A new unified full-wave approach is used to determine the like and cross polarized cross sections of composite random rough surfaces with significantly different scales of roughness. Traditionally, the solutions for such composite rough surfaces are expressed as sums of solutions based on the perturbation and the physical optics approaches. The full-wave solutions account for both specular point scattering and Bragg scattering in a unified, self-consistent manner. With the new unified full-wave approach, the incoherent radar cross sections are obtained by regarding the composite (multiple scale) rough surface as an ensemble of individual large scale scattering elements (patches) with arbitrary orientations. Thus the scatter cross sections, obtained from the original full-wave solution in a tilted co-ordinate system, are averaged over the large scale slopes (in and perpendicular to the plane of incidence) to obtain the cross sections of the composite surfaces. The rough surface height joint probability density function is assumed to be Gaussian.