In Montana, footings have to sit below the frost depth set by the building department that issues your permit, because soil that freezes expands and lifts anything sitting in it. That required depth varies by jurisdiction and site, so the only safe number is the one confirmed for your lot. Footings that stop short get heaved every winter and crack.
Frost depth is the single most important number on a western Montana foundation, and the one most often guessed at. This guide explains what it is, what happens when a footing is too shallow, who decides how deep yours must go, and the engineered alternatives. Handy's Construction digs and pours foundations across the Missoula Valley, the Bitterroot and the Mission Valley and confirms the required depth before every dig.
What is frost depth?
Frost depth, often called the frost line, is how far down the ground can be expected to freeze in winter at a given place. Above it, soil can freeze and thaw; below it, the ground stays above freezing. A footing placed below that line bears on soil that does not freeze, so it stays put all year.
How deep frost reaches depends on climate, elevation, snow cover, soil type and moisture. Bare, wet, fine-grained soil in a cold spot freezes deeper than dry gravel under a blanket of snow.
What does frost heave do to a shallow foundation?
Frost heave is the damage that comes from building too shallow. When water in the soil freezes it expands, and in silty soils it can pull in more water and grow layers of ice that push the ground up. A footing sitting in that frozen zone gets lifted, rarely evenly, and settles back when the ground thaws. Repeat that for a few winters and the signs show up:
- Cracks in footings, foundation walls and slabs
- Doors and windows that stick or will not latch
- Drywall cracks at corners of openings
- Decks, porches and additions pulling away from the house
Repairing a heaved foundation almost always costs more than digging it to the right depth the first time.
Fine silt and clay are the most frost-susceptible soils, and parts of the Missoula Valley floor are exactly that, left behind by Glacial Lake Missoula. Wet ground near the Clark Fork and the Bitterroot adds water to the equation.
Who decides how deep footings must be where you build?
The building department that issues your permit does. In western Montana, that may be a city building department inside city limits or the county outside them, and the required frost depth can differ from one jurisdiction to the next. That is why Handy's Construction does not quote a single statewide number: the right answer for a lot in town may not be the right answer for acreage up a valley. The plans, the permit and the inspection all check it, and Handy's confirms it before the first bucket of dirt comes out.
Tip: required depth is a minimum. Poor soil, fill, a slope or high groundwater can all call for footings that go deeper or are designed differently. The site visit is where that gets decided. See what a foundation costs in Montana for how depth feeds the price.
Are there alternatives to deep footings?
Yes, but they are engineered, not improvised. A frost-protected shallow foundation uses rigid insulation around and under the footing so the ground beneath it does not freeze, which can allow a shallower footing where the design and the building department allow it. Slabs for shops and garages are often built with thickened edges and designed for the site. Pole buildings get their frost protection from posts or post footings set below frost, as covered in our pole barn vs. stick-frame comparison.
Whatever the approach, it has to be designed for the building and approved for the lot. Concrete and foundations lists the foundation types Handy's builds, and excavation and site prep covers the dig.
Can footings be poured when the ground is frozen?
Footings should never be poured on frozen ground, because the soil underneath will settle when it thaws. The concrete itself can be poured in cold weather if it is protected: fresh concrete must not freeze while it gains strength, so cold-weather pours use heated mix water, insulated blankets and enclosures. Handy's Construction pours through winter when a schedule demands it, but spring through fall is the easier and less expensive season for foundation work.
What else do people ask about frost depth and footings?
How deep do footings have to be in Montana?
Deep enough to sit below the frost depth set by the building department that issues your permit. That depth is not the same everywhere in the state, and it can differ between jurisdictions, so confirm it for your specific lot before you dig. Handy's Construction checks it with the city or county before every foundation.
What is frost heave?
Frost heave happens when water in the soil freezes, expands and forms ice lenses that push the ground upward. A footing that is sitting in that frozen layer gets lifted in winter and drops back in spring, which cracks concrete and racks walls, doors and windows out of square.
Do pole barn posts need to go below the frost line?
Yes. Embedded posts or post footings on a pole building need to reach below frost just like a conventional footing, or frost can lift them unevenly. See pole barn vs. stick-frame shop for how the two building types differ.
What is a frost-protected shallow foundation?
It is an engineered foundation that uses insulation around and under the footing to keep the ground beneath it from freezing, which can allow a shallower footing. It must be designed and approved for the site; it is not a shortcut for simply digging less.
Can concrete be poured in freezing weather?
Yes, with protection. Fresh concrete must not freeze while it gains strength, so cold-weather pours use measures such as heated mix water, insulated blankets and enclosures. Footings should also never be poured on frozen ground.
