text: Green roofs can be categorized as intensive, semi-intensive, or extensive, depending on the depth of planting medium and the amount of maintenance they need. Extensive green roofs traditionally support  of vegetation while intensive roofs support  of vegetation. Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are considered intensive because they are labour-intensive, requiring irrigation, feeding, and other maintenance.  Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.
  Extensive green roofs, by contrast, are designed to be virtually self-sustaining and should require only a minimum of maintenance, perhaps a once-yearly weeding or an application of slow-release fertiliser to boost growth.  Extensive roofs are usually only accessed for maintenance. They can be established on a very thin layer of soil (most use specially formulated composts): even a thin layer of rockwool laid directly onto a watertight roof can support a planting of Sedum species and mosses.  Some green roof designs incorporate both intensive and extensive elements. To protect the roof, a waterproofing membrane is often used, which is manufactured to remain watertight in extreme conditions including constant dampness, ponding water, high and low alkaline conditions and exposure to plant roots, fungi and bacterial organisms.
  Advances in green roof technology have led to the development of new systems that do not fit into the traditional classification of green roof types. Comprehensive green roofs bring the most advantageous qualities of extensive and intensive green roofs together. Comprehensive green roofs support plant varieties typically seen in intensive green roofs at the depth and weight of an extensive green roof system.
  Another important distinction is between pitched green roofs and flat green roofs.  Pitched sod roofs, a traditional feature of many Scandinavian buildings, tend to be of a simpler design than flat green roofs.  This is because the pitch of the roof reduces the risk of water penetrating through the roof structure, allowing the use of fewer waterproofing and drainage layers.