What is a Green Roof?
A green roof is a layer of vegetation planted on top of a waterproofing system that is installed on top of a flat or slightly sloped roof. Green roofs are also known as green or ecological roofs. They are divided into three main categories: extensive, intensive, and semi-intensive. Although there are no precise definitions of them, a sprawling green roof has a shallow growing medium, usually less than six inches, with a modest roof load, limited plant diversity, minimal watering requirements, and is often not accessible. . Intensive green roofs have more soil and a deeper growing medium, sometimes several feet, that can support a more diverse plant selection, including small trees. Therefore, they have more substantial structural loads and need more frequent maintenance and watering. They are usually accessible. Semi-intensive green roofs include features of both types. The proper depth of any green roof depends on the roof structure, plants chosen, annual rainfall, and stormwater performance requirements.
All three types of roofs require specific layers of roofing materials that are not found in normal roofs. The basic anatomy of a green roof consists of vegetation, growing medium, filter membrane, drainage layer, root repellent / waterproof layer, roof membrane support for top plantings, thermal insulation, vapor control layer and support of structural roof. Each of these layers performs a specific function to keep the plants alive and protect the structure underneath.
The growing medium is not the same material that is used for a houseplant or a garden. Traditional soil is heavy and tightens after repeated rains, reducing water retention and aeration of plant roots. The growing medium for green roofs is made up of mineral aggregates and only a small amount of organic material. It must have good drainage and aeration consistent with a structure that allows it to retain water. It must also be lightweight, resistant to decomposition and compression, and physically and chemically stable.
Some green roofs are installed in a large integrated section, while modular green roofs use small portable planting beds placed together to create a larger green roof. Modular units are usually metal or plastic trays filled with growing medium. Modular green roofs can be installed gradually and are easily removed for maintenance and inspection of the roof layers underneath. Additionally, modular sections can often be grown in a greenhouse and “ready to plant” on the green roof. Since the plants are already established, there are fewer problems with replacement plants that don’t take root or thrive. However, a drawback is that moisture cannot flow between the units, which increases the impacts of wet and dry seasons on plants.
