Total posts : 45366
I was speaking about an RF ground. The transmitter is isolated from the mast and the roof. You can still ground the mast itself to the roof ground for lightning protection.
I don’t know either if my experiences translate to the AM band. But, as stated, it might be worth the effort to find out – it would be pretty easy to just mount the transmitter close to the roof and see what happens. I seem to recall another post where someone had mounted an AM transmitter close to a metal roof in a barn(?), he disconnected the ground wire and actually got a better signal. And, as also stated, you don’t have to worry about the FCC disconnecting your ground to see if it radiates.