double-decker vs roof

double-decker and roof both are nouns.

Word	Noun	Adjective	Verb	Adverb
double-decker	Yes	No	No	No
roof	Yes	No	Yes	No

double-decker (noun)	roof (noun)
a vehicle carrying many passengers; used for public transport	a protective covering that covers or forms the top of a building
	protective covering on top of a motor vehicle
	an upper limit on what is allowed
	the inner top surface of a covered area or hollow space

double-decker (verb)	roof (verb)
	provide a building with a roof; cover a building with a roof