Ice Dams in Idaho

The weather half of the condition is present at McCall on about 47.3 days in a typical winter, concentrated in December. In the winters that have one, the first spell of 3 or more of those days in a row lands around Dec 1 and the last around Feb 16. Those are counts from NOAA daily records for 1991-2020, not a forecast, and they describe the weather rather than any particular roof.

Heat escaping from the house. NDSU Extension puts it in one sentence: "Ice dams occur when escaping heat melts the snow on the roof and the water freezes when it reaches the cooler surface near the eaves." University of Minnesota Extension, whose ice dam page is the fullest published account of the mechanism, adds the temperature condition behind that: snow has to be lying on the roof while the upper roof surface runs above freezing and the eave runs below it, sustained over days rather than for an hour, which also requires the outdoor air to be below freezing. Its page is linked from this one.

McCall leads this page's stations at 47.3 setup days a winter; Boise sits at the other end with 7.6. The mountain north and east are ice dam country and the Snake River Plain around Boise is not. McCall runs six times Boise's setup count and more than ten times its melt-refreeze count.

Not always, but agencies read a row of them as a signal about the building rather than about the storm. NDSU Extension writes that ice dams "indicate that your house is leaking heat and does not have adequate attic ventilation," and the University of Alaska Fairbanks Cooperative Extension Service calls ice dams at the eaves "another sign that warm air is leaking into the roof cavity or that it is inadequately insulated."

By changing the building, not the roof surface. NDSU Extension: "The best way to reduce the potential for ice dams is by providing adequate attic insulation and ventilation that keep the roof cold and to minimize any air leaks from the living space into the attic." University of Minnesota Extension puts an order on those two: seal the ceiling first so warm indoor air cannot reach the attic, then add insulation depth, because insulation laid over open leaks still lets the air through. It also says the work belongs to weatherization contractors and that the contractor should be measuring the ceiling with a blower door test rather than guessing. Both pages are linked from this one.

That is the part that hurts people, and it is work for a professional. OSHA's winter weather guidance says employers should protect workers "by using snow removal methods that do not involve workers going on roofs, when and where possible," warns that "snow covered rooftops can hide hazards such as skylights that workers can fall through," and notes that "electrical hazards may also exist from overhead power lines or snow removal equipment." A surface loaded with snow, OSHA adds, "must be inspected by a competent person to determine if it is structurally safe." Removal is work for a professional.

It stops being an ice problem and becomes a moisture one. NDSU Extension describes the damage as water behind the ice entering the house, making insulation ineffective, rotting structural wood, staining wall coverings and "creating conditions conducive for mold growth." EPA is the federal authority on indoor mold and its guidance is that "the key to mold control is moisture control" and that "it is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth."

NOAA daily station observations. No agency publishes an ice dam climatology, so this one is derived from the daily record and labelled as such: for every day of the 30 winters from 1991-92 through 2020-21, the station is checked for an inch or more of snow on the ground and a daily high at or below 32 F, and those days, the runs of them, and the separate melt-and-refreeze days are counted. Winters missing more than 10 percent of their Nov to Mar reports are dropped rather than guessed, and a station needs 20 clean winters before any page may quote it.