Are you currently faced with a large block of ice covering your roof? If you are looking at that, you should be making arrangements to deal with the immediate problem - get rid of the snow and melt the ice. Following that you will be best to look at the insulation and ventilation part of your attic.

Icicles such as these are formed when there is not enough ventilation in the attic to remove the warm air from the house. The warm air rises up and melts the snow, which then promptly freezes into ice. It can get very deep, and move quite high along your roof. This can damage your shingles, and also leaves no way for the water to run off the roof as it melts.

So what to do - first get the snow off the ice dam. Once that is done, get some salt up there to help melt the dam and get your eaves opened up.

That will fix the symptom, but the problem lies inside your attic. Possibly you have added insulation over the years - and it may have been pushed deep along the outside crevices. This is the most common reason we see for the lack of ventilation.