Why Do I Have Icicles Hanging From My Roof?
Icicles form when water melting from ice or snow on the rooftop drips through cold air. When the roof's surface temperature climbs above freezing, the snow layer above it melts. As water drips over the eaves, cold air freezes the water stream. Icicles grow when warm water flowing over the ice freezes in the updraft the icicle's relative warmth creates. Icicles on the eaves could signal ice dams on the roof and insulation problems. Does this Spark an idea?
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Normal Icing
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Icicles that form during thawing weather don't always indicate trouble on the roof. As weather warms, the dark surface of the rooftop heats in the sunlight and thaws the snow and ice built up on top. If the entire surface of the roof heats evenly, a layer of water runs unobstructed beneath the snow and over the eaves. Ice dams do not form in these conditions, but icicles do, because the roof warms before air temperature rises above freezing. A warming roof could cause snow and ice to break loose and slide over the edge. Knocking icicles loose reduces strain on eaves and gutters, but always stand at a safe distance to avoid any unexpected ice-falls.
Ice Dams
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Ice dams, or ridges of ice building up several feet from the edge of the eaves, form because the upper roof warms above freezing while the overhang past the eaves does not. As the snow melts, the runoff hits the cold section of roof and freezes again. Ice layers build up, forming a dam that could be several inches high. As runoff backs up behind the dam, the water seeps under shingles and leaks into the home's interior. Trying to break ice dams loose could damage shingles or bring heavy ice plummeting over the roof's edge. Special roof rakes remove snow built up behind ice dams, but not the dam itself.
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Ventilation
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By ventilating the attic correctly, the upper roof stays at the same temperature as the exposed eaves and no ice dams form. For every 300 square feet of ceiling area in the home, you'll need 1 square foot of open ventilation to the outside air, says engineer Brian Holmes of the University of Wisconsin Extension. Weatherproof vents shield the openings from rain without blocking airflow. The best airflow system uses screened vents in the soffits below the eaves and weatherproof vents near the peak of the roof.
Insulation
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Air leaks from the living area into the attic and poor insulation in ceilings could also contribute to ice dam problems. If enough warmth escapes to heat the upper roof, ice dams could form despite ventilation in the attic. Drain vent stacks and chimneys may be weatherproofed at the roof, but not sealed at the attic level. Hot air rises through any gaps. Stove pipes and chimneys require fireproof sealants and insulation, but drain stacks seal easily with insulating foam. To prevent heat loss from living area ceilings, install attic insulation with a rating of R38, to a depth of 1 foot.
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