roof vs housetop

roof and housetop both are nouns.

Word	Noun	Adjective	Verb	Adverb
roof	Yes	No	Yes	No
housetop	Yes	No	No	No

As nouns, housetop is a hyponym of roof; that is, housetop is a word with a more specific, narrower meaning than roof:

roof (noun)	housetop (noun)
a protective covering that covers or forms the top of a building	the roof of a house
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

roof (verb)	housetop (verb)
provide a building with a roof; cover a building with a roof