Minnesota Roof Collapse

Does anyone know what kind of roofing material was employed. News stories imply that a plastic tarp material may have been used. Given the snow was 17 inches deep, and assuming that 1 in. of rain is equivalent to 10 in. of snow by weight, the load was 8.84#/sq.ft. on a uniform basis. Drifting by wind etc. would change these numbers dramatically. I am sure that the designers utilized all kinds of safety factors, because of the human element, plus acccounting for the worst snows in the areas history.