Ice Dams in the Cascade Foothills: Cause and Prevention

What causes an ice dam?

Heat. Warm air from the living space leaks into the attic through ceiling penetrations, recessed lights, bath fans, top plates, and hatches, and it warms the underside of the roof deck. Snow lying on that warmed section melts from below, even while the air outside stays well under freezing. The meltwater runs down the roof under the snowpack until it reaches the eave. The eave overhangs the exterior wall, so there is no heated space beneath it and it stays at outdoor temperature. The water refreezes there and builds a ridge of ice. Each subsequent cycle adds to that ridge, and eventually meltwater ponds behind it. Standing water on a roof does what standing water does: it works sideways and upward through laps, under shingles or panels, past nail holes and around flashings, and appears on the ceiling below. The damage is water damage, and it is happening above the freeze line, not at the icicles everyone photographs.

Why do foothills homes get ice dams when lowland homes rarely do?

Because ice dams need a specific set of conditions to line up: enough snow to insulate the roof surface, air temperatures cold enough to refreeze meltwater at the eave, and a heated attic underneath. The Cascade foothills supply all three regularly. North Bend, Snoqualmie, Enumclaw, Buckley, Granite Falls, Darrington, Morton, Randle and Packwood sit at elevations where snow accumulates and lingers through freeze and thaw cycles. Lowland Seattle, Tacoma and Everett get snow that usually melts before the cycle can repeat enough times to build a dam. House construction matters as much as geography. Older foothills homes often have shallow attics, insulation that has settled or been disturbed, blocked or absent soffit vents, and duct runs or recessed lighting in the ceiling plane. Two neighboring houses under the same snow can behave completely differently, and the difference is usually in the attic.

Does a metal roof prevent ice dams?

No, and it is worth being blunt about that. A metal roof does not stop heat escaping from the house, and heat loss is the mechanism. If the attic is warm, snow will melt from below and refreeze at the cold eave regardless of what the roof is covered with. What a metal roof does change is exposure time and the number of vulnerable points. A smooth interlocked surface sheds snow more readily, so the snowpack that feeds the melt cycle is often present for less time. Fewer accumulation cycles means fewer dam-building cycles. It also changes what water finds if it does back up. That is a real difference and it is covered in the last section. But anyone selling a roof covering as an ice dam cure is describing an outcome the material cannot deliver on its own.

How do you actually stop ice dams?

Keep the roof deck cold, which means stopping heat from reaching it. The work happens in the attic, in this order. First, air sealing. Seal the leaks in the ceiling plane: recessed light housings, plumbing and wiring penetrations, top plates, chases, the attic hatch and any ductwork joints. Warm air moving through a gap carries far more heat than the same area of missing insulation, so sealing comes before adding depth. Second, insulation. Bring attic insulation up to a consistent depth across the whole ceiling plane, including out over the top plate at the eave, without blocking airflow. Baffles at the eave keep the insulation clear of the vent path. Third, ventilation. A continuous intake at the soffit and exhaust at the ridge lets outdoor air wash the underside of the deck and carry away whatever heat still gets through, keeping the deck close to outdoor temperature. Blocked soffits are one of the most common findings on foothills homes with a dam history. Finally, make sure bath and kitchen fans and dryer vents discharge outdoors, not into the attic.

What does an interlocking aluminum roof do when ice forms?

It reduces the number of places backed-up water can find. Interlock panels engage on all four sides and are secured with concealed fasteners, so there is no exposed screw head and no rubber gasket sitting in the weather to be worked by repeated freeze and thaw cycles. Exposed-fastener roofs concentrate risk at thousands of gasketed penetrations, and those gaskets age faster where water freezes, expands and thaws against them. The aluminum itself is dimensionally stable and does not absorb water, and it will not rust where meltwater sits. Ice-and-water membrane at the eaves and in the valleys remains part of a correct assembly in snow country, and it is the layer that protects the deck when a dam does form. None of that substitutes for fixing the heat loss. Treat the panel system as the thing that limits damage during the years it takes to get around to attic work, and treat air sealing, insulation and ventilation as the cure. Interlock has completed 3,223 Washington installations since 1997, and on foothills homes the attic conversation belongs in the estimate alongside the roof.

Common questions