I looked at different roofing materials when I was building our brooder house. Metal is expensive, but it holds up way better than any of the fiber type roofing materials, and it will sheet off snow and ice much better. I think it is worth the price to use metal. At Lowe's I paid about $16...
Heather,
I would try to work with the roof line that you have already for ease of incorporating a roof. Since you already have wire, the only thing you really are looking for is the shelter aspect of the roof. I don't know what the side of the building looks like on the right of this view you...