Sourced from NAHB Best In American Living
Green roof” is one of many names given to an assembly of materials including plants that is installed over built construction. Also known as living roofs, eco-roofs, vegetated roofs, and rooftop gardens, a green roof is an extension placed above a typical roof surface. A green roof can also be at ground level over an underground built structure.
Green roofs have been around for thousands of years. In Mesopotamia, stepped pyramids called ziggurats were built with rooftop gardens as early as the third millennia BCE. The modern green roof system was developed in Germany in the 1960s, primarily in response to rising energy costs and the realization of the damage caused by urban stormwater runoff.
The components of a green roof include a root repellant barrier, a drainage system, filter cloth, a lightweight growing medium, and plants. All of these are placed above the waterproof membrane of a flat or shallow-pitched roof surface. An edge treatment of metal or plastic is typically installed to separate the vegetated area from the edge of the roof.
Growing medium is not soil. It is composed of shale and/or slate (approximately 50 percent) that is super-heated to produce a lightweight, engineered mixture that captures the nutrients in rain to sustain the plants. Other materials in the growing medium include sand, clay, and sanitized compost. The water that passes through is held in the drainage layer for plants to absorb. Any overflow runs under the assembly to the roof drains.
The components can be installed individually, known as “built-in-place,” or in a modular system of trays that are pre-grown at a nursery with growing medium, filter fabric, and an integral drainage layer. As the green roof industry continues to grow, new products and systems are developed and introduced into the marketplace.