I will respectfully disagree with you.
Understand and agree. But, the primary cause of ice dams is heat escaping from the house. When the sun contributes, it does so as a secondary factor. The primary factor being poor roof design, i.e., allowing form to lead function. DC
I strongly disagree with Mr. DC. The primary cause is the sun. Roof design contributes as does any heat loss or air leaks from the house structure.
Decades ago houses had very poor insulation in the attics if any. When my parents' house (NE Wisc, 25 miles from Green Bay) was built in 1955 my Dad said the accepted standard of the day was 4" of fiberglass between the ceiling joists. But he put in 6" fiberglass. Very little by today's standards.
Houses built in the last 30 or so years have lots of insulation in the attics, and still ice dams can occur.
If you have snow on the roof, eventually it will melt. Even with a perfectly insulated attic with
zero heat loss, the snow will eventually melt.
There are three different outside temperature situations:
1. Below say 25*F. The snow, the roof, the attic will likely be cold enough the snow won't melt. The temp is probably cold enough that any sun on the roof won't raise the roof temp to 33*F. So no melt, no ice dam.
2. Above say 45*F. Let say it snow 6", its cold (less than 25*F, the snow probably won't melt). If the temp suddenly goes to 45*F or more, the snow will melt. Its warm enough that the melt water will be running on portions of the roof, the gutters, and the downspouts that are above 32*F and hence there will be no freezing, no ice dams.
3. Between say 25*F and below 33*F. While the ambient temperature won't support the 6" of snow melting, during the daytime and especially with any sunny conditions the surface of the south side of the roof will quickly warm up to over 32*F, even the air space in the attic will warm up (in spite of a perfectly insulated attic) which will warm up the north side of the roof structure also melting the snow on the north side too.
So the snow starts to melt. As the melt water goes down the roof, it eventually gets to the gutters (usually aluminum) which are light colored and don't absorb the sun light and get warm. They are under 32*F, the melt water cools, and ice forms. As the ice builds the water starts to puddle on the bottom of the roof (overhang) next to the gutter. This area is often cold because there is no warm air trapped underneath it like there is attic. Ice will continue to built up on the overhang.
Then night comes. Temps drop to low 20's. The ice gets really cold. The next day, with the ice mass really, really cold ... any melt water that hits it quickly freezes.
This is usually worse on the north side of the house versus the south side because the south side has the benefit of sunlight which will tend to warm up the house structure which keeps the melt water liquid versus the cold north side.
