William Degnan says a roof may collapse with little or no warning.
Signs of the danger include sagging roof steel, leaks, cracks, buckling of interior walls, doors or windows that are difficult to open, and sprinkler heads pushed down below ceiling tiles.
Degnan advises people to be careful not to damage gas and oil service to the building. He said using a roof rake is recommended while keeping away from electrical lines.
Exits should be kept clear of snow, as well as windows, in case of a fire.