One viewer has recommended using Urea fertilizer to melt ice on the roof rather than using salt-based de-icers.
Actually, the research done, during the famous 1998 Eastern ice storm, confirms this idea. The salt will discolour the shingles and rust the gutters and generally not be good for either the roof or the plants below. Urea does not work fast, but it does melt ice. I have received one important caution from Lorraine living in Waukesha, Wisconsin. She confirms that fertilizer does melt snow, but can make a dog, who likes to eat snow, very sick, if not kill it! Hence, don't use it if you have snow eating animals!
One thing to keep in mind is that any de-icer, made up of large chunks, will tend to cut through ice fast, but not reduce the thickness of the ice. If it is more powdery, it will not cut through but will reduce the size of the ice itself. Choose your de-icer according to the results you want.