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Ice damming is a roofing problem unique to northern climates in which the low pitched roofs form ice dams and trap water causing water damage to the roof sheathing and seepage into the building. There are several conditions that has to exist before ice damming occur. Freezing temperatures as already mentioned, a low pitched roof, and a worm attic. When snow falls and accumulates on the roof it may sit there for days. During the day, temperatures may rise above freezing and with the help of the sun, melt the snow which accumulates at the eaves of the roof. At night, temperatures drop below freezing and the melted snow forms an ice dam at the eaves of the roof. As the cycle repeats, the melted snow, now water, enters the roof sheathing. A warm attic accelerates the process.
What can home owners do to prevent ice dams? The atmospheric temperature cannot be controlled but factors around the house can be. Changing the pitch of the roof is not feasible. What is controllable is the insulation and leaks in the attic. Keeping a cold attic is key. Seal the attic leaks and add more insulation to keep the attic cold in the winter are critical steps to preventing ice damming.
Additional measures to help address ice damming include adding eave protection, waterproofing membrane, along the first 18-24 inches of the roof from the eaves. Another anti ice dam measure is the use of heating cables along the eaves. Often, home inspectors view heating cables as an indicator that the house has had ice damming issues in the past. Another indicator of past ice damming issues is pike ax damage marks on the shingles along the eaves. Venting from soffit vents to the roof vents helps keep the attic cold.
The damage ice dams can have on a house can be extensive and mysterious as the water seeps through the roofing shingles and sheathing, and may run along framing members. If undetected, damages can include moisture damage to drywall, ceiling, flooring and creating of an environment friendly to mold.