Snow is an excellent insulator. Somewhere between the inside ceiling and the top of the snow on the roof the temperature goes from room temp to freezing. With enough snow insulating the roof the snow against the roof rises above freezing, melts, and the liquid water trickles down the roof until it runs into cold air. Nasty ice dams are the result. Even well-insulated roofs can have ice dams with enough snow on top. I don't live in Anchorage any longer but always hated having to fight the ice dams on my roof. An ice dam 10" high is bad juju.
