When you say "safe" you are talking about the safety of the scanner when transmitting, right? Roofs are really expensive and of course there are other factors to antennas and masts (power lines, neighbors, lightning etc...) a blanket "yes, its safe" isn't necessarily wise.
For example if you ask a doctor is taking an aspirin is "safe", it's reasonable to either get a "maybe", or an interrogation.
Sorry for being a wet blanket.
Thanks
Joel