Hello I have a 40x60 pole building that I am beginning to insulate with faced R-19 insulation. I would like a lightweight material to use in the roof to cover this up to prevent any damage being caused to the insulation. IE:Birds that happen to get into my shop via open garage door. I was looking into dry walling it but that would be over 2500 sq ft of drywall finishing to do while looking up. I'd prefer an alternate that is cost effective and lightweight. Any help would be much appreciated.

Fiberboard is out of the question because of the fire hazard it presents while being exposed.

Tyvek?

6 mil plastic?

If fire is a concern, then metal roofing?