This paper presents a numerical method for the calculation of the scattered field when a Gaussian beam meets a flat or slightly rough conducting surface at a grazing angle. When the beam is horizontally polarized, the surface acts like a perfect conductor whereas in the vertical polarization case the reflection coefficient depends heavily on the angle of incidence. In the first case we use the image medium method and in the second we introduce a modification in order to handle the changing reflection coefficient. The method is a solution to the parabolic approximation of the full wave equation and, in the case of the horizontal polarization, is tested against an integral equation solution to the full wave equation. The results of the two polarizations are compared for a flat and Pierson-Moscovicz sea surface.