There are many different varieties of green roofs, all of which may incorporate some form of living plant matter. In urban and suburban environments, a green roof can be a way to make use of space that would otherwise be left bare. Container gardens on rooftops may be used as either vegetable or flower gardens. In other situations, a green roof may refer to a structure that is covered in sod or other vegetable matter. This can include individual homes, high-rises, and other modern structures that are designed specifically to support heavy vegetation.
Green roofs were first traditionally composed of two layers of sod laid over birch bark. The sod could then extend its roots down into the bark and help to hold it in place, potentially resulting in a more stable structure. This was a common type of roof in many Scandinavian countries. A variant known as the sod house was also found in Canada and the US during the original settling of the prairies in those countries. Instead of just using sod as a roof covering, the walls of of these houses were constructed of stacked up pieces of turf.
Sod is also sometimes used in the roofing of modern structures, though the process may differ. Rather than initially covering the roof in birch bark, a variety of roofing felts and water-proofing materials may be used. Similar materials and processes may also be employed when creating an extensive green roof. This type of building covering, like the sod roof, is typically composed of living vegetable matter that often requires very little maintenance after being installed.
The other main categories of green roofs are known as semi-intensive and intensive. These roofs can include items such as container gardens and other installations that may require a substantial amount of money or labor to maintain on a regular basis. An intensive or semi-intensive green roof may need to be watered regularly, and can involve other potential yearly investments such as the purchase and application of fertilizers. The result may be a sort of rooftop park or garden.
Designing a structure to have a green roof, or retrofitting an existing building, can be costly, though there may be a number of benefits. A green roof may contribute to the energy efficiency of a building, while the roof itself may last longer than a traditionally constructed one. If the roof includes container gardens, there is also the possibility of growing fresh produce. Certain green roofs may even help increase the property value.