Ice build-up on roofs, especially metal roofs, can cause a serious situation. This ice build-up is known as an "Ice dam". Ice dams are continuous chunks of ice that form along the margins of your roof. While frozen, they're no more trouble than the icicles that hang down. But during the warmer parts of a winter day, water melting off the roof pools behind the ice, then seeps back up. Sometimes water can work its way 5 or even 10 ft. back up. The weigh of this water can push the ice off the roof falling on whatever is below. If there is a tall building that the ice falls off onto a person, it could cause serious injuries or death. The best way to prevent this from becoming a problem is to prevent the ice build-up from forming. On metal roofs, install snow guards above entrances and walks.
If an area is found to have ice build-up above entrances and walks, the entrances and walks should be closed to pedestrian use if possible. It is better to be safe than sorry.