Ice dams form when snow lying against the roof is melted by attic warmth from below rather than from the sun’s warmth from above. If water on the roof cannot drip off the eave because the gutter is blocked by leaves, ice, or snow, the water may re-freeze and push water up under the roof shingles and into the sheathing, soffit, and wall cavity. When ice dams form, water is forced inside and can flow down the interior walls, damaging decorative finishes. Plastic sheeting placed under the attic eaves may catch water and minimize interior damage. Ice dams often form on the north slope of a roof since this slope receives limited sunlight during the winter. Clearing an ice dam is difficult and dangerous as it requires accessing the ice dam and chipping a hole through the overlying blockage to let the water out.
There are several ways to prevent or lessen the occurrence of ice dams. Using a special long rake, snow can be raked off the eaves after each major snowfall to keep gutters free of overlying ice and snow. Alternatively, eave flashing, such as a course of roll roofing or rubber membrane ice shield installed under the roofing shingles, can prevent water infiltrating the roof sheathing. Finally, unheated portions of the attic can be air-sealed and then vented to keep household heat from warming the underside of the roof; opening attic windows or adding soffit vents, or vents in the gable ends of the house may be needed to cool the attic sufficiently.