From Longman Dictionary of Contemporary Englishthe roof falls/caves inthe roof falls/caves inSERIOUS SITUATION informal especially American English if the roof falls in or caves in, something bad suddenly happens to you when you do not expect it → roof
Examples from the Corpus
the roof falls/caves in• It may not be long before the roof falls in.• The Warriors were leading, with only a few minutes of the game to go, when the roof fell in. 