A theoretical model based on enthalpy formulation and fully implicit finite difference method, has been used for one-dimensional model to predict the time-wise variation of latent heat stored and melt fraction of PCM in the designed model. The objective of this paper is to predict the thermal performance of a latent heat thermal energy storage system roof, a building component which contributes maximum heat load inside the building and comparison of heat load reduction due to addition of PCM wall. Stored heat can easily be utilized for any other application or can be flushed to the ambient .