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I’ve had similar interesting experiences with a metal roof on FM. I had first installed a (Canadian certified) Part 15 equivalent FM transmitter (a Decade MS-100, a really excellent transmitter by the way) in a waterproof enclosure on a tripod about 5 feet from the roof – FM is actually somewhat usable in Canada, as we are allowed 4 times the field strength at 3 meters than the U.S. Range was about what I expected line of sight.
I was forced to move the antenna (box) when I moved studios, and this time installed it about 6 inches from the metal roof – this was not planned, but I had to use a different mount. To my surprise, the signal was significantly stronger – I assumed that there was some sort of capacitive thing happening and that the roof was acting was a huge ground counterpoise. I actually went back to the original mount as I was afraid that I was over the maximum field strength allowed (and as has been discussed previously here, you need expensive equipment to measure the actual field strength).
So yes, in my view it’s possible that the roof is acting as your counterpoise. But maybe something else is happening. I’m sure others of a more engineering bent will have opinions.