Two advantages of green roofs are their ability to cool the building below and the air above, and to reduce stormwater runoff volumes, thanks to the transpiration of moisture through their leaves. The most common green roof plant are species of Sedum. Those plant can survive drought conditions in the rooftop environment in part because they transpire relatively little water through their leaves.
To what extent does the characteristic that helps Sedums survive, their low transpiration rate, negate their environmental benefits? Is the performance of a Sedum-planted roof significantly higher than that of a non-planted roof with the same construction and soils? If not, does the Sedum offer other benefits that still make its use worthwhile? If it offers no real benefits, and a higher-benefit roof is not feasible, is the only alternative a conventional roof, or is a plant-free soil-covered roof a realistic alternative?*
*Green roof people discourage the use of the word soil in favor of “growing medium.” In a world of where manufactured soils are more and more the norm for built landscapes, I am not sure there is a meaningful distinction.