Metal is not copper. Also, does anyone know what the overhang should be (for shingles on bay window roof).
Thanks

When your saying "metal roof" are you talking about an actual corregated roof of metal or are you talking about an undelayment of metal sheeting?

No, it is the actual roof for the bay window that has been up there for years (roof on the rest of the house is shingles). Some sort of metal. Roofer is proposing to just lay shingles over the metal, but I am concerned that may not be the proper thing to do (he also proposes clipping the existing metal roof so that it no longer "curls" over the edges of the bay window roof and laying shingle just beyond that).
I don't know if weather matters, but I live in South Carolina where it is very hot, very humid, and we have frequent heavy rains in the summer.

If it is sheetmetal then yes you can lay shingles ontop of it. But if its the corregated kind I wouldn't recommend shingles on top without stripping off the roofing. Another solution if the metal is still in decent shape is to coat it. You can buy rhino liner in a can and spray it on the sheetmetal to create a sealed extremely durable roof element. Mostly its an estetic look for your home as to how it blends in or not.