An asymptotic solution is presented for the diffraction by a resistive strip which is useful in the simulation of thin dielectric layers. Up to third-order diffraction terms are derived which include the surface wave field effects in a uniform manner. Extensions to the case of conductive and impedance strips are also given in Appendix C. The derivation of the higher order terms is based on the extended spectral ray method. New first-order diffraction coefficients for the impedance, resistive and conductive half planes are also presented. The last are uniform everywhere, including the surface wave field boundaries.