Sandwich materials are currently much valued in industry, especially in transport (automotive, aeronautics, shipbuilding and railroads) and civil engineering. Because of this rise of interest, it becomes more and more important to develop analysis tools able to take their specificities into account. Herein. we introduce a very simple and efficient post-processing method especially developed for heterogeneous materials such as sandwich structures. The method, based on Reissner principle, permits to fulfill the force equilibrium at interfaces between different layers, the skins and the core whose mechanical properties are very different, and yields accurate results for continuous stress components even with a coarse meshing through the thickness of the structure. Only small matrices are involved in the computation, leading to an easy programming and implementation even in existing finite element packages. Moreover, the problem caused by nodal extrapolation of stress components from Gauss points is removed. The present formulation, detailed in 2D for a simple illustration, is also valid in 3D. Finally, the method is assessed in the classical case of a simply supported beam under uniform pressure as well as in the non-standard case like a U-beam.