This paper aims at providing an analytical model, suitable to highlight the mode coupling due to scattering on small one-dimensional irregularities (parallel ridges) of the surfaces of isotropic solid plates, when shear horizontal waves polarized along the ridges propagate perpendicularly to them. An impedancelike boundary condition at the interface between the teeth (the ridges) and the inner plate (bounded outwardly by the ridges) accounts for the inertia of the teeth to describe the roughness. Using the integral formulation, the displacement field is expressed as a coupling between eigenmodes of the inner regular-shaped plate.