The like polarized and the depolarized backscatter cross sections for composite rough surfaces are evaluated using the full wave approach as well as a perturbed physical optics approach. The full wave approach which accounts for both specular point and Bragg scattering in a self-consistent manner is expressed as a weighted sum of two cross sections associated with the large-scale and small-scale spectral components of the rough surfaces, respectively. Provided that the assumed condition for deep phase modulation is satisfied, the full wave solution for the total backscatter cross section is insensitive to the precise choice of the wavenumber where spectral splitting is assumed to occur. The solution based on the perturbed-physical, optics approach is shown to be suitable even for moderately large mean square slopes provided that the angle of incidence is larger than 30°.