Abstract: A casing for making concrete construction components. The casing comprises at least one flat casing element consisting of a composite material with a metal grid as a carrier and at least one foil attached to this carrier. In order to improve a casing of this type in such a manner that an increased stiffness of the composite material employed as the casing is achieved while at the same time maintaining its lightweight construction for easy handling, the invention provides a stiffener element. The stiffener element is formed as a three-dimensional grid structure or grid girder comprising at least three longitudinal rods of structural steel extending in the longitudinal direction of the stiffener element and disposed side by side at spaced intervals. These longitudinal rods are interconnected by a plurality of cross members made of structural steel.