We're doing a rooftop barbecue today.
I worked out on the rooftop in the morning even though I work the night shift.
Why don't we go up to the housetop and catch the star light shining down from the heavens?
Above the roof is the rooftop. What else would be up there?
This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 屋 though, so here's a mnemonic to help you:
One place in the world has more rooftops than any where else, and that's Oklahoma (おく). They get so many tornadoes that they need extra rooftops for when the first few blow off.