A stiffener has an elongate web element and an elongate foot element connected by a curved elbow that has a radius greater over a first portion than over a second portion of the stiffener. The stiffener is formed by feeding an elongate member through a roll forming die having first and second rollers, the first roller having a shaped section arranged to be located within a correspondingly shaped recess in the second roller, the first and second rollers defining a gap there between corresponding to the desired shape to be imparted on the elongate member, the desired shape including a curved elbow between a web and a foot portion, wherein the shapes of the recess and the shaped section vary such that the radius of curvature of the gap between the rollers corresponding to the curved elbow varies between a minimum and a maximum value.