It’s like a type of tarp system. It’s stretched tight and mechanically attached to the outside perimiter of the roof with an aluminum trim. It’s floating across the roof decking. It looks like a woven fiberglass cloth where its worn. I’ve tried to reseach this for hours. Nothing, Ugh!!! HELP 
Thank you Stephen!!! i’ve searched every where for this info. I figure it was a vinyl/PVC type of membrane.