in Northern Kentucky and the eastern mountains. Kentucky also lets builders reach the required depth by backfilling with non-compacted soil to the frost depth plus 4 inches, and lets footings bear directly on solid rock exposed during site prep.
| Frost depth | Counties |
|---|---|
| 24" | All counties not listed below |
| 27" | Bell, Clay, Knox, Lawrence, Owsley |
| 30" | Boone, Breathitt, Campbell, Harlan, Johnson, Kenton, Leslie, Magoffin, Perry |
| 33" | Floyd, Knott, Letcher, Martin, Pike |
Every Other State: Your Building Department Sets It
In the rest of the country the frost depth is a local number, set when a city or county adopts the IRC and fills in Table R301.2. We checked the state-amended residential code for the states in the first row and confirmed the frost-line cell is left blank for local jurisdictions. To get your number, call or search your city or county building department for its "climatic and geographic design criteria" (Table R301.2) or "frost line depth". Ask for the depth to the bottom of the footing, below finish grade. If you are outside any city, the county or state building office covers you.
| Status | States |
|---|---|
| State code confirmed blank: set locally | Arkansas, California, Georgia, Idaho, Louisiana, New Hampshire, New York, Ohio, Oklahoma, South Carolina, Utah, Virginia, Washington |
| Set locally (no statewide value found) | Alabama, Alaska, Arizona, Colorado, Delaware, District of Columbia, Hawaii, Illinois, Iowa, Kansas, Maine, Maryland, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, North Dakota, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Vermont, West Virginia, Wyoming |
What Needs a Frost-Depth Footing (and What Doesn't)
Frost depth applies to permanent supports of buildings and structures. Several common projects are exempt, and some are not covered by the building code at all. Where the code is silent, setting posts below the frost line is still the surest way to stop them heaving. Your jurisdiction may amend any of these exceptions, so confirm before relying on one.
| Project | Frost depth required? | Governing rule |
|---|---|---|
| House and attached garage footings | Yes, plus 12" minimum | IRC R403.1.4, R403.1.4.1 |
| Deck attached to the house | Yes | IRC R507.3.3 |
| Freestanding deck (not attached) | No, 12" minimum still applies | IRC R507.3.2–R507.3.3 |
| Detached shed, garage or workshop | No if ≤600 sq ft light-frame (≤400 sq ft other) with eave ≤10 ft | IRC R403.1.4.1 Exceptions 1–2 |
| Fence posts | Not code-regulated (fences ≤7 ft need no permit); below frost is best practice | IRC R105.2 item 2 |
| Poured concrete or block retaining wall | Yes, footing below frost | IRC R403.1.4.1 / engineered design |
| Segmental block retaining wall | Usually no; built on a compacted gravel leveling pad | NCMA segmental wall design guidance |
| Paver patio or walkway base | No; base thickness is for load and drainage, not frost | ICPI / manufacturer spec |
| Buried water service pipe | Yes: at least 12" deep and 6" below the frost line | IRC P2603.5 |
Frost-Protected Shallow Foundations: The Code Path Around Deep Footings
A frost-protected shallow foundation (FPSF) uses rigid foam insulation around and beside the footing to keep the ground under it from freezing, so the footing can sit well above the frost line. IRC R403.3 allows the prescriptive version only for heated buildings kept at a monthly mean of at least 64°F. It cannot be used for unheated porches, utility rooms, garages or carports, or attached to an unheated basement or crawl space. The insulation must be labeled to ASTM C578. Required R-values and dimensions come from Table R403.3(1), keyed to the site's Air Freezing Index (Figure R403.3(2), based on 100-year return-period data from 3,044 weather stations). Unheated buildings need the ASCE 32 design method. Massachusetts publishes its Air Freezing Index by county: 2,000 for Berkshire, Franklin, Hampden and Worcester, and 1,500 for all others.
Why Frost Depth Matters: Heave and Adfreeze
Frost heave needs three things at once: freezing temperatures reaching into the soil, a frost-susceptible soil (silts and clays that wick water upward), and water. As the soil freezes, ice lenses grow and lift whatever sits above them, then let it down unevenly in the spring. A footing below the frost line sits in soil that doesn't freeze, so it stays put. Posts have a second problem, called adfreeze: frozen soil grips the sides of a post or pier and jacks it upward even when the bottom is deep enough. That is why fence and deck posts are often set on a flared or belled footing, with gravel backfill around the shaft. The most common mistakes are digging to a number from an internet map instead of the local code, measuring from a temporary grade instead of finish grade, and pouring onto loose backfill instead of undisturbed soil.
Standards & Citations
| Standard | Code / Section | Requirement |
|---|---|---|
| IRC Minimum Footing Depth | 2021 IRC R403.1.4 | Exterior footings at least 12 inches below undisturbed ground surface. |
| IRC Frost Protection | 2021 IRC R403.1.4.1 | Extend below the frost line in Table R301.2, build per R403.3 or ASCE 32, or bear on solid rock; accessory-structure exceptions. |
| IRC Climatic and Geographic Design Criteria | 2021 IRC Table R301.2, note b | The jurisdiction fills in the frost line depth as the minimum depth of footing below finish grade. |
| IRC Deck Footing Depth and Frost | 2021 IRC R507.3.2–R507.3.3 | Deck footings 12 inches minimum; below the frost line where attached to a frost-protected structure. |
| Minnesota Footing Depth for Frost Protection | Minn. R. 1303.1600 | 60 inches in Zone I counties, 42 inches in Zone II counties, unless an engineer determines otherwise. |
| Wisconsin UDC Frost Protection | Wis. Admin. Code SPS 321.16 | Footings below frost penetration or at least 48 inches below adjacent grade, whichever is deeper; FPSF per ASCE 32. |
| Indiana Residential Code Design Criteria | 2020 Indiana Residential Code Table R301.2(1) | County-by-county minimum footing depth of 24, 30 or 36 inches below finish grade. |
| Kentucky Minimum Frost Protection Depth | 2018 Kentucky Residential Code Table R403.1.4 | 24 inches statewide, 27 to 33 inches in 19 listed counties. |
| Oregon Residential Specialty Code Design Criteria | 2023 ORSC Table R301.2 | County frost line depth of 12, 18 or 24 inches; 12 inches below 2,500 ft in Jackson, Josephine and Multnomah. |
Apply These Requirements
Open a calculator to run this guide’s requirements against your own project.
Footing Size Calculator
Footing width and thickness come from IRC Table R403.1(1); the depth to the bottom of the footing comes from your frost line.
Deck Footing Calculator
Attached-deck footings go below the frost line (R507.3.3); the frost depth sets pier length and concrete per footing.
Deck Calculator
Uses frost depth to size post footings and concrete for the whole deck.
Fence Calculator
Post hole depth and concrete bags depend on setting posts below the frost line to prevent heave.
Retaining Wall Calculator
Rigid poured or block walls need a footing below frost; segmental walls use a gravel leveling pad instead.
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Frequently Asked Questions
How deep is the frost line in my state?
Eleven states publish it in their residential code: Connecticut 42", Florida none, Indiana 24–36" by county, Kentucky 24–33" by county, Massachusetts 48", Michigan 42", Minnesota 42" or 60" by county, New Jersey 30" south and 36" north, North Carolina 12" minimum, Oregon 12–24" by county, and Wisconsin 48" minimum. Everywhere else, your city or county building department sets the number in its adopted Table R301.2.
What is the minimum footing depth if there is no frost?
Twelve inches. IRC R403.1.4 requires every exterior footing to be at least 12 inches below undisturbed ground no matter the climate, so in Florida and other no-frost areas the 12-inch rule is the governing depth. Where the frost line is deeper than 12 inches, the frost line governs.
Is the frost line map on the internet accurate?
It is not a code document. The IRC contains no frost-depth map, only an Air Freezing Index map used for frost-protected shallow foundations, and none of the state codes we checked adopt a national frost map. Those images are rough reference contours. The legal number is the one your jurisdiction puts in Table R301.2, which is often deeper than the map suggests.
Do deck footings have to go below the frost line?
If the deck is attached to a house (or another frost-protected structure), yes: 2021 IRC R507.3.3 requires attached-deck footings to extend below the frost line, sit on solid rock, or use another approved frost-protection method. A freestanding deck only needs the 12-inch minimum in R507.3.2, but many building departments still require frost footings, so confirm locally.
Does a shed need footings below the frost line?
Usually not. IRC R403.1.4.1 exempts freestanding accessory structures up to 600 square feet of light-frame construction (400 square feet otherwise) with an eave height of 10 feet or less. Separately, R105.2 exempts one-story detached accessory structures up to 200 square feet from needing a permit. Minnesota also allows slabs on most soils for detached garages and sheds up to 1,000 square feet.
How deep should fence posts be set for frost?
The IRC does not set a fence post depth, and fences up to 7 feet tall need no building permit under R105.2. To prevent frost heave, set posts so the bottom of the concrete sits below your local frost line, bell or flare the bottom of the hole, and backfill around the post with gravel so frozen soil cannot grip it.
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