We deal with the following question: how can the composite nature of a boundary condition formulated for a periodically inhomogeneous surface and involving the composite surface parameter, be treated analytically? We show that when the appropriate Fourier transformation is applied, the composite boundary condition reduces to a specilic eigenproblem condition, which constitutes the spectrum of eigenvalues of an "effective" surface parameter, a novel quantity we introduced to account for the nonhomogeneity of the surface.