Ice dams in Seattle form when warm attics melt snow that refreezes at eaves, forcing water under shingles

https://hectorumwp775.capitaljays.com/posts/how-to-stop-moss-and-algae-on-seattle-roofs-2

Ice dams in Seattle form when warm attics melt snow that refreezes at eaves, forcing water under shingles. Continuous soffit-to-ridge ventilation, air sealing, and eave ice-and-water shield stop hidden winter leaks.