The conventional wisdom that the incoherent portion of the forward-scattered signal off a rough surface approaches a constant asymptote as the roughness parameter (RP) grows large is reexamined and shown to be incorrect. Essential use is made of the fact that the incoherent-scattered component has been observed to come mainly from the specular direction. With this approximation, a remarkably close fit to the data is achieved when one further assumes that the surface is illuminated by a very narrow beam. Asymptotic expressions show that the rms of the incoherent portion of the signal first rises linearly with RP, and then eventually falls off as the inverse square root of the RP. This general shape is independent of the form of the surface autocorrelation function.