What kind of roof is this?

If metal, then I'd suggest a vertical, the longer the better (no traps please
!).

At the U of Wis in Milwaukee we had all our antennas up on a 5 story high
concrete/metal roof. A 40 meter ground plane worked extremely well, dipoles
worked great if they were WAY off the roof. Low wires were junk.

jw
wb9uai