How blue green roofs can help developers to achieve biodiversity net gain

In January 2024, Biodiversity net gain (BNG) will become mandatory for developers in England. For small sites it will apply from April 2024. This new legislation will help ensure natural environments are not just protected but actually ‘nature positive’. It is a positive step for wildlife, supporting the development of habitat and contributing to the recovery of nature. The new biodiversity net gain mandate will, however, present a challenge for house builders and developers. This article explains how blue green roofs can be part of the solution. 

Key takeaways from this article

• How blue green roofs can provide biodiversity units for BNG calculations
• How a blue green roof is designed and planted will determine the level of biodiversity units (BU) provided
• Blue green roofs can be particularly useful for habitat restoration of dry grasslands, open mosaic habitats on previously developed land and for developments in urban areas
• Wavin PolderRoof transforms flat roofs into climate resilient intensive or extensive blue green roofs.
• The ‘blue’ aspect of PolderRoof provides a positive water balance with intelligent and controlled water reservoirs and an integrated irrigation function to maintain climate resilient habitats and avoid drought disturbance
• Best practice states that onsite mitigation for BNG including blue green roofs should be prioritised over off-setting
• Blue green roofs provide a range of additional climate resilience, sustainability and well-being benefits for developments

What is biodiversity net gain?

In a nutshell, biodiversity net gain means that for developments in the Town and Country Planning Act 1990, developers will need to:

• Try and avoid habitat loss on a piece of land that they plan to develop
• If this is not possible, create habitat onsite or off site or where this is not possible buy statutory credits from the government
• Work with an ecologist to calculate biodiversity value of a site and plan to achieve a net gain of 10% using the biodiversity metric (a habitat based approach)
• Get approval of BNG plan from local planning authority before starting to build

Where do blue green roofs fit in?

Research has shown that blue green roofs represent a fundamental means of ecological compensation within the built environment and provide an opportunity for habitat creation as well as providing a host of other benefits including sustainable drainage, improved building performance, carbon reduction (photosynthesis) and reduction of urban heat island effect with evapotranspiration.

Martin Lambley, Wavin Global Urban Climate Resilience Product Manager explains “The Wavin PolderRoof blue green roof system can create multifunctional water storage on flat roofs to prevent flooding during heavy rainfall and reuse for both irrigation and grey water. Rainfall is changed from waste into a resource and roofs can be used to provide habitats to boost biodiversity, sustainability gains and surface water management. This can help to meet planning legislation using roof space that might otherwise not be utilised.”

CIEEM, IEMA and CIRIA have created a guide for the industry, Biodiversity Net Gain Good Practice Principles for Development which suggests for small-scale low impact developments and those seeking a biodiversity net gain with no impact, both green roofs with a diversity of plant species and brown roofs with a range of substrates can be utilised.

Why are blue green roofs useful for increasing biodiversity?

The BSI Little Book of Biodiversity Net Gain explains how net gain is measured using the type, condition and area of habitats in terms of ‘Biodiversity Units’ (BUs) measured before and after a development. In practice this means creating more and/or better-quality habitats that will align with the metric requirements, support local conservation objectives for species, be suitable given local site conditions and be resilient to the effects of climate change. The Local Government Association Planning Advisory Service advises that green infrastructure features including blue green roofs can create a climate resilient habitat on site which contributes to BNG metric calculations.

Blue green roofs can be particularly beneficial on small urban sites where it can be difficult to find enough space to achieve biodiversity net gain and developers need to work to maximise the benefits of every available space including roof tops. With development land being a valuable commodity, careful utilisation of roof space will provide blue green infrastructure that can deliver multiple benefits including biodiversity.

Research has found that even the relatively few Sedum green roofs present in London provide effective habitat for a large number and diversity of invertebrates, but there was also evidence of rare invertebrates colonising green roofs in London. Dry grassland, a habitat area that is particularly at risk in the UK can be recreated on rooftops by combining grasses and wildflowers with the traditional sedum coverings and adding and decaying wood and sandy mounds to attract insects and birds.

Buglife’s best practice guide for creating green roofs for invertebrates advises that green roofs host a high percentage of species of conservation concern with all green roof categories (sedum and biodiverse) 15% of spiders and 10% of beetles recorded being of local or national importance, including notable and red data book species. Diverse spider faunas were observed including 10% of the whole UK fauna and 20% of greater London spider fauna.

A city of London rooftop survey in 2014 found that intensive green roofs, even as high as 16 floors above street level, do provide both nesting and foraging habitat for common breeding species of birds. Biodiverse green roofs made from crushed concrete and brick have been successful in providing habitats for and establishing breeding pairs for the Black Redstart, a target species for The City of London biodiversity action plan. This is attributed to the presence of the shrill carder bee, a rare ground nesting bee that burrows into shale on a roof garden. This is a perfect example of the complexity of ecosystems that can be supported on roof tops in urban areas. Studies of green roofs in New York have found that birds and arthropods use green roofs as a stopover habitat during migration and a foraging habitat during the breeding season helping to mitigate the loss of habitat due to increasing urbanisation.

Blue green roofs can be particularly useful for habitat restoration on brownfield sites

Certain habitats such as open mosaic habitats on previously developed land (OMH) score highly for biodiversity metrics and can be difficult and potentially expensive to compensate for offsite. These can often be found on brownfield land (land that has had a previous industrial use but can be built on) where the periodic disturbance and abandonment of the land can ‘restart’ succession and create a variety of habitats which are often richer in biodiversity than farmed countryside. According to the Association of Geotechnical & Geoenvironmental Specialists OMH is assigned a ‘high distinctiveness’ level and therefore losing OMH habitat will result in the loss of at least 6 units/ha in comparison to 2 units/ha that would be lost for bare ground. This represents a challenge for developers when restoring, off-setting or compensating in order to achieve biodiversity net gain. Blue green roofs can offer a possible source of biodiversity net gain credits to help compensate for this loss.

In their guide to creating green roofs for invertebrates, Bug Life explain how biodiverse roofs can replicate habitats like those found on wildlife-rich, low nutrient, free draining sites such as brownfield. It suggests using a local substrate and site spoil such as crushed brick and concrete to replicate local ground conditions on the roof. This has been shown to have a positive influence on the diversity of beetles and spiders which colonise roofs. The guide also recommends using a variety of materials such as sand and shingle. However, all materials used on a blue green roof should first be screened to prevent issues such as damage, over loading, effect on drainage and thermal properties and ecology e.g., highly fertile soils encouraging growth of invasive species.

How can developers maximise the biodiversity net gain with blue green roofs?

Green roofs can help to achieve biodiversity net gain however not all green roofs are equal! How the roof is specified, designed and planted will affect its biodiversity and the BNG credits it is assigned. Green roof structures can be specified as either extensive or intensive.

A green roof is classified for the BNG metric as being a roof or deck where vegetation is intentionally grown or habitats for wildlife are established. This can include brown roofs that are not purposefully planted but have a growing medium selected to allow indigenous plant species to inhabit the roof over time.

Extensive green roof systems are often planted with sedum plants which can be low in biodiversity. They can also be planted with mosses, grasses and wildflowers to increase biodiversity.

To be classified as a biodiverse green roof for the BNG metric, a variety of substrate depth is required (between 80 & 150mm, with at least 30% of roof 150mm deep) and planting or seeding with a variety of dry grassland species and sedums (with a ratio of 60:40 and more than 25 wildflower species). Additional credit can be obtained for other habitat features such as bricks, logs and decaying wood.

Intensive green roofs are classified as high maintenance green roofs that are designed as a park or garden and include shrubs, trees, perennials and grasses (more than 50% native and 30% non-native or pollinator interest). Importantly though, it stipulates at least 70 per cent of the roof should be soil and vegetation (including water features) with only 30% as hard standing or paving.

Blue green roofs are ‘green roofs’ that also features water storage to help manage surface water and prevent flooding during periods of high rainfall and provide water reuse for irrigation and grey water. Typically, they also conform to sustainable drainage systems (SuDS) regulations. They can also be classified as ‘biodiverse’ and ‘intensive’ for BNG calculations where they meet the required criteria.

BS 8683 is a new British Standard for the process of planning and implementing net gain , it can help with planning applications by proving that correct procedures have been followed, and this standard also offers guidance for how to effectively deliver net gain.

When planning for blue green roofs for biodiversity net gain, early expert advice should be sought regarding the building design, range of plant species used, BNG calculations and design based on sound ecological principles to optimise the roof function and local biodiversity value.

Wavin PolderRoof - the intelligent solution

The Wavin PolderRoof transforms flat roofs in existing or new buildings into intelligent and controlled water reservoirs with integrated irrigation function, forming an indispensable foundation for intensive or extensive (including biodiverse) green roofs.

How can blue green roofs help to protect habitats from drought associated with climate change?

Nature-based solutions, green infrastructure, and especially rooftops have been identified as critical components on the path to climate mitigation and adaptation, but water is critical to the ongoing maintenance of this green infrastructure. As climate change affects weather patterns periods of drought and extreme rainfall, green infrastructure will be maintained by water storage and intelligent systems that can ensure the right levels of water are available at the right times. This is where the ‘blue’ aspect of blue green roofs really come in!

Drought disturbance has been found to be one of the main controlling factors of assemblage development on blue green roofs with cover-abundance of perennial wildflower species in particular being strongly influenced by drought disturbance. 

Martin Lambley, Wavin Global Urban Climate Resilience Product Manager explains “Wavin’s PolderRoof features intelligent controlled water reservoirs with integrated irrigation functions that can help to avoid drought disturbance and achieve a positive water balance; by storing water for plants and evaporation, building cooling, or water reuse. During the growing season integrated irrigation can help to maintain optimum growing conditions.”

Blue green roofs as an alternative to off set

Good green space in a development adds value and makes it more desirable to homeowners. Planning early can help to maximise the effectiveness of green infrastructure and can help to achieve BNG in conjunction with other planning requirements such as amenity and sustainable drainage (SuDS) which is likely to be soon required under Schedule 3 of the Flood and Water Management Act 2010.

CIEEM’s best practice guide states that Offsetting is part of the mitigation hierarchy and should only be used if other options for reducing residual impacts on affected habitats (including retention, restoration or enhancement on-site) have been fully explored. Whilst onsite mitigation including blue green roofs may not always provide enough credits to achieve the required 10% net gain in biodiversity (as measured by the biodiversity metric), blue green roofs can still provide a contribution to an overall plan that will help to minimize the cost of offsetting, achieve best practice and add additional landscape value to the site.

Anthony Kolanko, Wavin UK Commercial Manager for Urban Climate Resilience commented “Rooftops offer extensive opportunities for climate adaptation and more sustainable buildings, both in the private and public sectors. They can play a key role in holistic solutions for more sustainable and climate resilient built environments. In our crowded urban environments where space is limited, it makes sense to utilise this space carefully for multiple gains.”

Blue green roofs bring more benefits than just biodiversity

In addition to biodiversity net gain blue green roofs can also provide an approved method of source control for SuDS (sustainable urban drainage) contributing to a holistic system which can boost both sustainability and climate resilience onsite and beyond.

Some of the key additional benefits include:

• Improvements to air quality
• Mitigating the urban heat island effect
• Reducing CO2
• Improving life expectancy of a roof
• Increased energy efficiency
• Improved acoustics