This antiquities museum in France houses more than 3,000 collectibles, all housed under a state-of-the-art green roof planted with native species. Opened in 2006, this green roof consists of roughly 86,000 square feet. Green roofs can initially require a considerable up-front investment. Because they often require extensive design and careful structural analysis, some experts estimate that they can cost anywhere from $9 to $40 per square foot. However, the up-front cost is eventually softened by a high return in energy efficiency. One Canadian study reveals that even a 6-inch extensive green roof can reduce summer energy demands by 75 percent, according to Discovery.