upper side vs rooftop

upper side and rooftop both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper side	Yes	No	No	No
rooftop	Yes	No	No	No

As nouns, rooftop is a hyponym of upper side; that is, rooftop is a word with a more specific, narrower meaning than upper side:

Other hyponyms of upper side include crest, crown, poll, crown, pate, poll, desktop, tiptop.

upper side (noun)	rooftop (noun)
the highest or uppermost side of anything	the top of a (usually flat) roof