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I might add that, if there is no ground wire from the metal roof to a ground rod, there will still be an RF current path to the earth. Depending upon the area and height of the roof, there will be several hundred pF of capacitance to ground. The same situation occurs when using a vertical antenna mounted on a car. There will be capacitive coupling between the body of the car and the pavement.
Note added 8/10/08:
Weak’s proposal of an inverted vertical antenna hanging down from the metal roof is very interesting. This kind of antenna will radiate, but poorly. In the antenna circuit, the displacenent curent flows from the vertical element to the roof, and from the roof to the ground. These displacement currents flow in opposite directions, producing a low radiation resistance and high antenna reactance. The performance is much better if the antenna is pointed in the upward direction from the roof.