How Can Green Roofs Contribute Biodiversity Net Gain?

Green roofs are increasingly recognised as a critical tool in the pursuit of nature-positive construction practices. This article seeks to explore the role of green roofs in contributing to Biodiversity Net Gain (BNG), a concept introduced by the UK Government in the Environmental Act 2021.

Introduction to Biodiversity Net Gain

The concept of Biodiversity Net Gain (BNG) was introduced to ensure that new construction projects not only safeguard the natural environment but also actively enhance it. Despite being part of English law, BNG enforcement isn’t anticipated until 2023 or later.

BNG works on the principle of ‘measure-to-improve’. It uses a system known as The Biodiversity Metric 3.1 to calculate biodiversity ‘units’ both before and after the completion of a development project. For a project to be considered a success in terms of BNG, the post-development score must be at least 10% higher than the pre-development score. This system ensures that credits are earned through the correct identification of genuinely diverse or distinctive habitats.

The Appeal of Green Roofs

The concept of green roofs, whether integrated into buildings, bus stops, or cycle shelters, is increasingly popular as a means of bolstering a project’s green credentials. But the question remains: do they contribute to Biodiversity Net Gain?

The answer lies in the quality and distinctiveness of the new habitats created. The Biodiversity Metric assesses the quality of new habitats based on the following criteria:

1. Distinctiveness
2. Condition
3. Strategic significance

When it comes to green roofs, these parameters play a critical role in determining their contribution to BNG.

Evaluating Green Roofs for Biodiversity Net Gain

Green roofs can be classified into three main types: sedum blankets, intensive green roofs, and biodiverse green roofs. Each type varies in terms of distinctiveness and condition, which impacts their BNG score.

Sedum Blankets

Sedum blankets are described as “A 60mm deep green roof (20mm sedum 40mm substrate); and are of limited value as an offset for loss of biodiversity.” While they can contribute to BNG, they score quite low in terms of distinctiveness, especially when compared to biodiverse green roofs or intensive green roofs.

Intensive Green Roofs

Intensive green roofs are high-maintenance green roofs designed to mimic a park or garden. They include shrubs, trees, perennials, and grasses, and often feature water elements. To meet the biodiversity standard, at least 50% of the roof area should be native wildflowers, and 30% non-native wildflowers, with the remaining 20% consisting of soil, vegetation, and water features.

Biodiverse Green Roofs

Biodiverse green roofs are designed specifically for biodiversity. To be classified as a biodiverse roof and meet the ‘Good’ condition score, the roof should have a varied depth of 80 – 150mm, at least 50% of which is at 150mm. It should be planted and seeded with wildflowers and sedums or pre-prepared with these. Additional habitat features, such as sand piles or logs, should also be present.

The Shift towards Quality over Quantity

When it comes to BNG, developers are likely to score better for biodiverse green roofs than sedum blankets. This could lead developers towards designing biodiverse roofs, as they can reduce the need for purchasing more BNG units while improving the overall quality of green roofs in urban environments.

By creating fully biodiverse green roofs, we can better support a wider range of wildlife, including bees and rare bird species. Combining these measures with cycle hubs and shelters can also support both active travel and the environment simultaneously.

The Role of Blue Green Roofs

Blue green roofs offer an innovative means of ecological compensation within the built environment. They not only provide opportunities for habitat creation, but also offer other benefits such as sustainable drainage, improved building performance, carbon reduction through photosynthesis, and mitigation of the urban heat island effect via evapotranspiration.

Blue green roofs can be especially beneficial on small urban sites where space constraints make it challenging to achieve biodiversity net gain. By utilising every available space, including rooftops, developers can make significant strides towards achieving their BNG goals.

Key Takeaways

The introduction of the BNG mandate is a positive step towards sustainable development. It encourages developers to adopt strategies that not only protect but enhance biodiversity. Green roofs, particularly biodiverse and blue-green roofs, can play a significant role in achieving these goals.

By focusing on quality rather than quantity, and by prioritising distinctive and diverse habitats, developers can significantly contribute to BNG while also creating healthier, more sustainable urban environments. With the right approach and strategic planning, the mandate for Biodiversity Net Gain can usher in a new era of nature-positive development.