Placing roof sheathing vertically
Like the other guys said, any type of sheet materials should be run perpendicular to their supports. This goes for drywall also. Many people do not believe that there could be any difference in drywall, but you can see for yourself easily. Cut a 12" piece across the 48" width of the sheet, and cut a 48" piece from a 12" rip of the sheet. Set them across anything so they are supported at the ends, and begin puting equal amounts of weight in the center of each, and compare the sag.