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. 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.
HOW DO ICE DAMS FORM?
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.
WHAT DAMAGE DO ICE DAMS CAUSE?
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.
HOW CAN YOU DEAL WITH AN ICE DAM?
There are two avenues of attack: dealing with an existing ice dam and preventing one in the first place.
DEALING WITH EXISTING ICE DAMS
1. Remove the ice dam by breaking it free in small chucks. Do NOT use an ax or other sharp tool! You’ll cut through the shingles. Instead, tap lightly with a blunt mallet. This is slow, dangerous work, so hire someone experienced at roofing. Even if you do it safely, the chunks of ice can take pieces of shingle with them.
2. Clear out gutters and downspouts. Again, this is ladder work and an easy way to damage either plastic or metal gutters and spouts.
3. Melt troughs through the ice dam with calcium chloride ice melter. Do NOT use rock salt! It will damage paint, metals, and plants beneath the eave and wherever the salty water drains.
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.
PREVENTING ICE DAMS
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.