This paper describes the entire analysis of a double screen FSS. Two single layers compose the structure. Each single layer is composed of conducting patches on isotropic dielectric materials. The relation between scattered fields and the current sources is presented. The analytic procedure to get the dyadic Greens function and the incident fields is detailed. The numerical procedure to get the induced density current on patches and, so, the scattered fields is also presented. Finally, we show how to transfer the impedance seen by the current sources for the top and the base of the structure.