the hangar weren't planning on the roof caving in. The main reason for parking airplanes inside is to avoid the cost to de-ice the airplanes after they've been exposed to freezing temps and snow and ice. $400-900 for an overnight hangar fee is much less than $1500-5000 just to have the airplane de- iced. Not to mention I'm sure some of the airplanes belong to companies that are based there.
That particular hangar belongs to Dulles Jet Center which is a fixed base operator and the whole facility is only a few years old. I fly into Dulles a fair amount and I do business with these folks as they're service is top notch. Definitely a costly mishap.