But there remains massive potential to put the UK's rooftops to better environmental use. A report by English Nature published just over a decade ago reported that there was scope to green more than 20,000 hectares of urban roofscape across the UK as a whole.
That report clearly identified the many benefits of green roofs, which included: slowing rainwater run-off rates; providing precious wildlife habitat and attractive open space within towns and cities; and protecting the building fabric from sunlight and temperature fluctuations. Developer British Land has long been conscious of such benefits. It first included green roofs in its sustainability brief for development more than a decade ago in 2004. Since then it has created green roofs on 12 new buildings, and it has been retrofitting existing buildings using the Pocket Habitat modular vegetation system developed for it by consultant Arup. The developer has now made green roofs standard on its central London developments and birds, insects and occupiers alike reap the benefits of the diverse high-level green spaces being created.
Sarah Cary, sustainable developments executive at British Land, says green roofs are evolving from the simple sedum mat. As an example she cites the developer’s Clarges Mayfair office and residential scheme in London’s West End, which is currently under construction and will boast a rooftop heather moor. Cary adds: "People are getting much more innovative and creative as the industry has got better at coming up with products and architects have got better at doing it".
One of British Land’s major green roof installations is at Regent’s Place, close to London’s Warren Street station. This new quarter of the capital comprises a series of buildings - a mix of offices, retail space and apartments - with some 50,000sq ft of green roof space and gardens, with buildings having either a natural or an intensively managed green roof.
Green roof design on the development was informed by a two-year pilot project with consultant Arup. This involved laying out the roof in a ‘puzzle’ pattern, comprising different growing mediums, seed mixes and plant arrangements, the pattern being selected according to sunlight and shading influences. Biodiversity is promoted by locating diverse micro-habitats adjacent to one another. As well as having biodiversity value, the roofs are attractive to look at and provide welcome amenity space.
The Regent’s Place green roofs include:
• Sparsely-vegetated ground and sedum
• Wildflower mix and organic substrate for birds and butterflies
• Border plants and shrubs for garden areas
• The largest insect hotel for a London commercial building. Occupier Lend Lease installed the 24 habitat walls of its building, 20 Triton Street building
• Four beehives on the eighth floor terrace of one building, 10 Triton Street, which were installed by occupier Aegis Group.
• Cary says that initially British Land was sceptical about green roofs, but that has clearly changed: "We now market our green roofs. Occupiers like the visual interest, and people like having the space outdoors. They’re one of our key selling points."
• The occupiers’ enthusiasm for their green roofs is evident in the fact that some have enhanced their environmental value by adding beehives and insect walls
• Studies have found the roofs add significant biodiversity value. For example, soon after they were planted two buildings, 10 and 20 Triton Street, became home to insects including bees, hoverflies and ladybirds, and notably a male black redstart was sighted there
• The ‘puzzle’ pattern layout proved successful in optimising biodiversity and is now being repeated on British Land’s new 5 Broadgate development.
The advantages of green roofs like those at Regent’s Place could go further. New research from the University of Melbourne in Australia, suggests that green roofs could benefit occupiers by boosting their workers’ attention spans. The university found that workers who had a 40-second glimpse of a green roof performed better in some basic tests. The full results are published in the Journal of Environmental Psychology.
• Cary says it is important to consider substrate levels to ensure that disabled access from building to roof garden can be maintained
• It is also important to give careful consideration to wind. Cary says, "We did some planting in a few windy areas that unfortunately didn’t survive."
In 2010, British Land produced its report, Encouraging biodiversity. Its learning points included:
• Consider biodiversity as early as possible in the property cycle, and then at every stage from acquisition, design, development and management, through to refurbishment or sale
• Landscape architects and ecologists need to work together at all stages of the project
• Locate varied habitats close together, such as bare substrates and short vegetation, to encourage invertebrates
• Choose native plant species and appropriate wildflower mixes.
This case study is from Horticulture Week's Landscape4Places campaign hub. Landscape4Places is a new campaign which seeks to highlight the contribution of quality landscaping to great placemaking. For more on the campaign, go to www.horticultureweek.co.uk/landscape-for-places