Frost Depth, Frozen Ground, and When Egress Cuts Get Scheduled in Montana

September 4, 2026

The City of Billings designs footings to a 36-inch frost depth, and once the ground is frozen anywhere close to that, an egress excavation stops being a morning with a mini excavator and starts being a fight. That single number, more than anything about windows or code, decides when egress work happens in the Yellowstone valley. This post walks through the actual climate numbers, what the ground does through the year, and how to plan a Billings basement bedroom project so the cut lands in the right month.

The numbers behind the season

Billings is not the coldest place in Montana, but it is cold enough. NOAA’s 1991 to 2020 climate normals for Billings Logan International Airport put the January mean at 27 degrees, with a typical January high around 36 and low around 18. July averages 73. Snowfall runs about 57 inches a year, with measurable snow in every month from September through May: roughly 10 inches in January, 9 in February, 8 in March, and, the number that surprises newcomers, 7.5 inches in April.

The saving grace is the chinook. Warm, dry winds off the mountains break the cold open several times most winters, which is why Billings ground rarely stays frozen to full depth for months at a stretch the way it does in Fargo or Sioux Falls. Chinooks are also why we will not tell you an egress cut is impossible in January. We will tell you it is not something to plan around.

The city’s design criteria set the local frost line at 36 inches to the top of footings, and the Billings building ordinance uses the same figure unless a foundation investigation shows otherwise. Two things follow from that. Your footing sits at least that deep, which sets how low a new sill can go. And the drainage under a window well has to work below that depth, or it freezes solid exactly when snowmelt needs somewhere to go.

What the ground does, month by month

December through February. Ground frost builds from the surface down, deeper on north-facing walls and under bare ground, shallower under snow cover and on sunny south walls. A chinook can thaw the top foot for a few days. Excavation is possible in a warm spell on a south wall with an open side yard, and miserable everywhere else. Concrete cutting itself does not care about cold, but the dig comes first.

March. The reliable season opens. Frost comes out of the ground from both directions, the surface thaws, and the clay goes soft. Machine access can be the limiting factor: a mini excavator on saturated clay makes ruts a landscaper will charge you to fix. We watch the yard, not just the calendar.

April and May. Peak runoff. The mountains send the Yellowstone up, the valley’s clay terraces are saturated, and this is when window wells that were installed without drainage announce themselves. The record crest at Billings was 16.2 feet on June 15, 2022, high enough to shut down the city water plant; the previous record was 15 feet in 1997. Most homes are nowhere near the river, but the same pattern of wet ground plus spring rain shows up in every well in the valley. Good months to dig, and the best months to see exactly where water goes.

June through September. Prime season. Dry ground, long days, one-to-two-day installs. Also the busiest months, and the ones where a fall listing should already be scheduled.

October and November. The pre-listing rush and the last reliable digs. The first hard freezes come in October, but the ground stays workable well into November most years. This is when sellers with a spring listing get the cut done so the finish can happen inside all winter.

Why frost matters more on clay

The USDA soil survey for Yellowstone County maps the terraces between Billings and Laurel, and northeast of Shepherd and Huntley, as deep silty clay loams and silty clays over stratified clay. It counts roughly 82,000 acres of saline, saline-sodic, or sodic soils in the county, and it explains what sodium does: the clay disperses, permeability to water and air goes slow, and shrink-swell potential goes high.

Frost heave needs three things: freezing temperatures, water, and fine-grained soil that can pull water up to the freezing front. Yellowstone valley clay supplies the last two in abundance. A window well set on bare clay with a few inches of gravel gets lifted a little every winter and dropped a little every spring. Add the shrink-swell cycle from irrigation and summer drought, and a corrugated well installed plumb in 1985 is leaning by 2005 and pulling its anchors by 2020. That is the mechanism behind most of the well replacement work we do in the Heights.

It is also why the drainage bed goes below 36 inches. Gravel that stays above frost freezes into a plug. Gravel below frost, wrapped in fabric and tied to a footing drain or dry well, keeps moving water through the winter thaw cycles. The covers and drainage page has the full build-up.

How a well-planned Billings project runs

Spring listing. Measure in September. Permit through the City of Billings CityView portal in October, four to seven business days for the initial residential review. Cut and set the window before Thanksgiving. Frame, wire, and drywall inside from December through February. Final inspection in March. List in April with a conforming bedroom.

Summer finish. Measure in January or February, when we have time to walk the basement carefully. Permit in March. Cut in April or May once the yard will hold the machine. Finish through summer.

Flagged bedroom with a closing date. In season, two to three weeks inside the city from signed quote to passed final; less in Lockwood, Shepherd, and Huntley, where no permit exists. If the flag comes in January with a March closing, call immediately; we will watch for a chinook window and be honest if the ground says no. The code compliance upgrades page covers that scenario.

Failed well. Wells that flooded in April are best rebuilt by late summer, when the ground is dry and the excavation stays open. Waiting until it floods again next April means digging in the worst month.

What runs in winter

Frozen ground stops excavation. It does not stop the rest of the trade.

• Basement window swaps in existing openings: no dig, no cut, a morning’s work in January. See basement window replacement.
• Well covers: install any month. A cover sized for Billings snow load matters most in exactly the months you cannot dig.
• Measures, quotes, and permits: the whole paperwork side. The people first on the March schedule are the ones who got measured in December.
• Concrete cutting from inside, in rare cases where an existing well is deep enough and drainage already exists, but that is a case-by-case call at the measure.

The short version

• Billings designs to 36 inches of frost. The dig has to go below it for drainage, and the ground has to be thawed to dig.
• Reliable season: roughly March through November. Chinooks open winter windows we do not promise.
• Clay plus water plus frost is why wells heave and why drainage is the line item that matters.
• Spring listing means a fall cut. Summer finish means a winter measure.
• Window swaps, covers, and paperwork run all year.

Full installs run $3,500 to $6,500 and the rest of the numbers are on the pricing page. If you want the cut done before the ground closes, the free on-site measure is the first step, and September is the month to take it.