Those pretty icicles forming on your roof may cause a lot more damage than you think. Not only could they fall and cause damage, but they can also cause ice dams. Ice dams occur after periods of heavy snow and several days of freezing temperatures. They can cause water damamge and gutter collapse. Icicles that are greater and 2 inches in diameter are usually a good indicator of an ice dam.
Here are some tips to help the prevention of ice dams.
1. The most common way to avoid ice dams is to use a snow rake to help clear your roof.
2. While roofs with lots of angles may look nice, they are much more complicated, and thus, ventilation and drainage become much more difficult.
3. Reducing heat sources in your attic can also help prevent air leaks that lead to ice dams. Insulation on its own is not enough.
4. In new home development or when reroofing, install a rubberized roofing underlay that extends at least 6 feet up along the edges of the roof.
5. Deicing cables, or heating cables, that fit into the roof and gutters are an economical way to help drain a roof of snow melt
If you already have an ice dam, it is best to call in a professional contractor to use a steamer to help remove the ice. If it is an emergency and you can not wait for a contractor, you can use a hammer to create holes in the dam and allow the water to drain. However, that method is not recommended because it can cause further damage.
Here's an example from the Boston Globe about what ice dams look like.