A method for manufacturing rigid panels made of a composite material requires a caul sheet having a smooth surface that is formed with a plurality of grooves. A first layer of the composite material is laid on the caul sheet, and is cut to create flaps that extend into the respective grooves. Strips of composite material are then placed along the edges of the groove to extend and overlap each other in the groove. Next, a unidirectional ply is placed along the length of the groove, and this combination is then covered with a second layer of the composite material. Together, the combination of the first and second layers, the strips and the unidirectional ply are co-cured to create a rigid panel with integral stiffening members.