Electromagnetic (EM) reflection properties of building materials and structures have been investigated through practical measurements. However, the finite thickness of measured materials makes it challenging to resolve the rays reflected from the front and the back surfaces. In this paper, we therefore present a three-step minimum least squares-based algorithm to resolve two closely adjacent rays reflected from the front and the back surfaces of a board-shaped material. Our analytical and numerical results show that the proposed algorithm achieves the Cramér-Rao lower bound. The proposed algorithm is validated using measurement data for various materials and incident angles in the 40–50 GHz frequency band. The validation results show that the proposed algorithm is capable of resolving two closely adjacent rays with a root-mean-square deviation that is smaller than 0.08. Main applications of the proposed algorithm can be found in the frequency domain measurements of the EM wave reflections by typical building structures, e.g., window glass, doors, ceiling, and floors.