What is the primary purpose of implementing green roofs as BMPs in urban areas?

• A. To increase urban soil compaction.
• B. To replace the need for drainage systems entirely.
• C. To reduce stormwater runoff and provide insulation.
• D. To attract birds only.

Answer: (C)

Green roofs act as a natural sponge in urban systems, reducing the amount and speed of stormwater entering already stressed drainage networks. The growing medium and vegetation hold rainwater, slowly releasing or absorbing it, while plants transpire water, which lowers runoff during rain events. This storage and delayed drainage mean peak flows are reduced and water quality can improve as water passes through the substrate and plant roots. At the same time, the layered roof provides insulation, helping keep buildings cooler in hot weather and warmer in cold weather, which lowers energy use and can further support water management by reducing demand on climate-controlled systems. This combination—lower runoff and added insulation—fits the primary purpose of green roofs as a BMP. The other options don’t align with their goals: they don’t increase soil compaction, they don’t eliminate the need for drainage systems, and attracting birds is not their primary aim.