Green Roof
Practices that capture and store rainfall in an engineered growing media that is designed to support plant growth.
Functional Elements of a Green Roof System
A green roof is composed of up to nine different systems or layers that combine to protect the roof and maintain a vigorous cover. The relative placement of various layers may vary depending on the type and design of the green roof system.
1. Deck Layer: The roof deck layer is the foundation of a green roof. It may be composed of concrete, wood, metal, plastic, gypsum, or a composite material. The type of deck material determines the strength, load bearing capacity, longevity, and potential need for insulation in the green roof system.
2. Leak Detection System (optional): Leak detection systems are often installed above the deck layer to identify leaks, minimize leak damage through timely detection, and locate leak locations.
3. Waterproofing Layer: All green roof systems must include an effective and reliable waterproofing layer to prevent water damage through the deck layer.
4. Insulation Layer: Many green rooftops contain an insulation layer, usually located above, but sometimes below, the waterproofing layer. The insulation increases the energy efficiency of the building and/or protects the roof deck (particularly for metal roofs).
5. Root Barrier: Another layer of a green roof system, which can be either above or below the insulation layer depending on the system, is a root barrier that protects the waterproofing membrane from root penetration.
6. Drainage Layer and Drainage System: A drainage layer is then placed between the root barrier and the growing media to quickly remove excess water from the vegetation root zone. The selection and thickness of the drainage layer type is an important design decision that is governed by the desired stormwater storage capacity, the required conveyance capacity, and the structural capacity of the rooftop.
7. Root-Permeable Filter Fabric: A semi-permeable needled polypropylene filter fabric is normally placed between the drainage layer and the growing media to prevent the media from migrating into the drainage layer and clogging it. The filter fabric must not impede the downward migration of water into the drainage layer.
8. Growing Media: The next layer in an extensive green roof is the growing media, which is typically 3 to 6 inches deep (minimum 3 inches). The recommended growing media for extensive green roofs is typically composed of approximately 70 to 80 percent lightweight inorganic materials, such as expanded slates, shales or clays; pumice; scoria; or other similar materials.
9. Plant Cover: The top layer of an extensive green roof typically consists of plants that are nonnative, slow-growing, shallow-rooted, perennial, and succulent. These plants are chosen for their ability to withstand harsh conditions at the roof surface.