An Ice Dam is a hump of ice that forms at the edge of a roof under certain wintertime conditions. An ice dam can damage both your roof and the inside of your home. It will put gutters and downspouts at risk too.
Ice Dams are a common sight in winter. There are several things you can do to avoid getting an ice dam or to reduce the risk of damage after one has formed, but there’s really only one cure: a combination of better sealing, insulation, and venting in the attic and eaves.
An ice dam forms when the roof over the attic gets warm enough to melt the underside of the layer of snow on the roof. The water trickles down between the layer of snow and the shingles until it reaches the eave of the roof, which stays cold because it extends beyond the side of the house. There, the water freezes, gradually growing into a mound of ice.
The flatter the pitch of the roof, the easier it is for an ice dam to get a grip. Gutters at the eaves can also trap snow and ice. If snow and ice build up high enough in the gutter, it can provide a foundation for an ice dam.
When an ice dam gets big enough, melted water backs up behind it and seeps underneath the shingles. Eventually, it will drip into the insulation and down into the ceilings and exterior walls beneath the eave, ruining sheetrock and paint. If the ice dam breaks free, it can pull shingles and gutters off with it, and it will damage anything it falls on: shrubs, windowsills, cars, pets, and people. If the roof sheathing stays wet, it can form mildew and start to rot.
There are two avenues of attack: dealing with an existing ice dam and preventing one in the first place.
NOTE: A good trough-maker is a tube of cloth (a leg from an old pair of panty hose works well). Fill it with calcium chloride, tie off the top, and lay it vertically across the ice dam. It will slowly melt its way down through the dam, clearing a path for the underlying water to flow free.
You can scrape snow from the roof whenever it falls, using a snow rake from below or a broom or plastic shovel from above. BE CAREFUL: The first method can bury you in snow, while the second can send you slipping off the roof. Hire someone who knows how to use a safety line.
You can replace your shingle roof with standing seam or other metal roof. Or you can replace the bottom three feet or so of your shingle roof with a wide metal drip edge. Whatever you do, install a water-repellant membrane under any new roofing.
NOTE: If your roof is not very steep, an ice dam can still form on metal roofing and drip edges.
All of these methods treat the symptoms, not the underlying problem, which is the warm roof, caused by poor insulation and venting of the space under the roof. We have found that the only way to cure an ice dam – and prevent one in the first place – is to:
The diagram on this page gives a good idea of what needs to be sealed, insulated and ventilated:
The ice dam cure will also cure a significant loss of heat from your home. It’s a win-win situation: The money you save on fuel bills will pay for the work to protect yourself from ice dams, and your tighter home will feel more comfortable during these cold winters.