How deep footings must go in Minnesota, and why

Every Minnesota winter, the cold works its way down into the ground, and a footing sitting in soil that freezes is exposed to frost heave. Minnesota's building code answers this with a minimum footing depth, and the number depends on the county you build in.

This post covers what the rules say, where they live, and the alternatives to a deep footing that the code itself allows.

What frost heave is

Frost heave is the ground lifting as it freezes. A builder's guide to frost-protected foundations, published by HUD and prepared by the NAHB Research Center, says frost heave can only happen when three conditions are present at the same time:

the soil is frost susceptible (the guide points to soils with a large silt fraction)
enough moisture is available, roughly 80 percent saturation or more
below-freezing temperatures are reaching into the soil

Remove any one of the three and, according to the guide, frost damage can't occur. That is the thinking behind the two main approaches in the code: put the footing below the depth where the ground freezes, or keep the soil under it from freezing.

Minnesota's rule: 5 feet or 3½ feet

The depth comes from Minnesota Rules, part 1303.1600, "Footing depth for frost protection." Chapter 1303 holds the state's "Minnesota provisions," which the rules describe as requirements needed, among other reasons, "to address Minnesota's climatic conditions."

Unless an engineer competent in soil mechanics determines otherwise, the rule sets the minimum footing depth "due to freezing" at five feet in Zone I and 3-1/2 feet in Zone II.

Zone I lists 34 counties, among them St. Louis, Itasca, Crow Wing, Otter Tail, Clay and Polk.
Zone II lists the other 53. Le Sueur County is on the Zone II list, along with Scott, Rice, Nicollet, Blue Earth, Hennepin and Ramsey.

So for a footing in Montgomery, or anywhere else in Le Sueur County, the starting number is 3½ feet.

The rule also says less depth may be permitted when an engineer competent in soil mechanics presents supporting evidence. A shallower footing is possible, then, but it takes an engineer's soils work to justify it.

Wooden footing forms set on a cleared building site — Footing forms on a cleared site.

How the residential code points to that rule

Minnesota adopts the 2018 International Residential Code (IRC) through Minnesota Rules, chapter 1309, with state amendments. Two of those amendments connect the IRC to the frost rule:

In the IRC's climate table, Table R301.2(1), Minnesota fills in the "frost line depth" entry with a reference to part 1303.1600.
Minnesota rewrote IRC section R403.1.4.1, "Frost protection." Its version opens with "Footings shall not bear on frozen soil" and lists five ways to protect foundations from frost:
1. Extend below the frost line in Table R301.2(1).
2. Build to IRC section R403.3 (frost-protected shallow foundations).
3. Build to ASCE 32.
4. Erect on solid rock.
5. Build to Minnesota Rules, chapter 1303.

The 2018 IRC also sets a general floor: exterior footings go at least 12 inches below undisturbed ground. Minnesota's frost depths are well past that.

Codes do change. The Department of Labor and Industry (DLI) is considering amendments that would adopt the 2024 edition of the IRC. Its request for comments was published in the State Register on Feb. 2, 2026, and the rest of the timetable is listed as to be determined, so confirm the current version when you apply for a permit.

Frost-protected shallow foundations

The second method on that list is the main alternative to a deep footing. Under 2018 IRC section R403.3, footings don't have to extend below the frost line when they're protected with insulation laid out according to the code's figure and table. That applies only to buildings kept at a monthly mean temperature of at least 64°F. The section says these foundations can't be used for unheated spaces such as porches, utility rooms, garages and carports, and that insulation used below grade for this purpose must be labeled as complying with ASTM C578.

The HUD guide explains why it works: the insulation keeps the soil under the footing from freezing. It notes that an inch of polystyrene insulation rated R4.5 has an R-value equal to about 4 feet of soil, on average.

Exceptions, and where they differ

This is where the model code and Minnesota's version part ways, so it pays to read closely.

The model IRC. The 2018 IRC's own section R403.1.4.1 lists exceptions for free-standing accessory structures with an eave height of 10 feet or less: 600 square feet or less for light-frame construction, 400 square feet or less for other construction. It also says decks not supported by a dwelling don't need footings below the frost line.
Minnesota's amended section. The version of R403.1.4.1 in Minnesota Rules, part 1309.0403 does not list those exceptions.
Minnesota's slab provision. Subpart 2 of part 1303.1600 has its own exception: slab on grade construction may be placed on any soil except peat or muck for detached one-story private garages, carports and sheds no larger than 1,000 square feet. For other buildings, it allows a structurally designed slab on grade when the soil is sand, or pit run sand and gravel, to the zone's footing depth, with limits on how fine that sand can be unless an engineer approves it.

How these pieces apply to a particular garage, shed or addition is a question for the building official who reviews your permit.

What this means for your project

Know your zone. Le Sueur County is Zone II: 3½ feet, unless an engineer's soils work supports less.
Heated or unheated matters. The insulated shallow foundation in R403.3 is for heated buildings only.
Soil matters. It decides whether frost heave can happen at all, and Minnesota's slab provision turns on soil type.
Ask your building department. Under part 1309.0010, the residential code is administered by municipalities that have adopted the Minnesota State Building Code, so your city or county is the place to confirm requirements and permits.

Planning a footing, foundation or slab in or around Montgomery? Estevez Construction does concrete and excavation work for residential and commercial customers. Call us at (612) 601-3995 or request a free estimate.