The heat island effect has resulted in higher urban temperatures. Green roofs could contribute to cooling, providing insulation to buildings, and lead to substantial savings in energy for temperature control. This study compared the effects of hydroponic green roofs on the reductions of rooftop temperature and heat amplitude. Treatments include water depths, plant types and growth mediums. The experimental site was located in Taichung, the third largest city of Taiwan, which has a subtropical climate. The results indicate that, first, a water depth of 10 cm is sufficient to provide an ideal hydroponic green roof system that reduced rooftop temperatures and heat amplitude by 5 degrees C and 55%, respectively. Second, when vegetation was added to the ideal hydroponic roof, the rooftop temperature was further reduced by 3 degrees C to 5 degrees C and the rooftop heat amplitude was further reduced by 16%, compared with the corresponding values for the roof without vegetation. However, between the two types of plants tested, no differences were observed in the reduction of rooftop temperature and heat amplitude. Third, the solid-type growth medium slightly outperformed the hydroponic-type growth medium in reducing the rooftop temperature; however, the solid-type was more challenging regarding system installation, maintenance, and weed control. (C) 2016 Elsevier B.V. All rights reserved.