Freeze-Thaw Cycling: What It Actually Does to Roofing Materials
Freeze-thaw damage is not really about cold. It is about water getting inside a material and expanding as it freezes — which is why some materials come apart and others simply do not care.
Every contractor in this province blames freeze-thaw for something. It is usually correct, and it is almost never explained. Freeze-thaw is not a vague synonym for a hard winter. It is a specific, mechanical process that requires liquid water inside a material and a temperature that crosses the freezing point in both directions. Understand those two requirements and you can predict, fairly accurately, which parts of a house will deteriorate and which parts will still look the same in thirty years.
A Freeze-Thaw Cycle Is Not a Weather Forecast
The temperature that matters is the temperature of the material, not the number on your phone. A dark south-facing slope can sit well above freezing in direct February sun while the air temperature never rises above zero, then drop below freezing the moment the sun leaves it. That surface cycled. The north slope of the same roof, in the shade all day, may sit frozen solid for a week without cycling once. This is why two planes of one roof age at completely different rates, and it is why any published count of freeze-thaw cycles for a town is close to meaningless for a specific building. What matters is how often the wet surfaces on your house cross zero, and that depends on orientation, shading, wind exposure, colour and how well the material holds water.
Water Expands When It Freezes, and That Is the Whole Mechanism
Nearly every liquid contracts as it solidifies. Water does the opposite: as it crystallizes it expands by roughly nine percent. In an open bucket that expansion simply pushes upward and nothing happens. Inside the pore structure of a material, there is nowhere for it to go. The ice presses outward against the pore walls, and it presses hard enough to open a microscopic crack. Repeat that and the crack widens. Damage happens where water is confined, which is why a saturated material freezing is destructive and a dry material freezing is not.
Porous Versus Non-Porous: The Distinction That Decides Everything
Once you accept that the water has to be inside the material, the list of vulnerable products writes itself. Anything that absorbs, wicks or holds water in an internal structure is exposed. Anything that sheds water at a solid surface is not.
- ✓Vulnerable, because they absorb water: concrete, mortar, brick, stone, clay and concrete tile, wood, oriented strand board and plywood, and the asphalt-saturated mat inside a shingle.
- ✓Effectively immune, because they have no pore structure to fill: steel, aluminum, zinc, copper, glass, cured factory-applied coatings and most solid plastics.
- ✓Vulnerable by geometry rather than by material: laps, seams, joints and crevices where liquid water is held between two non-porous surfaces. The material is fine, but the trapped ice still has to expand somewhere.
- ✓Special case: any horizontal surface that ponds. Standing water in a flat-bottomed eavestrough, a clogged downspout, a poorly sloped valley or a low spot in a walkway freezes as a solid block and pushes against whatever contains it.
Why the Damage Ratchets Instead of Reversing
The reason freeze-thaw is so effective is that it does not undo itself on the thaw. The crack it opens on Tuesday night is still open on Wednesday afternoon, only now it is slightly larger, so it holds slightly more water than it did before. The next freeze works on a bigger volume of water in a bigger void, and it opens the crack a little further. Each cycle is small and each cycle is permanent. Ten winters of that is why a mortar joint that looked perfect goes from sound to sandy in what feels like a single season — the damage was accumulating invisibly the whole time and only became visible when the surface finally spalled off.
The rate of freeze-thaw damage is set by how wet the material is when it freezes, not by how cold it gets. A cold, dry winter is gentle. A mild winter that hovers around zero with repeated rain, melt and refreeze is the hardest one on a building.
Where It Does Its Work on a Southern Ontario House
Once you know what to look for, the pattern of deterioration on an older home stops looking random.
- ✓The bottom edge of the roof. The eave and the first metre or so above it are where meltwater collects, refreezes and sits. It is the wettest part of the roof for the longest part of the year, and it fails first on almost every asphalt roof we tear off.
- ✓Shingle edges and keyways. Water wicks into the exposed cut edge of the mat, freezes, and lifts and cracks the tab.
- ✓Chimney crowns, mortar joints and parging. Cementitious material, fully exposed, usually with a crack already in it to let water in.
- ✓Eavestrough seams and end caps. A trough that does not drain fully holds a slug of ice all winter that works the seam open a little more each year.
- ✓Concrete steps, porches and walkways, especially where de-icing salt keeps the surface wet longer than it would otherwise be.
- ✓Wood fence pickets, rails and post tops, which soak up water end-grain first and split from the top down.
- ✓Post footings and anything shallow in the ground, which get lifted by frost rather than cracked by it.
Frost Heave Is the Same Physics, Underground
When soil freezes, water in the soil turns to ice and expands, and in silty soils it also draws additional water up toward the freezing front, forming ice lenses that grow and push the ground surface upward. Anything shallow gets carried up with it and does not come all the way back down when the ground thaws. That is why fence posts lean, gates stop latching in March, and patio slabs go out of level. The fix is to get the bearing surface below the depth at which the ground freezes at your location, or to use a footing designed to shed the heave. Frost depth is a local value set by your municipality — confirm the required footing depth with your building department rather than assuming a figure you read somewhere.
What Freeze-Thaw Actually Does to a Metal Roof
Internally, nothing. There is no pore structure for water to enter, so a steel or aluminum panel can be frozen wet and thawed indefinitely without any internal stress at all. What the same weather does impose on metal is thermal movement: a long panel is measurably longer on a July afternoon than on a January night, and it makes that trip twice a day in shoulder season. That is a real load, and it is the reason a properly built metal roof uses concealed clips that let the panel float, allows for expansion at the panel ends, and avoids pinning the panel rigidly at both ends. It is also why exposed-fastener systems need attention over time — the fastener holds a hole in the panel that the panel is trying to slide past, and the washer under the head takes that abuse cycle after cycle. Metal solves the freeze-thaw problem outright; it replaces it with a movement problem that good detailing handles.
Frequently Asked Questions
How many freeze-thaw cycles does Southern Ontario get in a year?
There is no single honest answer, and you should be sceptical of anyone who gives you a confident number. Cycle counts depend on whether you are measuring air temperature, ground temperature or the temperature of a specific surface, and a shaded north wall and a sunlit south roof on the same house will produce wildly different counts. What is reliably true is that our climate spends a great deal of the year near the freezing point rather than well below it, and near the freezing point is where the damage happens.
Does a metal roof suffer freeze-thaw damage?
Not in the way porous materials do. Steel and aluminum absorb no water, so there is no internal expansion and no cracking or spalling. What they do experience is thermal expansion and contraction, which is a movement issue rather than a deterioration issue, and it is handled through the clip and fastening design rather than through the material itself.
Why does my south-facing slope look worse than my north slope?
It gets far more ultraviolet exposure and far more thermal cycling, and in winter it repeatedly melts and refreezes on days the north slope stays frozen through. On asphalt those two mechanisms compound: the sun stiffens the asphalt binder, and the cycling cracks it. On a north slope the more common problem is moss and algae, because it stays damp and shaded.
Is a mild winter easier on my house than a cold one?
Not necessarily. A steady cold winter locks water up as ice and leaves it there. A mild winter with repeated thaws, rain on snow and refreezing is much harder on masonry, shingles and eavestroughs, and it is the classic ice damming winter as well.
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