Wildlife on your roof

A new NBS section on green roofs was published as part of update 43. Technical author Mike O’Brien gets his green finger out.

When considering the topics of biodiversity and wildlife habitat, the roof of the building you’re sitting in is not usually the first place you would consider but, in an urban environment, there can often be few other appropriate sites. The benefits of green roofs are numerous, including energy conservation and fuel efficiency, noise reduction, pollution reduction, storm water detention, extended roof life and improved building aesthetics. But one of the most compelling reasons for including a green roof on your building is that wildlife can be encouraged to colonize what could otherwise be a barren wasteland.

Wildlife

The Rio Earth Summit in 1992 led to the development of the UK government’s Biodiversity: The UK Action Plan in 1994, and the establishment of a steering group which published national biodiversity targets in 1995. Initially, few of these targets related to habitats or species in urban environments but, over the past five years, local action plans encouraging the development of urban habitats for black redstarts, peregrine falcons and long-tongued bumblebees (amongst others) have been published.

Urban brownfield sites that have been vacant for decades are now being rapidly developed. This regeneration is displacing a surprising diversity of wildlife that has adapted to the derelict industrial and waste ground environments found on many of these sites. One species whose habitat needs are driving an expansion in urban green roof construction is the black redstart. A small songbird, listed in Annex 1 of the European Habitats Directive and Schedule 1 of the Wildlife and Countryside Act 1981, its natural habit in continental Europe includes cliff faces. It is at the north western limit of its range in colonizing brownfield sites in London and Birmingham. It tends to concentrate its activities near areas of human habitation, on sparse stony ground, so an extensive green roof would be a suitable habitat.

Many other bird, insect and spider species could benefit from green roof habitat. They could be translocated to a suitably large-scale roofscape or encouraged to colonize a roof by providing a vertical link to surrounding ground level habitats in the form of a planted gabion wall or climbing vines. For more detailed information on the development of green roof design for the benefit of black redstarts, and other species, see the website www.blackredstarts.org.uk.

The Technology

Advances in flat roof waterproofing technology in the 1970s encouraged the development of green roofs and the expansion of specialist green roof companies in Germany and Switzerland. Since then the technology has extended throughout Europe and to North America. However, the majority of green roof research is still carried out in Germany and Switzerland. The document Guideline for the planning, execution and upkeep of green roof sites is the definitive standard for green roof construction, published in English by the German Landscape Development and Landscape Construction Research Facility in 2004 (see www.f-l-l.de or www.roofmeadows.com).

There are several manufacturers in the UK market who supply all of the components required to establish an extensive green roof, from the waterproofing and insulation to the growing medium and plants. These ‘systems’ often contain artificial constituents in the protection layers and growing medium. Standard sedum (stonecrop) planting mixes have been developed to suit the UK environment, but are not specifically designed to encourage particular threatened species. These mixes are available as either pre-planted vegetation mats or as individual plug plantings. The main advantage of the ‘system’ installation is the assurance that adequate drainage is included and that a warranty for the waterproofing layer is provided. However, recent research from Switzerland indicates that the most environmentally diverse roofscapes can be created by using natural soils and installing plants of local origin and provenance.

Conclusion

The English Nature report Green roofs: Their existing status and potential for conserving biodiversity in urban areas (www.english-nature.org.uk) promotes the development of ‘building-integrated habitat’ encompassing planting, growing medium, perches, roosts, nesting containers and other wildlife-related apparatus. Further research into the specific needs of threatened species in the UK should be encouraged and local authorities should promote the inclusion of green roof technology in urban regeneration schemes. Green roofs are likely to play an increasing role in maintaining, and expanding, habitat for threatened urban species in the UK and in preserving the overall extent of green space in our city centre.

Confirmed as current June 2007