In areas where nature has been destroyed by construction works and the ground is sealed, extensive green roofs can partially compensate for lost green areas and can provide replacement habitats for flora and fauna. Wild bees, butterflies and ground beetles find food and shelter there. However, the development of biodiversity depends to a great extent on how the habitats that are provided for the flora and fauna on a roof are structured. Pure sedum green roofs that are frequently installed in conjunction with very shallow substrate depths are not suitable for exploiting this potential. Indeed, the biotope function of greened roof areas can be specifically fostered with very little work using various design features and applying basic biodiversity principles during the planning and implementation stages.
These are for example extending the range of plant species by using various substrate depths. While a depth of 8 cm is sufficient for low-growing, low-maintenance sedum types and other succulents, the root area for biodiverse herb, grass and wildflower vegetation can be increased with individual landscape mounds to a depth of between 12 and 15 cm. When choosing the plants, particular attention can be paid to their significance as forage plants for insects and birds. Dead branches and logs are particularly valuable structural elements for biodiverse roofs. They are used as a habitat by moss, lichens, fungi, beetles and flies. Vegetation-free spots with pebbles, boulders, gravels, and sands are an important enrichment, as they are used by insects and other roof inhabitants as a refuge, a breeding place and a sun trap. Additional nesting aids such as bug hotels serve to encourage the ongoing insect colonisation. Finally, even temporary water areas can be included in order to improve the availability of water for insects and birds. In some places, sheeting is laid out and covered with sand in order to retain rainwater on the roof for a longer period of time.
