Ice Dams happen when the roof deck is warm enough to melt snow, and the outside temperature is cold enough to freeze that melted snow
In this Building Science Fundamentals excerpt, Dr. Joe Lstiburek discusses the R-value of snow and how it complicates ice dam avoidance.
Video transcript:
All right, this is a phenomenal graph. It comes from the Cold Regions Research Lab from the Army Corps of Engineers in Hanover, New Hampshire. This was provided by Wayne Tobias who, until he passed away, was of equal status and stature in roofing as René Dupont. Anyway, this is the R-value of snow.
R-value of snow explained
The R-value of snow, based on density, is between R1 and R2 per inch. So remember this: the R-value of snow is between R1 and R2 per inch. Now, Canadians know this because we all live in igloos, right?
So if I've got, you know, 10 inches of snow on my roof, I'm going to have between R10 and R20 on the top of the roof shingles. What does that do to the temperature of the OSB? It warms it up, and if it goes above freezing, I'm going to get an ice dam.
The higher the amount of snow on your roof, the more likely you're going to get an ice dam.