An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow, also know as water, from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. This is common in the spring.
What damage does it 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.
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.