We present a numerical evaluation of the regime of validity for various rough surface scattering theories against numerical results obtained by using the method of moments. The theories are the small perturbation method, the Kirchhoff approximation, the momentum transfer expansion, the two-scale expansion, and a new theory, which we call the unified perturbation method. Both the field and phase perturbation expansions for these theories are examined. It is outlined how all these theories can be derived in a unified manner starting with the extinction theorem. We consider the contribution of each theory up to second order in the perturbation expansion for the surface current. The rough surfaces considered are oceanlike surfaces, which exhibit height power law spectra.