I was replacing a roof on a bank branch in northern virginia. The western part, not close to DC. Sloped Flat roof on bar joists, it was a gypsum deck with a membrane on top. The scope was to R&R the membrane and a few other minor items. Good old boy contractor hired by the landlord, they stripped everything off and went home for the day. Stoopid. Everybody know if you strip a roof you tarp it, even more important for a gypsum deck. And storm were predicted that night. And it did.
Gypsum roofs are interesting. Compressed fiberboard, like ceiling tiles, were laid on the joists, then gypsum was mixed up and poured over the surface about 2” thick, then the waterproof membrane is applied. As long as the gupsum stays dry it has enough strength to walk on. If i5 gets wet, it turns to mush. Bank employees came to work the next morning, the entire roof had collapsed into the branch. Made a terrible mess.
