In this study, the thermal performance of three kinds of roofs with different heat capacity and thermal conductivity under different external conditions has been investigated using a numerical method. For this purpose, the combined solar radiation, conduction and convection heat transfer were calculated implicitly in terms of a one-dimensional finite difference method. Different high and low solar radiation conditions in two common climates in the Middle East, including hot-humid and hot-dry, were considered. The effect of roofing materials was investigated in terms of their thermal storage and overall heat transfer coefficient. Moreover, the time lags and decrement factors were evaluated to compare the performance of the roof. The numerical model has been validated using EnergyPlus. The results indicate that the roof with high thermal storage and low thermal conductivity has better performance in comparison to others. However, the total heat gains are not linearly proportional to the overall heat transfer coefficients, e.g. here, the ratios of a total load of roof 1 to roofs 2 and 3 are about 12 percent lower than the ratio of overall heat transfer coefficients. Furthermore, the solar radiation intensity had considerable effects on time lags. Finally, it can be concluded that the external conditions have no significant effect on the decrement factor.