Statistics have confirmed that a lawn placed on a roof top can reduce the cooling load. In this study, the VISUAL DOE 4.0 energy simulation program was used to determine the effects of roof lawn gardens (RLG) on the annual energy required for cooling, and annual electrical usage in an experimental model. The overall heat transfer coefficient (U-Value) of the RIG was estimated by using data from site measurements and the effect of the RIG on the building's energy consumption was calculated by the simulation program. The effect of different thicknesses of soil was also investigated. From the field measurements, it was confirmed that the RIG could reduce building heat gain. Thus, an experiment was set up to find the energy benefits of the RIG. With evaporative cooling by the RIG, it was found that the consumption of energy in a building with a RLG with 0.20 m of soil was 37.11% less than in a building with a bare roof surface (without evaporative cooling). Moreover, a building with 0.10 m of soil in the RIG consumed 31.07% less energy than a building with a bare roof surface. The results show that an increase in the thickness of the soil layer reduces the building energy consumption. (C) 2011 Elsevier Ltd. All rights reserved.