The rooftop is the upper surface of a roof — the part that faces the sky. If you climb up and stand on a building, you are on the rooftop. Though actually, roof and rooftop sit very close, but people do not use them in quite the same way. Roof is the whole cover over a house. Rooftop makes you picture the top as a place you could use: a rooftop garden, a rooftop bar, a party on the rooftop. In cities with flat roofs that picture is natural. On a sharp, pointed house roof, speakers still say “on the roof” more often than “on the rooftop.” It is an ordinary physical word, not fancy, and it is at home in both everyday talk and writing.