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Just in case you can access this:
You’re going to need some type of radial system to maximize your signal. Since you are on a roof use as many as you can and run them to the edge of the roof if possible. Ideally you would have some kind of earth ground where you mount the antenna such as a metal vent pipe where you can strap your ground connection.
Your biggest obstacle may be what the building owner will allow in the way of ground radials on the roof and what type of roof is there. If you were really lucky the roof would be metal and you could just tie into it. (Then you could skip the radials.)