Freeze-Thaw Cycles in Western Montana and What They Do to a Property

How many freeze-thaw cycles does Missoula get?

NOAA's 1991-2020 normals for the Missoula airport count an average of 172.5 days a year with a low at or below 32 degrees Fahrenheit, but only 42.8 days when the high stays at or below 32. Every day with a freezing high also has a freezing low, so the difference, roughly 130 days, is the number of days that freeze and then rise above freezing. Hamilton's normals give 172.6 freezing nights and 29.1 freezing days, a gap of about 143 days. Stevensville and Darby are in the same range.

That is a rough count, since a single day can cross the line more than once and a mild night may barely touch 32. But it gives the scale: in the valleys, crossing the freezing mark is the normal state of affairs for about four months of the year.

Which months are worst?

Not the coldest ones. In January the Missoula airport averages 29.8 freezing nights and 13.6 days that never get above freezing, which leaves about 16 crossing days. December is similar. February rises to about 20 crossing days and March to about 24, because nearly every night still freezes (25.0 on average) but only about one day in March stays frozen. November also runs near 20.

So the heaviest freeze-thaw stress falls in late winter and early spring, exactly when snowmelt puts the most water on the ground. That is why potholes, heaved pavers and cracked concrete tend to appear in March and April rather than January. Hamilton follows the same pattern, with 25.7 freezing nights in March and only 1.2 frozen days.

Does elevation or exposure change the count?

Yes. Even the two Missoula stations differ. The National Weather Service office station northwest of town (Missoula 6 NW WFO) averages 160.7 freezing nights and 39.9 freezing days a year, a gap of about 121 days, and its median first fall freeze is October 4 against September 26 at the airport. A few miles and a little terrain change the numbers.

On a single property the bigger factor is sun. A south-facing slab can thaw every afternoon in February while a north-facing walk two hundred feet away stays frozen for weeks and then goes through its cycles in March. Shaded areas also hold ice longer, which matters for footing. When you look for freeze-thaw damage, look first where sun and shade meet and where water collects.

Why does freezing water do so much damage?

Because water expands as it freezes. The American Red Cross puts it simply in its frozen pipes guidance: that expansion puts tremendous pressure on whatever is containing it, including metal or plastic pipes, and no matter the strength of the container, expanding water can cause it to break. The same force works inside the pores of concrete, the joints of a paver patio, the cracks in a rock wall and the top layer of soil.

A single freeze rarely does much. The damage comes from repetition with water present. Each thaw lets water move deeper into whatever opened up last time, and each freeze pries it a little wider.

What does freeze-thaw do to concrete driveways and walks?

Concrete is protected from freeze-thaw mainly by tiny air bubbles mixed into it on purpose. A Federal Highway Administration tech brief on deicers and concrete explains that freeze-thaw durability has traditionally been tied to the size and volume of entrained air bubbles, that the most severe exposure is when deicers are present, and that typical air contents range from 5 to 8 percent. The bubbles give freezing water somewhere to go.

The same brief makes a point every property owner should know: if concrete freezes and thaws while critically saturated (above roughly 86 percent saturation), damage can occur within even a single cycle, regardless of the air content. Saturation is most likely at joints where water and salt solution pool. The practical takeaway is drainage. Slabs that shed water, joints that are sealed, and downspouts that do not empty onto the concrete last far longer. New concrete and de-icers have their own page: protecting a new driveway from de-icers.

What happens to pavers, patios and retaining walls?

Segmental paving and dry-laid walls move instead of cracking, which is both their strength and their weak point. When water sits in the base or behind a wall and freezes, it lifts, and the material does not always settle back where it was. Uneven pavers, a tipping wall cap or a gap opening at the edge of a patio are usually freeze-thaw and drainage problems rather than material problems.

Our guide to hardscaping that survives freeze-thaw covers base depth, drainage and material choices in detail, and snow damage to fences, walls and patios covers what plow piles and snow load add. When repairs are needed, our hardscaping and retaining wall crews rebuild on a base built for this climate.

How does freeze-thaw affect lawns, beds and plants?

The top layer of soil heaves. University of Wisconsin-Madison Extension explains that a lack of snow cover and fluctuating temperatures lead to freezing and thawing cycles of the upper soil, and that this soil heaving action damages the roots of perennials, bulbs, ground covers and strawberry plantings. It notes that snow is an excellent insulator of the soil and suggests winter mulching to protect plants from heaving.

Trees get a different freeze-thaw injury. University of Minnesota Extension describes sunscald on bark, usually on the south or southwest side of a tree, and winter browning on evergreens on the south, southwest and windward sides. The pattern of those sides points to sun and wind exposure, so look there first. Our guide to winter burn on evergreens covers the needle side, and our winter tree care and snow load guide covers young trunks.

Can freeze-thaw damage pipes and irrigation?

Yes, and these failures are the expensive ones. The Red Cross lists the pipes that freeze most often: those exposed to severe cold such as outdoor hose bibs, swimming pool supply lines and sprinkler lines, supply pipes in unheated areas like crawl spaces, attics and garages, and pipes against exterior walls with little or no insulation. Its advice includes draining sprinkler supply lines following the manufacturer's or installer's directions, removing and draining hoses, and closing inside valves to outdoor hose bibs while leaving the outside bib open to drain.

Irrigation backflow assemblies and valve boxes sit right at the surface and freeze first. If yours was not blown out, see our sprinkler blowout and winterization service and the guide to frozen backflow preventers and irrigation damage. Plumbing inside the house is a job for a licensed plumber.

Why does freeze-thaw make walkways so slippery?

Because every thaw makes water and every freeze turns it to ice. A drive that was cleared at noon can have a glaze across it by dusk where meltwater ran from a pile, a roof or a shaded edge. The roughly 130 crossing days a year are, in practice, 130 chances for a refreeze. A common cause is snow piled uphill of a walk, which is why where you pile snow matters so much.

The fix is partly timing and partly layout: clear to the surface before temperatures drop, keep meltwater from crossing walks, and treat the predictable ice spots on every visit. We go through the details in black ice on driveways and walks.

How do I spot freeze-thaw damage in spring?

Walk the property once the snow is gone and before the irrigation is turned back on. On concrete, look for surface flaking, crumbling at joints and new cracks. On pavers and walls, look for pavers that rock underfoot, joints that have opened, caps that have shifted and any lean in a wall. In beds, look for perennials pushed up out of the soil. On trees, look for split or discolored bark on the south and southwest sides.

Then check the water side: a hose bib that drips when shut off, a backflow assembly with a cracked body, wet spots in the lawn above an irrigation line. Note what you find with photos, because it makes the conversation with a contractor or an insurer easier. Our guide to yard cleanup after the snow melts puts this into a spring routine.

What can I do before winter to limit freeze-thaw damage?

Most of it is about water. Extend downspouts so they discharge away from walks, steps and slabs. Clear leaves from drains and gutters in the fall so meltwater has somewhere to go. Fill and seal open concrete cracks and joints in dry weather. Drain hoses, shut off hose bibs from inside and have the irrigation blown out. Mulch perennial beds once the ground has frozen. Mark the edges of drives and beds so snow is not piled where it will melt across a walk.

Our fall cleanup service handles leaves and drains before snow, and our snow and 24-hour storm services handle clearing and ice management through the winter. This guide is part of our Montana Winter Property Guide, and the full winter property prep checklist for Montana puts every step in order. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate for winter service in Missoula and the Bitterroot Valley.

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