It often gets very hot on clear days in the mountains and very cold on clear nights. Melt water produced during such days may refreeze on eaves because of the drastic temperature drop from day to night.
Ice accumulation at the eave can happen quickly on a cold night if there is snow on the roof and excessive heat loss from the building into the roof system. Ice dams have long icicles hanging down from them. Water ponds up slope from the ice dam. This water is insulated from the cold by the snow above, and it receives heat from the building below. This heat often prevents the water from freezing behind the ice dam. The water is located above an exterior wall. In high altitude areas, the water behind the ice dam will freeze at night if the heat loss from the building is low.
Ice dams that accumulate over a longer period of time from minor heat loss from the building or the effects of high altitude look more like the ice dam shown in the next illustration. Water freezes at the outer edge of the ice dam and backs up the additional water flowing down the roof. When the water overflows the buildup at the edge, additional water freezes and builds the ice dam a little higher and farther out.
Water ponded behind an ice dam penetrates at head and side laps. The water often turns to ice beneath the roofing material on the cold eaves. As ice forms, it expands;
