A tonneau cover assembly that selectively covers a truck bed of a truck is provided. The tonneau cover assembly includes a cover and a longitudinally extending rail located adjacent a sidewall of the truck bed. The longitudinally extending rail includes an attachment member. The attachment member extends longitudinally along with the rail and at least a portion of the attachment member is located over a portion of the bed. The cover includes a stiffener located adjacent the attachment member of the longitudinally extending rail.