noun
BAD : In the bedroom, the roof looked very damp.
GOOD : In the bedroom, the ceiling looked very damp.
BAD : The bedroom roof was one huge mirror.
GOOD : The bedroom ceiling was one huge mirror.
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roof = the covering on top of a building that protects it from the weather: 'If the rain is coming in, there must be a hole in the roof.' 'All the rooves were covered in snow.'
ceiling = the surface of a room that is above your head: 'When he stands on tip-toe, he can almost touch the ceiling.'