How deep do footings have to go to get below the frost line where you live? This guide explains what the frost line is, how the building code sets your footing depth, typical frost depth ranges for every US state, and how to find the exact number for your town before you dig.
| Code minimum anywhere | 12 in below undisturbed ground |
|---|---|
| South | About 6–18 in |
| Mid-Atlantic, lower Midwest | About 18–30 in |
| Northeast, Midwest | About 30–48 in |
| Northern tier | About 42–72 in |
What is the frost line, and why does it matter?
Water in soil expands when it freezes. If a footing sits in soil that freezes, that expansion (frost heave) can lift it, then let it settle again in spring. Over a few winters that cracks foundations, racks deck posts and tilts fences. Putting the bottom of the footing below the deepest normal freeze keeps it on stable ground.
How the building code sets footing depth
- 12 in minimum: the International Residential Code (IRC R403.1.4) requires the bottom of exterior footings to be at least 12 in below undisturbed ground, even where the ground never freezes.
- Below the frost line: IRC R403.1.4.1 requires footings to extend below the frost line, unless they are protected from frost another approved way (for example a frost-protected shallow foundation).
- Your jurisdiction sets the number: each city or county fills in its frost line depth in IRC Table R301.2(1), the local "climatic and geographic design criteria" table. Some states set it statewide; others by county or town.
- Frost-protected shallow foundations (IRC R403.3) use rigid insulation to keep the soil under a shallow footing from freezing. They need a specific design, so ask your building department first.
Two official examples
| Where | Required footing depth for frost | Source |
|---|---|---|
| Minnesota, Zone I (northern counties) | 60 in (5 ft) | Minnesota Rules 1303.1600 |
| Minnesota, Zone II (southern and central counties, incl. Hennepin and Ramsey) | 42 in (3½ ft) | Minnesota Rules 1303.1600 |
| Anne Arundel County, Maryland | 30 in for attached-deck footings | County 2021 IRC Deck Construction Guide |
Frost line depth by state (typical ranges)
These ranges show how deep frost typically goes in each region, so you can plan before you call. Depth varies a lot within a state with elevation, latitude and soil: northern and higher areas are deeper. Use them for planning only.
| State | Typical frost depth | Region |
|---|---|---|
| Alabama | 6–18 in | South |
| Alaska | 42–100+ in | Alaska |
| Arizona | 12–60 in | Mountain West (varies with elevation) |
| Arkansas | 6–18 in | South |
| California | 12–24 in | Pacific Coast (deeper in the mountains) |
| Colorado | 12–60 in | Mountain West (varies with elevation) |
| Connecticut | 30–48 in | Northeast & Midwest |
| Delaware | 18–30 in | Mid-Atlantic & lower Midwest |
| District of Columbia | 18–30 in | Mid-Atlantic & lower Midwest |
| Florida | 0–6 in | Florida & Hawaii |
| Georgia | 6–18 in | South |
| Hawaii | 0–6 in | Florida & Hawaii |
| Idaho | 12–60 in | Mountain West (varies with elevation) |
| Illinois | 30–48 in | Northeast & Midwest |
| Indiana | 30–48 in | Northeast & Midwest |
| Iowa | 30–48 in | Northeast & Midwest |
| Kansas | 18–30 in | Mid-Atlantic & lower Midwest |
| Kentucky | 18–30 in | Mid-Atlantic & lower Midwest |
| Louisiana | 6–18 in | South |
| Maine | 42–72 in | Northern tier |
| Maryland | 18–30 in | Mid-Atlantic & lower Midwest |
| Massachusetts | 30–48 in | Northeast & Midwest |
| Michigan | 42–72 in | Northern tier |
| Minnesota | 42–72 in | Northern tier |
| Mississippi | 6–18 in | South |
| Missouri | 18–30 in | Mid-Atlantic & lower Midwest |
| Montana | 42–72 in | Northern tier |
| Nebraska | 30–48 in | Northeast & Midwest |
| Nevada | 12–60 in | Mountain West (varies with elevation) |
| New Hampshire | 42–72 in | Northern tier |
| New Jersey | 30–48 in | Northeast & Midwest |
| New Mexico | 12–60 in | Mountain West (varies with elevation) |
| New York | 30–48 in | Northeast & Midwest |
| North Carolina | 6–18 in | South |
| North Dakota | 42–72 in | Northern tier |
| Ohio | 30–48 in | Northeast & Midwest |
| Oklahoma | 6–18 in | South |
| Oregon | 12–24 in | Pacific Coast (deeper in the mountains) |
| Pennsylvania | 30–48 in | Northeast & Midwest |
| Rhode Island | 30–48 in | Northeast & Midwest |
| South Carolina | 6–18 in | South |
| South Dakota | 42–72 in | Northern tier |
| Tennessee | 6–18 in | South |
| Texas | 6–18 in | South |
| Utah | 12–60 in | Mountain West (varies with elevation) |
| Vermont | 42–72 in | Northern tier |
| Virginia | 18–30 in | Mid-Atlantic & lower Midwest |
| Washington | 12–24 in | Pacific Coast (deeper in the mountains) |
| West Virginia | 18–30 in | Mid-Atlantic & lower Midwest |
| Wisconsin | 42–72 in | Northern tier |
| Wyoming | 42–72 in | Northern tier |
How to find your exact frost depth
- Call or check the website of your city or county building department. Ask: "What frost depth do you require for footings?" Many publish it in a residential design-criteria sheet or deck guide.
- Look for your jurisdiction's IRC Table R301.2(1). Some states post every county's table online.
- Check your permit handout or deck guide. Deck and fence guides almost always state the required footing depth.
- If you are near a boundary or at high elevation, ask specifically; mountain towns often require more than the valley below.
Frost depth for decks, fences and sheds
- Decks attached to the house need footings below the frost line, because frost heave would lift the deck against the house. See our deck footing size calculator.
- Free-standing decks may qualify for exceptions in some codes; ask your building department before you rely on one.
- Fence posts are not always regulated, but a post that stops above the frost line can heave. See bags of concrete per fence post.
- Sheds on skids or a gravel pad can often float with the frost; a shed on footings needs them below frost. Check local rules for accessory buildings.
FAQ
What is the frost line depth?
It is the deepest point the ground normally freezes in winter. Footings must sit below it, and always at least 12 in below undisturbed ground under the IRC.
How deep is the frost line in Minnesota?
Minnesota Rules 1303.1600 require 60 in (5 ft) in Zone I, the northern counties, and 42 in (3½ ft) in Zone II, the southern and central counties, unless an engineer justifies less.
How deep should footings be if the ground does not freeze?
At least 12 in below undisturbed ground. That IRC minimum applies everywhere, including Florida and Hawaii.
Who sets the frost depth for my house?
Your local building department. Each jurisdiction lists it in its IRC Table R301.2(1) design criteria, and some states set it statewide.
Do deck footings have to be below the frost line?
Footings for decks attached to a house generally do. Some codes allow exceptions for certain free-standing decks, so check with your building department.
References: International Residential Code (IRC) R301.2(1), R403.1.4, R403.1.4.1 and R403.3; Minnesota Rules 1303.1600; Anne Arundel County (MD) 2021 IRC Deck Construction Guide. State ranges are approximate planning values, not code requirements.
Unlock an ad-free experience plus a growing library of ready-to-use civil-engineering calculators, IS-code cheat-sheets and formula PDFs. From just ₹99.
Discover more from The Civil Studies
Subscribe to get the latest posts sent to your email.
