I've used Plastruct shingle sheets, individual strip shingles, simulated tar paper, cork-based strip shingles, simulated tar paper, and Evergreen styrene standing seam "metal" roofing. All have their place. I don't believe most barns use standing seam roofing due to initial cost.
Bar Mills Gravely Building with their self adhesive, strip shingles painted weathered black and weathered with Dr. Brown's weathering powders.
Plastruct sheet shingles painted weathered black, individual shingles picked out with lighter shades with red and green, dusted with Dr. Brown's weathering powders.
Bar Mills Idaho Hotel with Evergreen Standing Seam roofing. Painted oxide red with dry brushing and weathering powders.
Scratch-built "Shingles" cut from 400 grit emery paper strips and contacted cemented to roof of Gloor Craft Coaling Tower
Sterling Tower by Model Tech, Inc. with their included cork-based, self adhesive shingle strips.
Bar Mills Saulena's Tavern with simulated, self-adhesive, tar paper strips again painted with weathered black and aged with powders.
In my opinion, every one of these methods makes a believable roof. It now depends on what you want the barn to look like.
