Seattle is not an ice dam region, and most national advice about them does not apply here. But we do get freeze events, they do produce ice dams occasionally, and when they happen they are almost always telling you something specific about your attic. Understanding the mechanism matters more than preparing for the event.
The process requires three conditions together: snow on the roof, a roof surface warm enough in its upper portion to melt that snow from below, and eaves cold enough to refreeze the meltwater.
Heat escaping from the house into the attic warms the underside of the deck across the main slope. Snow in contact with that warmed surface melts and runs down as liquid water. When it reaches the eave — which overhangs the exterior wall and therefore has outdoor temperature both above and below it — there is no heat, and the water refreezes.
That ice accumulates into a ridge along the eave. Subsequent meltwater arrives and pools behind it, and pooled standing water finds its way under shingle courses, because shingles are designed to shed flowing water down a slope, not to resist standing water sitting on them.
This is why the topic belongs in a ventilation discussion rather than a snow discussion.
An ice dam cannot form on a roof whose upper surface is the same temperature as its eaves, because nothing melts. A well-sealed, well-insulated, properly ventilated attic keeps the entire deck close to outdoor temperature, and snow simply sits there until it sublimates or slides.
So an ice dam is direct visual evidence that heat is escaping from the living space into the attic and warming the deck. The same heat loss is raising your heating bill, and in Seattle the same air leakage carrying that heat is carrying moisture that condenses on the sheathing — which is the far more common and more damaging local problem.
Icicles hanging from the eaves are the mild version of the same signal. They look picturesque and they are a diagnostic.
Our winters are mild, we get limited snow in the lowlands, and snow that does fall often does not persist. Most winters pass with no ice dam anywhere.
But we do get periodic significant snow events, occasional sustained cold snaps, and elevation variation across the region — higher neighbourhoods and areas east and north of the city see more. When a snowfall is followed by a cold clear spell, the conditions exist, and homes with poor attic performance produce dams.
The consequence of rarity is that many Seattle roofs are not detailed for it. Self-adhered ice-and-water membrane at the eaves is not required here the way it is in genuinely cold climates, so it is frequently omitted — which means that when a dam does form, there is no secondary defence beneath the shingles.
Do not chip, hack, or hammer at the ice. This is how roofs get destroyed. Chipping breaks shingles, gouges the deck, and does more damage than the water would have.
Do not use a torch or open flame. Obvious, and it still happens.
Do not use rock salt directly on the roof. It corrodes flashings and fasteners and damages plantings below.
Do consider a roof rake, from the ground. Removing snow from the lower few feet of the roof eliminates the material that feeds the dam. Use a proper roof rake with a long handle, work from the ground, and never from a ladder in icy conditions — and be aware that raking can damage shingles if done aggressively.
Do manage the interior. If water is entering, contain it, protect belongings, and photograph everything before cleanup.
Do call a professional for significant dams. Steam removal is the correct method for serious ice — low-pressure steam melts ice without damaging roofing. It is not something to improvise.
Since the cause is heat loss, the fix is the same sequence that addresses condensation, which is the more pressing Seattle problem anyway.
Air seal the ceiling plane so heat and moisture stop entering the attic. Ensure insulation is adequate and, critically, that it is not blocking soffit intake at the eaves. Restore balanced ventilation so the attic runs close to outdoor temperature. Confirm every bath and dryer duct terminates outside rather than into the attic, since those deliver both heat and moisture directly.
And at re-roof time, install self-adhered membrane at the eaves and in the valleys. It is inexpensive when the roof is already open, it is not required by code here, and it is the difference between an occasional ice event being a nuisance and being a ceiling repair.
Heat cables along the eaves are sometimes proposed. They manage the symptom, consume electricity indefinitely, have a limited service life, and do nothing about the heat loss causing the problem. They are a last resort for a roof geometry that genuinely cannot be fixed any other way, not a first response.
This post is part of our in-depth coverage of moss removal & gutter care in Seattle.
Gutters, Downspouts & Drainage → Moss Removal & Gutter Care →
Occasionally, when snow is followed by a cold clear spell. More importantly, an ice dam is evidence that heat is escaping into your attic and warming the deck - the same heat loss and air leakage that causes the far more common local problem of attic condensation.
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