Jorg – The nature of sedums, stone crops and C4 plants is limited transpiration through stomata closure and “waxy” textures which is great for temperate climates, but also limits any claims of cooling through transpiration like other plants. While is refreshing to sit on a cool green patch of grass it is not so much on a field of sedums. Then green roofs present the problem of trying to balance enough water in the medium to allow plant survival in the harshest of locations on a roof while not filling the mediums “gas tank” with applied water such that when a storm comes in it can absorb and detain the event. A much more effective approach is to use a segmented, evenly distributed blue roof mechanism which can capture a full 2” rainfall event without the building owner being faced with the burden if trying to keep plants alive in the roof. The GSA has documented many spectacular failures of green roofs in North America as there is insufficient knowledge and skill to maintain within the building crew tasked with keeping it alive. Finally, stone crops are not the optimum plants for bees and pollinators often cited as a benefit of such green roof systems. Europe and its success with succulent green roofs is not the environment here in the states.