• roof [countable]
• 1 the structure that covers or forms the top of a building, vehicle, tent etc:
• They finally found the cat up on the roof.
• roof of
• We can probably strap the cases to the roof of her car.
• slate/tiled/thatched etc roof
• a flat roof
• a pitched roof (=sloping roof)
• red-roofed/metal-roofed etc
• a wooden-roofed theatre
• 2 the top of a passage under the ground:
• Suddenly, the whole tunnel roof caved in.
• 3

  a roof over your head

   somewhere to live:
• I may not have a job, but at least I've got a roof over my head.
• 4

  go through the roof

  informal
• a) also hit the roof to suddenly become very angry:
• Put that back before Dad sees you and hits the roof!
• b) if a price, cost etc goes through the roof, it increases to a very high level
• 5

  the roof of somebody's mouth

   the hard upper part of the inside of your mouth
• 6

  under the same roof/under one roof

   in the same building or home:
• If we're going to live under the same roof, we need to get along.
• Here you can buy food, clothes, and electrical goods all under one roof.
• 7

  under somebody's roof

  spoken in your home:
• As long as you live under my roof, you'll do as I say.
• 8

  the roof falls/caves in

  informal especially American English if the roof falls in or caves in, something bad suddenly happens to you when you do not expect it

• ➔ raise the roof

  at raise1 (23), sunroof

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.