When we were building the shop the boiler went out and the mechanical contractor wanted to heat the place to avoid issues with the in floor tubing (as I recall it was a pretty good cold snap). He brought in a big salamander heater, and of course there was no insulation in the lid yet (and no lid for that matter). Overnight it built a 14" thick ice dam the entire width of the overhang, which was not quite 3 feet. I was insulating the walls at the time and so much water came in that the insulation was falling out of the bays.
I spent 10 hours up on a ladder breaking ice over my head with a framing hammer. Make a channel, get the water draining, rinse and repeat.
Ice melt or water softener salt stuffed in a leg of pantyhose and laid over the dam will help a lot to dig a channel. Beware that when a 1 foot by 3 foot chunk of ice finally lets go for a 20 foot drop, you AND your ladder should be out of the way.
Heat trace applied at the eave is worthwhile on a metal roof. They make a specific heat trace for shingle roofs exactly to prevent ice damming. However much prevention it takes is worth having to apply even a little bit of the cure.
