An increasing number of studies now demonstrate the benefits of rooftop greenery in reducing building energy consumption and improving the urban microclimate. However, the extent to which such thermal benefits are influenced by the components and design of rooftop greening systems has not received adequate attention. We report in this paper, results of a study to evaluate the effects of growing substrate and water retention layer on the thermal performance of a modular green roof system. The evaluation focused on surface temperatures of the green roof and plant responses in a field experiment with two main treatments: substrate types (normal garden top soil compared to soilless lightweight growing substrate) and the presence or absence of a water retention layer incorporated into the green roof system. Surface temperatures at three positions in the vertical profile of the green roof system, and plant responses (evap-otranspiration and stomatal conductance) were compared under different levels of water availability in the growing substrate, which were imposed by sequentially subjecting the green roof to three phases of irrigation over a two-month period: irrigation to achieve well-watered condition, irrigation withheld to induce drought conditions, and irrigation restored to pre-drought conditions. Results show that surface temperatures tend to be higher with topsoil as the growing substrate. The water retention layer also reduced surface temperature at the mulch layer throughout the day. However, the water retention layer may lead to slight increase in concrete surface temperature, possibly due to heat stored in the water retention layer. The study suggests that incorporating a water retention layer can be beneficial to green roofs in sustaining soil moisture, evapotranspiration rates and plant health. We discuss other implications of the results in relation to irrigation needs and optimising the benefits of green roofs for cooling. (C) 2017 Elsevier B.V. All rights reserved.