You need to provide some insulators to keep the feed line suspended at least 6" to a foot above the metal roof. ...
I believe that this separation is both frequency and power dependent. Six inches is awful close if you're talking about 1 KW @ 3.5mHz.
I'm thinking of putting on a metal roof. ... Will this have an adverse effect?
Scott E. Fields, K8AEC
Depends what you mean by 'adverse'. Certainly, there will be coupling of electromagnetic energy from the dipole to the metal roof as it is relatively close, especially in terms of wavelength, and so therefore on the lower bands will be affected more.
This is difficult to determine - things are pretty complex in the near field. Perhaps on one band the metal roof may actually enhance the signal in the desired direction; on another it may substantially alter the dipole's pattern. May be some of the modeling gurus can chime in and provide some insight.
Finally, I didn't see what were your operating interests/goals nor the parameters of the dipole - these would help us better answer your question.
GL, 73, Rich, K3VAT