I would advise you to use the building-roof as a ground for your 5BTV. Try to mount is as close to the roof as you can. I would say up to 6 inches is OK but much more might give you resonance problems. I've seen a few of these antennas mounted incorrectly, perhaps 4-5 feet off a roof, which means you would resort to a different feeding style that were used at the time.
It is correct, that if the metal plates on the roof are not well connected to each other, and particularly if the roof is very old, you might experience a capacitance effect that could give you a problem. I would though not worry about that at this moment.
The 4BTV and 5BTV have been popular in this country for the last 20-30 years and most of them are mounted on top of a roof with metal plates. You might consider guying your antenna (we must do it up here in the north) but take care to use a non conductive thread for the guying. You can find a guying formula in the handbook.
73 de TF2JB.