The aim of this study is to find time lag (TL), decrement factor (DF) and total equivalent temperature difference (TETD) values for multilayer walls and flat roofs of buildings using experimental and theoretical methods, and to compare the experimental results with theoretical ones. The TETD is a method for calculating cooling load due to heat gain from the walls or flat roofs, and it can be obtained using values of inside and outside air temperatures, solar radiation, TL and DF. The TL and DF depend on the highest and the lowest temperatures at the inner and outer surfaces of the walls or flat roofs, and the time periods involved in reaching these temperatures. Hence, two testing rooms each consisting of four multilayered walls and a flat roof, air conditioner, measuring elements are built to measure all required temperatures. The required temperatures, which are hourly inside and outside air temperatures, and surface temperatures of each structure layer, are measured in every minute during testing periods of the 2007 summer season of Gaziantep, Turkey. Hourly solar radiation values on the walls are computed using hourly measured solar radiation on a horizontal surface. The TL, DF and TETD values of eight different walls and two different flat roofs commonly used in Turkey are computed utilizing the measured temperature and solar radiation values. The computed values for the TI, DF and TETD are compared with theoretical results obtained numerically using periodic solution of one dimensional transient heat transfer problem for the same structures. (C) 2008 Elsevier Ltd. All rights reserved.