by Bohdan Rewko (Bo of T.O.)

Created on: January 12, 2010   Last Updated: February 06, 2010

Ice dams can occur on shingled sloped roofs where there is significant snowfall and blanket accumulation. Whether the heat source is the sun or building interior, it will eventually start to melt snow on the roof. When it refreezes, it can create ice buildup on the lower portion of the roof, known as an ice dam.

A certain degree of ice damming can be tolerated if a roof is designed properly, but roof damage will occur if the the condition is allowed to persist. This is because melting ice at a dam location will make its way into the building structure over joints of shingles or over flashing in a roofing system.

Roof damage caused by ice dams can be avoided in all three phases of a building's life: design, construction and maintenance. During the design phase a sloped roof can be designed with a substantial overhang, thereby creating a cool zone. The less thawing that occurs at the edge, the less chance there is for ice build up.

Wherever a dam occurs, flashing extending at least three feet above the overhang underneath shingles will keep ice from leaking into the building once it starts to melt. The shallower the roof pitch, the wider the flashing should be. Flashing material should  be self-sealing around nail penetrations.

Roof slope should also be a design consideration in regions where there is significant snow accumulation. Steeper pitches tend to drain quicker and keep roofs from collecting snow as they receive more direct sunlight when the sun is low in winter months.

Finally, sloped roofs should be designed to minimize heat buildup underneath shingles and allow for the free movement of air. This is where larger overhangs are again beneficial as they provide more vent space underneath. At the upper end, continuous ridge vents are preferable over intermittent venting units as they do a better job of evenly venting roofs. Of course, insulation below vented spaces should be adequate to prevent any heat buildup that cannot be readily dissipated.

Roof design is critical, but during the construction phase, steps need to be taken to ensure the integrity of that design. For example, where narrowing of ventilated space occurs at the junction of the exterior wall and roof rafters, baffles need to be properly installed or measures taken to provide unobstructed airflow between the various vented spaces under the roof. If dead zones exist, the buildup of warm air can create uneven roof melt that contributes to ice damming. The minimal allowable air space required by municipal building departments and building codes for venting purposes is usually two and a half inches, but more is always better.

Sometimes, where snow conditions or building design are such that ice dams become inevitable, the only solution to the problem may be to properly maintain the roof. Roof heating cables are available at hardware stores and should be considered in locations where snow damming is known to occur. Also clearing roofs of snow with a broom or other device can at times be the only remedy to prevent damage from ice dams.

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