hit the roof > to become very angry

hit the roof also go through the roof... =To suddenly become very angry. Ex- I'm afraid he'll hit the roof when he finds out our vacation is canceled. Ex-Officials went through the roof when a local newspaper published the report.

He'll hit the roof when he finds out about it - He'll be so worked up (aroused, upset, or angered) when he finds out about it.

The idiom "hit the roof" means to become very angry suddenly. Hence the cartoon scenes where a character becomes really angry and his/ her head starts steaming and they fly and literally hit the roof, if that makes sense to you. Note that I was not positive of the meaning of this word until I searched for it, but I deduced it from the sentence you wrote. Reading more will help you acquire this skill, but reading in context can be very beneficial for understanding idioms and expressions.

Hit the roof means to be very angry / upset. Imagine being so mad that you explode like a bomb and that would cause you to be thrown up and through the ceiling.....hitting the roof.