Why would a woodpecker be attracted to a steel roof?
In the spring, many members of the woodpecker family set up a territory to ward off other woodpeckers and attract mates; a steel roof would make a satisfying racket. Both male and female downies drum and will also use hollow trees, utility poles, and other handy noisemakers while moving around to various sites in a chosen territory and calling attention to themselves.
There’s probably not much that can be done to scare one off permanently. In any case, the racket should let up once the birds settle down to breeding, probably by mid-May. After that, they’ll be kept busy making their familiar and more attractive drumming sounds as they hunt for grubs and insects in hollow and decaying trees.