Take a 20- to 30-inch snowfall, which can easily hold 2 to 3 inches of water equivalent. For each square foot of roof surface, a weight of roughly 10 to 15 pounds must be borne by a building's roof and other structural members.
With surface areas of schools and commercial buildings reaching many thousands of square feet, tons of snow burden quickly piles up. Under-engineered roofs can give way under the load.
Strong winds and drifting can heighten the danger all the more. Wind-drifted snow can become highly concentrated over irregular roofs with loads locally much greater than the average load.
Story by AccuWeather.com Senior Meteorologist Jim Andrews.