In this study, periodic solution of unsteady heat transfer problem for building walls and flat roofs is firstly used to find the CLTD (cooling load temperature difference) values for these building elements. The solution is obtained by applying CFFT (complex finite fourier transform) technique. The solution of periodic nature is a novel approach in applying CFFT technique for calculating CLTD values for these elements. A computational procedure based on the periodic solution is developed, and a program in Matlab is prepared for the numerical calculations. The CLTD values are compared with those values given in ASHRAE handbooks. There is a considerable agreement between computed results and CLTD values provided in ASHRAE handbooks for the selected walls and roofs. It is obtained that differences in CLTD values for Roof 2, Roof 13 and Wall 3 selected for main directions change between 0 and 2.42 degrees C, 0 and 0.94 degrees C, and 1.8 and 43 degrees C, respectively. It is found that differences between estimated heat gain values and those given in ASHRAE by RTS (radiant time series) method change between 0 and 5 W/m(2). The numerical calculations are validated with ASHRAE standard data. (C) 2015 Elsevier Ltd. All rights reserved.