Two types of attic water damage are common in cold climates: ice dams and condensation of water vapor on cold surfaces in the attic.
Ice dams sometimes occur on sloping roofs in climates with freezing temperatures. When the temperature in your attic is above freezing, it causes snow on the roof to melt and run down the sloping roof. When the snowmelt runs down the roof and hits the colder eaves, it refreezes.
If this cycle repeats over several days, the freezing snowmelt builds up and forms a dam of ice, behind which water ponds. The ponding water can back up under the roof covering and leak into the attic or along exterior walls.
The right weather conditions for ice dams is usually when outside air temperatures are in the low 20s (°F) for several days with several inches of snow on the roof.
Research shows keeping the attic air temperature below freezing when the outside air temperature is in the low 20s can reduce the occurrence of ice dams. Research has shown sun exposure in the winter has little effect on attic air temperature. Warm air from living spaces below penetrating into the attic is usually the culprit in the formation of ice dams.
Condensation of water vapor on cold surfaces in attics can cause attic wood products to rot, which can lead to costly repairs. Condensation typically occurs when warm, moist air migrates into the attic from living spaces below. Research indicates unusually high humidity levels in the home's living spaces is strongly associated with attic condensation problems.
Building codes have some requirements that attempt to prevent the problems of ice dams and attic condensation. But codes do not address all the issues, and many houses are built without following building codes. It is the builder or designer's job to understand the relationship of humidity and air movement when designing and constructing the house so these problems don't occur.
The following are short-term fixes. In order to avoid the same problem in the future, a qualified contractor should assess the amount of attic insulation and attic ventilation (especially around where the ice dam formed), and look for holes in the air barrier between the living spaces and the attic.