A simple extension to the Fresnel-Kirchhoff theory of double knife-edge diffraction leads to expressions for the attenuation due to an obstacle with vertical sides and a flat, but not necessarily level, top. The method is to find the sum of the double knife-edge field and the field due to a wave that is reflected by the top of the obstacle as well as diffracted by the sides. It is assumed that the diffraction angle is small and that the top is perfectly reflecting. The attenuation is found in terms of the Fresnel surface integral in much the same way as for double knife-edge diffraction. The results of some calculations are displayed.