all these replies vary all over the map and so should show you that there can be danger in high power rf. as an extra class ham, old 1rst class commercial license holder, ele engr, I can tell you there can be danger to your health by those antennas on top of the 60' building. I also can tell you no AM broadcast antenna (25,000-200,00 watts) is there (yes, they are the tower and 160-250 FOOT high so NOT on top of that building as well as require beging LOW by the ground not high to work well, the normal antennas on that building are line of sight 30-480mhz band at typically 20-250watts, with maybe some microwave (wireless for internet stuff antennas at typically 0.5-25 watts. Did you know that a microwave oven is about same frequency as wireless internet stuff (2-3ghz) and this also is the frequency that vibrates water (or why would it cook food?) and also your body fluids? Do you worry about your kitchen microwave? Of course you don't beam it from an antenna toward some other location. I used to light a 4' florescent bulb fully with my 200 watt heathkit on 80, 40, or 10 meter ham band (3, 7, 30 mhz) within about 2 feet of my antenna. I also worked for years on all kinds of 2 way radios while going to college and probably spent more time overall than you will on top of some of those same kind of buildings as you are talking about,and I am still alive.
With your concern I would recommend you ask the building owners who want you to some work up there to give you a written ok to do so in safe way as you are concerned.
IMHO just stay 5-10 ft away from any of the antennas and you will probably be fine, but ask for written doc from building folks if you want to be sure.
