Elongated side rails for attachment to a sidewall of a truck cargo box as a portion of a tonneau cover assembly. Each assembly includes a flexible cover that can be operatively secured to side rails. One side rail includes a securing portion that can be operatively connected to the sidewall; an inclined portion extending from the securing portion, the inclined portion having first and second sides; a connecting member extending downwardly away from the inclined portion from a position located between the first and second sides; and a longitudinal portion extending from the connecting member opposite the inclined portion. To secure the cover to the side rails, hook fastener is preferably attached to loop fastener on the cover. One preferred side rail has a ridge to prevent premature engagement of the fasteners. Another preferred side rail has directional hook fastener to prevent premature engagement of the fasteners.