Ice Dams and Gutters: What Causes Them and What Helps

How a dam forms

The Extension explains it with roof temperatures: for a dam to form there must be snow on the roof while the upper roof surface is above 32 degrees and the lower edge is below. Snow melts high, runs down, refreezes at the edge, and the ridge of ice grows. Water then backs up behind it and leaks through the roof covering. In many homes, it says, air leakage is the major source of the heat.

What gutters have to do with it

• Gutters full of leaves freeze into a solid block that the dam builds on.
• Ice-filled gutters are heavy and pull hangers out of the fascia.
• Downspouts frozen shut send meltwater over the gutter onto walks, where it refreezes.

Prevention, in order of value

1. Air seal and insulate the attic; PNNL's Building America guidance details how.
2. Keep attic ventilation clear.
3. Clean gutters and valleys in late fall; see ice dam prevention.
4. Rake snow off the lower roof after big storms, from the ground.

Removal costs (HomeGuide)

Salt or melt tablets $200 to $300; chipping $800 to $1,800; pressure washer $400 to $2,000; steam $800 to $2,400, the most effective method.