The lower order modes are derived for a parallel plate waveguide loaded with a sheet of parallel conducting strips. Using a first order perturbation method, we derive the scattering parameters due to a small deformation of the sheet. Results for scattering within and between the novel slow mode and the ubiquitous TEM-mode are presented. With increasing angle between the strips and the direction of propagation, the interaction between the slow mode and the deformation shifts downwards in frequency, due to the reduced phase velocity.