[[File:YorkU1.jpg|thumb|Recently laid ''Sedum'' mats on a ground level podium roof.]]
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[[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]]
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<h4>Water quantity</h4>
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Green roof performance has not been reported to reduce over time. Controlled studies have instead indicated that maturing green roofs may have improved water retention properties <ref>http://www.sciencedirect.com/science/article/pii/S0022169417300768</ref>.