What is an Ice Dam
An ice dam is not something frozen beavers create during winter months. It is ice buildup on the edge of roofs, that can backup water on the roof forcing the water inside the house. These ice dams are common and we see them all the time here in Minnesota.
An ice dam is formed by the melting of snow on the roof. That melting snow then freezes towards the edge of the roof, and over time, will build up into an ice dam. The main cause of an ice dam is the melting of the snow. So we need to prevent the melting of the snow on the roof. This is usually caused by heat loss from the house, escaping into the attic area, which then warms the roof. This warm roof then melts the snow. When the melted water hits the edge, presto, instant ice dam.
What damage can ice dam do?
Ice dams can cause some pretty significant damage. Since it is a dam, it holds water. The water can be high enough to seep in behind shingles and into the roof space. This water then usually runs down the rafter, and into the exterior walls. The water that is in the walls can damage the insulation and can lead to mold growing in the walls. It can also ruin ceilings and windows and trim. One of our neighbors had a ceiling fall down in the living room due to ice/snow problems.
How to prevent ice dams
Since the main cause of ice dams is attic bypasses and heat loss, the main prevention is to seal up the attic better, and to add insulation. Attic bypasses are places where the hole had been cut in the ceiling of the upper floor to install pipes or lights, and there remains a gap between whatever was installed and the sheetrock itself. These gaps allow heat to escape and should be sealed up. The second main cause is lack of attic insulation. Adding more insulation will help to keep the heat inside the home and out of the attic.
One thing to consider when re-roofing a home is adding the ice/water shield to the entire roof. This way, if an ice dam forms, and there is water buildup, the ice/water shield can help prevent the water from entering the home.
What to do if you can't prevent
Sometimes ice dams are impossible to get rid of. If you can't prevent them, what should you do?
I have heat tapes installed on my roof that I have on a 8 hour timer. I'll run these during the daytime where it is sunny enough to melt some snow, but cold enough for it to freeze. This is usually in the late afternoon / early evening time.
Other people use roof rakes to "shovel" snow off their roofs. This can work too. It can't melt and freeze if it isn't on the roof.
In the cases were I have had significant ice dams, I've gone up on a ladder to create channels for the water to flow out. I've used an ax to chop the ice down. I've also used a chisel and hammer which may be safer than the ax. I've also used a torch (propane) to melt ice. No matter what the method, you need to be really careful not to damage the roof, and not to fall down.
Here are a couple of resources from the U of MN Extension: