A building with a "green roof" or rooftop garden is an effective tool for creating environmental change in urban environments, where the heat caused by dark rooftops creates extreme temperatures. Garden roofs are useful in mitigating flood damage in climates where flooding is problematic as they absorb excess water. Unfortunately, while a rooftop garden addresses some environmental issues, the difficulty of installing and caring for it can outweigh the potential benefits.
There's little doubt that a green roof is environmentally beneficial, but installing and maintaining one is fraught with disadvantages. Green roofs are more expensive to install than traditional roofs, require intensive maintenance, irrigation and drainage systems, are limited by climate conditions, and can be costly to repair, should the need arise.
Extensive Green Roofs
An extensive green roof garden is one that is grown in shallow soil, under conditions that are less than ideal for most plant types. This type of green roof uses alpine and ground-cover-type plants and is less expensive than other roof gardens. The extensive roof garden has limited energy efficiency and the disadvantages of installing this type of garden include limitations on plant types, less rainwater retention and less energy efficiency than intensive-type green roofs. It also does not provide recreational gardening space due to limitations on plant types and reduces the home's exterior appearance during winter months. While this roof suffers many disadvantages, it is still the simplest and least costly green-roof option.
Intensive Green Roofs
Intensive green roofs use deep soil for planting deeper-rooted plants. The intensive green roof has a complex irrigation system, but allows for greater diversity among plants choices and is accessible for recreational gardening due to both grass and the wide variety of plant types available to this type of green roof. These seeming advantages are also some of the biggest disadvantages to planting this type of green roof. The complex irrigation system is expensive and difficult to maintain. The diversity of the plants used increases the cost and maintenance required, making it a more time-consuming garden. This kind of green roof also puts an immense amount of weight on the building's support structure and requires an intricate drainage system.
While standard roofs are expensive, the cost to install a green roof far exceeds the price of a standard roof. Cost can exceed 10 times that of more traditional roofs, and a green roof requires more insulation and waterproofing material than a traditional roof and more rigorous upkeep. Upkeep is important as waterproofing and irrigation must be rigorously maintained to prevent any moisture from a green roof from damaging the building's structure.