We study and exactly evaluate the transmittance of microwave energy—below S-band—that goes through thick building walls covered by an external thin layer of rain. The incident radiation penetrates the wet walls at normal incidence, twice in their back-and-forth paths typical of sensing-through-the-wall (STTW) operations. The walls are responsible for most of the loss through the structure in view of their greater thickness, but the water layer contribution is significant, and it is quantitatively evaluated here. The properties of the water layer are expressed in terms of Debye's classical formulation, and those of the walls are obtained from available measurements. A simple homogenization technique is used to generate an "effective permittivity" of the double layer to be able to alternatively treat it as a single equivalent layer—an approximation to the exact values, which yields acceptable results. It is also found that the transmittance T and the reflectance R of a single and double layer could also be expressed in alternative and apparently novel forms that may simplify future calculations. Further, if either R or T were known as well as the layer(s) properties, then either one could be used to obtain the other. Finally, representative plots are displayed, quantitatively showing the (two-way) transmittance degradation that occurs when there is a rain layer on top of the various considered wall materials, and we explain a resonance effect observable in the transmittance plots that shows an oscillatory frequency behavior.