top side vs rooftop

top side and rooftop both are nouns.

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

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

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

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