This paper addresses a three-dimensional scattering problem from a PEC object above a periodic conductive rough surface by using a decomposition method based on the separation of equivalent current density on the periodic surface as induced and perturbed current densities. The infinite solution domain is forcibly bounded by assuming that the perturbed current density caused by the object is intensively dominant in only a limited area on the periodic surface. The proposed decomposition method achieves considerable accuracy while providing an effortless computation without attempting a half-space Green's function or exercising a tapered wave illumination. This method provides a significantly superior solution to the tapered wave approach in illumination at low-grazing angles, although a plane wave is harnessed. Also, this proposed method can handle an arbitrary degree of roughness since the solution method does not contain an approximation of the surface roughness.