A green roof is a layer of vegetation planted over a waterproofing system that is installed on top of a flat or slightly sloped roof. It is also known as vegetative or eco-roofs.

Three main categories

Green roofs fall into three main categories – extensive, intensive and semi-intensive.

  • Although there are no precise definitions of them, an extensive green roof has a shallow growing medium 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 to support a more diverse plant selection. 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 appropriate depth of any green roof depends on the roof structure, the plants which are chosen, the annual rainfall and stormwater performance requirements.

Specific roofing materials required

All three types of roofs require specific layers of roofing materials that are not found on regular roofs. The basic anatomy of a green roof consists of vegetation, growing medium, filter membrane, drainage layer, waterproof/root repellent layer, roofing membrane support for plantings above, thermal insulation, vapor control layer and structural roof support.

Every layer performs a specific function to keep the plants alive and to protect the structure beneath. The growing medium is different from a house plant or garden, as traditional soil is heavy.

A green roof growing medium is composed of mineral aggregates, organic material, good consistent drainage and aeration. It is lightweight, resistant to decomposition and compression, and physically and chemically stable.

Large or small planting beds

Some green roofs are installed in one large integrated section, while modular green roofs use small portable planting beds placed together to create a larger green roof. The modular units are often plastic or metal trays filled with a growing medium. Modular green roofs can be installed incrementally and are easily removed for maintenance and inspection of the roof layers underneath.

Additionally, the modular sections can often be cultivated in a greenhouse and be “ready to plant” on the green roof. Since the plants are already established, there are fewer issues with replacement plants that do not take root or thrive. One drawback, however, is that moisture cannot flow between units, increasing the impacts of wet and dry spells on the plants.

Full acknowledgement and thanks go to https://www.nps.gov for the information in this editorial.

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