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overhang. As temperatures fluctuate, the melted snow freezes, causing the ice dam to progressively grow larger (Figure 100). When a thaw occurs, this ridge of ice causes a dam that prevents melt water from escaping down the roof. The trapped water backs up under shingles and could leak into the attic, ceiling and wall. To prevent ice damming from occurring, do pay attention to air-sealing at the ceiling, the
ceiling/wall junction and at all penetrations through the ceiling air barrier during construction. Provide sufficient ceiling insulation to comply with building code requirements for your location, and ensure that the attic space is vented to the exterior at the eaves. Skylights should be situated away from the areas where ice damming is likely. In addition, eave protection consisting of either type S
100 Eave protection No eave protection
snow trapped water backs up through the roofing membrane thin ice slab under snow melted snow running down underside of sheathing heat loss or outdoor temperature melts snow ice
ice in eavestrough inside face of wall insulation Eave protection
minimum 900 mm (36 in.) wide (measured along the slope) eave protection extending at least 300 mm (12 in.) (measured horizontally) beyond inside face of the wall studs
eave protection directs water off the roof