I’m here to regale you with tales about ice damming because lots of us spend time dealing with it. Ice damming occurs when the underside of the roof deck is above freezing and the outside temperature is below freezing, and there’s snow on the roof.
This is important to understand: You need a warm roof deck when it’s cold outside and you need snow. If you don’t have snow, ice damming is not a problem. We don’t have many ice damming problems in the Arctic because we don’t have much snow in the Arctic. You might think, “Wait, there’s snow all over the Arctic!” Yes, but it’s been there a really, really long time. More snow falls in Syracuse, New York, than in Barrow, Alaska. By a factor of 10—and that’s a big number. If somebody multiplied your salary by 10, you’d notice it. Anyway, you need snow.
Let’s assume we have snow. How much snow? Four or five inches of snow on the roof has got my attention. That’s prime ice damming conditions. To get that much snow on the roof, you typically need more than 30 pounds per foot of snow load on the ground in the wintertime. Most people know they live in an ice dam location when they notice their neighbors have ice dams. That’s a pretty good indication: You should worry about ice dams when there are ice dams.
What happens when you start getting an ice dam? The snow melts. In the liquid phase, the water is wicked upward into the snow. The water is actually pulled away from the roof deck. The ice layer that occurs on the roof is several millimeters above the shingles. The liquid water, before it freezes, has…