Composite structures with stiffeners and method of making the same

A method for assembling a stiffened composite structure includes a step of positioning a plurality of dry fibres 27 along a first side 34 of a pre-preg composite laminate skin element 20 which is dimensionally changeable (e.g. is at b-stage cure) so that it may conform to a desired configuration. An interlayer 38 is positioned between the plurality of dry fibres 27 and the first side 34 of the skin 20 and the dry fibres are infused with a resin. The skin and infused fibres with the interlayer between them are co-cured. The dry fibres may take the form of a stiffeners. The interlayer may comprise an impermeable or permeable barrier; an adhesive, textured or b-layer film; a veil; or define a plurality of perforations. Heat may be applied during infusion causing an intermediate cure of the pre-preg. Co-curing may include applying pressure or heat. Used in aircraft construction.