Ice Dams

What Causes Ice Dams on Your Roof?

Distressed by ice dams that have cause so much water damage during this especially severe winter, homeowners want more than ever to know exactly what causes them and what can be done to prevent them.

The Cause

The problem begins with snow on your roof. Daytime outside temperatures melts some of it, and heat radiating from inside your house contributes to the melting. So does sunlight— when strong enough it melts the snow from above, even if the outside temperature is below freezing.

The cycle repeats: melting water flows into the gutters during the day and then freezes at night. It doesn’t take long for the ice buildup to reach the level of the eaves. As more daytime melting reaches the face of the developing dam and freezes, the dam creeps up the roof. (Clogged gutters and the presence of gutter helmets accelerate the process).

Why don’t higher daytime temperatures or heat radiating from inside the house melt the ice as it moves up from the gutters? First: the eaves receive no heat from your living space. Second: daytime temperatures are not strong enough, alone, to melt the ice that has formed on them.

Now you begin to notice the effects. Snow higher up on the roof continues to melt during the day thanks both to the sun and the heat escaping from your living space below.  But because the water is melting over the warmer part of the roof, it doesn’t freeze. Instead, it backs up against the ice dam and then drains into cracks and other openings in your roof covering—thousands of nail holes for example. The water then flows into your attic, down your exterior walls and through your ceilings, winding up in your living space.

Note that un-drained or inadequately drained “valleys” in many roofs, as well as obstructions to water flow that are often built into roofs, provide conditions for the development of additional ice jams. I’ll speak about these as well as ice dam prevention in Part II of this bulletin.

Lonergan Construction

508-875-0052

www.lonerganconstruction.com.