What the frost line is
Water in the soil expands when it freezes, and wet ground freezing from the top down can grow layers of ice that push up on anything sitting in it. That's frost heave. A footing above the frost line gets lifted a little in winter and settles back crooked in spring, and after a few years a deck sags at one corner or a row of fence posts leans and rises.
The fix is simple: put the bottom of the footing deeper than the ground freezes. Below that depth the soil stays unfrozen all winter, and nothing pushes.
What the code says
The International Residential Code, which most of the US builds on, has two rules for footings. They go at least 12 inches below undisturbed ground, everywhere. And where the ground freezes, they go below the frost line too.
The code doesn't print a frost depth of its own. It leaves a blank in its climate table, Table R301.2(1), and each state, county or city that adopts the code fills in the minimum depth of footing below the finished ground. That's why the answer depends on your address, and why it can differ between two towns a few miles apart.
The number is to the bottom of the footing, not the top. A 42-inch frost depth means a 42-inch hole, at least.
Frost depths by state and city
These are 20 places, from shallow to deep, each read from the code or design table the state or local building department publishes. 4 of them set one rule for the whole state. Everywhere else it's the city or county.
| Place | Frost depth | Notes | Source |
|---|---|---|---|
| Apache Junction, AZcity | Final grade | No frost depth below the finished ground. | City of Apache Junction, amendments to the 2018 IRC, Table R301.2(1)Checked November 9, 2026 |
| Benson, AZcity | 0 in, or 24 in above 4,000 ft | Depends on elevation. | City of Benson, climatic and geographic design criteriaChecked November 9, 2026 |
| Gilbert, AZtown | 0 in | No frost depth: the table lists 0. | Town of Gilbert, 2018 IRC climatic and geographic design criteriaChecked November 9, 2026 |
| Douglas County, NVcounty | 18 in, or 24 in at 6,000 ft and up | Lake Tahoe's Nevada side and the Carson Valley. | Douglas County, building codes and design criteriaChecked November 9, 2026 |
| Boise, IDcity | 24 in | A local amendment, for houses and other buildings alike. | City of Boise, currently adopted building codes (March 2023)Checked November 9, 2026 |
| Twin Falls, IDcity | 24 in | – | City of Twin Falls, residential design criteriaChecked November 9, 2026 |
| Queen Anne's County, MDcounty | 24 in | Maryland publishes a table for each county. | Maryland Department of Labor, IRC Table R301.2(1), Queen Anne's County (amended 2020)Checked November 9, 2026 |
| Washoe County, NVcounty | 24 in | Reno and Sparks area, under the Northern Nevada amendments. | Washoe County Building, construction design requirementsChecked November 9, 2026 |
| Alexandria, VAcity | 24 in | – | City of Alexandria, residential design criteria (2022)Checked November 9, 2026 |
| Elkins, WVcity | 24 in | – | City of Elkins, climatic and geographic design criteriaChecked November 9, 2026 |
| Lawrence, KScity | 30 in | – | City of Lawrence, residential code ordinance 9267, Table R301.2(1)Checked November 9, 2026 |
| Salt Lake City, UTcity | 30 in | Unchanged across the last three code editions. | Salt Lake City Building Services, design criteriaChecked November 9, 2026 |
| Columbus, OHcity | 32 in | Under the Residential Code of Ohio. | City of Columbus, Construction Industry Communication 09, climatic and geographic designChecked November 9, 2026 |
| Mission Hills, KScity | 36 in | Kansas City area. | City of Mission Hills, amendments to the International Residential CodeChecked November 9, 2026 |
| Ely, NVcity | 36 in | High desert, about 6,400 ft up. | City of Ely, building department and building codesChecked November 9, 2026 |
| Anchorage, AKcity | 42 in heated, 60 in unheated | A deck counts as unheated. | Municipality of Anchorage, AMC 23.85, local amendments to the 2018 IRCChecked November 9, 2026 |
| Michiganthe whole state | 42 in | Every exterior footing, measured below actual grade. | Michigan Administrative Code, R 408.30522, minimum depthChecked November 9, 2026 |
| Minnesotathe whole state | 3½ ft in the south, 5 ft in the north | Two zones, listed county by county. The Twin Cities are in the 3½ ft zone. | Minnesota Rules, part 1303.1600, footing depth for frost protectionChecked November 9, 2026 |
| Massachusettsthe whole state | 48 in | Set in the state's own climatic table. | 780 CMR, Massachusetts Residential Code, Table R301.2(1)Checked November 9, 2026 |
| Wisconsinthe whole state | 48 in, or below the frost if deeper | One- and two-family homes, decks included. | Wisconsin Administrative Code, SPS 321.16, frost protectionChecked November 9, 2026 |
A few patterns stand out. The low desert of the Southwest has little or no frost, and there the 12-inch minimum is what governs. In the mountain West the depth follows the elevation, so the same county can have two numbers. Across the northern states and New England it's 42 to 48 inches, and more in northern Minnesota and Alaska.
How deep to dig where you live
Pick a place from our table, or type the frost depth your building department gave you. You'll get how deep to dig for a fence post or a deck footing.
Decks and fences aren't treated the same
A deck footing is structure. Wherever a deck needs a permit, the inspector usually checks the footing holes before the concrete goes in, and the bottom has to be at the frost depth. An attached deck that heaves pulls on the house, which is why codes are strict about it.
Fence posts are a different story. Most cities don't inspect fence footings at all, and plenty of fences are built with posts 2 feet down in places where the ground freezes deeper. They often get away with it, especially with gravel under the post so water drains away. But posts that sit above the frost line are the ones that heave, so going deeper is cheap insurance, above all for gate posts. Our post depth guide has the usual rule.
How to find the number for your town
Search for your city or county with the words "design criteria" or "climatic and geographic design criteria". Most building departments publish a one-page table with snow load, wind speed and frost line depth, the same table the code leaves blank. If you can't find it, call the building department and ask. It's a question they answer every day.
Then call 811 a few business days before you dig. It's free, and they mark the buried lines so a 4-foot hole doesn't find one.
Fair questions
My town isn't in the table. What do I use?
Your building department's number. Don't borrow one from a nearby town, because depths change with elevation and from one jurisdiction to the next. Our table is a sample to show the range, not a replacement for your local code.
Do I need frost footings for a freestanding deck?
Often, yes. Some places let a small, low freestanding deck sit on precast blocks or shallow footings because it can move without damaging anything. Others want frost footings under every deck, and Wisconsin's rules include decks explicitly. Ask before you build.
What if the frost line is deeper than I can dig?
Rent a power auger, or hire someone with a tractor auger. Holes 4 or 5 feet deep are routine in the northern states. For houses there's also a way to build shallower foundations insulated against frost, but it's meant mainly for heated buildings, not decks or fences.
Does the depth change from year to year?
The ground freezes deeper in a cold, dry winter with little snow on it, and less under a thick snow cover. The code number is set for a hard winter, so it already allows for that.
Sources
- International Residential Code, Sections R403.1.4 and R403.1.4.1, and Table R301.2(1): footings at least 12 inches below undisturbed ground, protection from frost, and the frost line depth each jurisdiction fills in for itself.
- Call 811: the national call-before-you-dig service, for when and how to have buried utilities marked.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
