Abstract The normal modes of finite periodic skin-stringer structures have been analysed in terms of free flexural wave groups that can exist in infinite periodic structures. The concept of an equivalent internal restraint has been introduced and this is useful in determining the width of the propagation zones of flexural waves. The values of the propagation constant at the natural frequencies have been determined for three different boundary conditions. It has been shown that the natural frequencies of these structures can be determined easily and rapidly from the diagram showing the variation of the propagation constant with the frequency parameter. The results of this work can be used to construct design charts to predict the natural frequencies, structural wavelengths and wave velocities for finite structures with any number of spans and for varying degrees of stringer stiffness.