There are 17 rooftop races spread throughout New Providence. These are marked by a chequered flag with a person running next to them (not to be confused with the road and wingsuit races).
Gaining silver on these is pretty straightforward. My biggest tip is to wait until you reach Agility level 5 and do them all. Not only will all of them unlock for you, but you will find the lower levelled races far easier to do with higher jumping abilities, triple jump and boosters.
Cakechieveables has made a video guide showing you where each of the races are and a run through of each of them: