Chinooks, freeze-thaw and your roof

The Canadian Encyclopedia defines chinooks as warm, dry, gusty westerly winds that blow down the eastern slopes of the Rocky Mountains onto the prairies. In Canada, the chinook belt sits almost entirely in southern and central Alberta.

The Canadian Encyclopedia gives two reasons the air is warm. First, the chinook pushes out cold arctic air and replaces it with much milder Pacific air. Second, the air gives off heat as it drops its moisture on the western slopes, then warms further as it sinks down the eastern side and gets compressed. That's also why it arrives dry.

Often you'll see a chinook arch before you feel it: a straight-edged band of clear sky under a blanket of cloud, sitting over the mountains to the west.

The warm-up at the start of a chinook can be abrupt. The Canadian Encyclopedia reports a documented rise of 27°C in two minutes at the onset of one event, and maximum daily temperature anomalies ranging from +13°C in the northwest of the chinook belt to +25°C in the southeast.

The wind matters as much as the warmth. The same source puts chinook wind speeds at 16 to 60 km/h, gusting to 100 km/h. That means the same event that softens snow and ice on your roof can also pull at shingle edges.

Water expands when it freezes. On a roof, meltwater finds its way into tiny cracks, lifted edges and gaps around nails. When it freezes, it pushes those openings a little wider. Next thaw, a little more water gets in. One cycle does nothing you'd notice. Hundreds of cycles over many winters is how small defects become leaks.

The seal between shingles is part of the story. According to the Asphalt Roofing Manufacturers Association (ARMA), most asphalt shingles use a thermally activated sealant, meaning the sun's heat melts the strip and bonds each shingle to the one below. ARMA also warns that shingles can crack or even break apart when handled in cold weather. So a shingle that was installed in the cold and never fully sealed, or an older shingle that's gone brittle, is exactly the one a gusty chinook will lift or snap.

Flashing is the sheet metal that seals the joints where a roof meets a wall, a chimney, a skylight or a vent. Metal, wood framing, asphalt and caulking all expand and shrink by different amounts as the temperature changes. Every big swing moves those materials against each other. That's called thermal cycling.

Over time, caulk and roofing cement that was used to seal flashing dries out and cracks, fasteners loosen, and pipe boots (the rubber collars around plumbing vents) split. Those details usually fail well before the shingles around them do.

An ice dam is a ridge of ice along the eaves that traps meltwater behind it. Building scientist John Straube's digest on ice dams explains the root cause: part of the roof is warm enough to melt snow, and the water runs down until it reaches a colder surface and refreezes. The eaves overhang the wall and sit over cold air, so that's where the ice builds.

The digest lists the common reasons the upper roof gets warm: too little insulation, warm air leaking into the space under the roof, heat sources such as poorly insulated ducts, and uneven snow depth combined with sun. Chinooks add another push, melting a lot of snow quickly and then turning cold before it drains. Water pooled behind the dam can back up under shingles and leak into the roof sheathing, walls and ceilings.

The long-term fix usually starts in the attic: better insulation and air sealing, with ventilation to help keep the roof deck cold. At the roof edge, a waterproof membrane called ice and water shield is the backup. Both are covered in more detail in ice dams in Calgary and underlayment and ice and water shield.