tin roof & base antenna questions

I’m thinking of putting in an A-99 either on or near my tin-roofed shed. The roof is 50’ x 25’, with the ridge of the roof running longways; top of the ridge is about 20’ above ground. The roofing metal itself is unpainted galvalume-coated steel -- about as conductive as a roof can get. Due to zoning/neighbors, I can’t go much higher than 40’-50’ with the entire height, so putting up a mega-mast to get well above and away from the roof isn’t an option here.

Here are the questions:

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tin roof & base antenna questions