I think a mettle roof would be perfect for that. We collect off our asphalt shingle roof and use for my garden. With asphalt they say don't use it on things where you are eating the leaves, but it's ok for fruit baring things like tomatoes. Wood shingle are the same issue if they are treated at all. I'm more concerned with the chemicals that are in some roofing materials than a little bird poo that may be there.