The use of the prepositional phrase is "on the roof".
On is a preposition and the phrase ends at roof.
I hope that helped!
The reader knows to follow a series of steps in order.
Okay, so think of the stage as titled down towards the audience. Closer to the audience would be down stage, upstage is the back if the stage. Then for stage left it will be the audience’s left and the actor’s right. Then the opposite for stage right (audience’s right, actors left)