We discuss the application of the multimodal transfer matrix approach (MMTMA) to evaluate metal and surface roughness losses in one-dimensional periodic structures. To this aim, we analyze the dispersion characteristics of a parallel plate waveguide periodically loaded with chamfered glide-symmetric corrugations. The metal and surface roughness losses are included and accurately evaluated. Furthermore, the attenuation in the stopband region, which cannot be calculated with the eigensolver of commercial software, is also retrieved with the MMTMA. The advantages of MMTMA with respect to a first-order perturbation method are discussed.