The roof of the Metrodome is air-supported. It requires 20, 90-horsepower industrial fans to pump it full of 250,000 feet of air a minute to keep the roof inflated. The roof is made up of two layers. The outer layer is very thin and is made of Teflon-coated fiberglass. The inner layer is made of fabric. There is lots of dead space between the two layers. This space contains the fans, that blast the space full of hot air to insulate the roof.
In the winter, the temperature of the roof-supporting air is turned up. This helps melt the heaps of snow that fall in Minnesota and builds up on the the roof… and as it turns out, it doesn't do that good of a job.
During the 2010 Minnesota Blizzard, well over 17 inches of snow fell in the Minneapolis-St. Paul area. The record snowfall caused the roof of the Metrodome to collapse.