This article shows the advantages of using the zig-zag function (-1) ^kξ_k (ZZF) in the bending analysis of unsymmetrically laminated sandwich flat panels with a soft core. Higher order theories are developed by adding ZZF to displacement fields of known theories. From linear to seventh order cases in displacement are considered. The main advantage of ZZF lies in the fact that it introduces a discontinuity in the first derivative (so called zig-zag effect) of the displacement distribution corresponding to the core-face interfaces. Results including and discarding ZZF are compared in the bending response of sandwich plates loaded by an harmonic distribution of transverse pressure at the top surface. Different values of face-to-core stiffness ratio (FCSR) as well as length-to-thickness ratio (LTR) have been analyzed. It is concluded that: (1) ZZF is highly recommended in the bending analysis of unsymmetrically laminated sandwich plates; (2) the use of ZZF makes the error almost independent of the FCSR parameter; (3) ZZF is easy to implement and its use should be considered with respect to other theories.