What is Biodiversity Net Gain?
Introduced with the Environmental Act 2021, Biodiversity Net Gain (BNG) ensures new building developments leave the natural environment in an improved state compared to before construction began.
Although the Act is officially law in England, Biodiversity Net Gain is not expected to be enforced until 2023 at the earliest.
So what about Green roofs? Whether added to buildings, bus stops, or cycle shelters, they have been commonly understood to add green credentials to any project.
But do they contribute to Biodiversity Net Gain?
Understanding Biodiversity Net Gain
To determine biodiversity gain, The Biodiversity Metric 3.1 is used to calculate total ‘units’. These units are measured before and after the development is finished.
The new score, post-development, must be at least 10% more than before to pass the new laws, and thereby lead to biodiversity ‘gain’.
It might seem on the surface a crude measurement of natural habitats, but the stringent metric aims to ensure units are earned through correct identification of genuinely ‘diverse’, or ‘distinctive’ habitats.
Image credit: Figure 1. Breakdown of Biodiversity Net Gain metric calculations
How Can We Calculate Biodiversity Net Gain For Green Roofs?
To assess the quality of new habitats, the following criteria are used under the metric to help calculate units:
- Strategic significance
Described in Fig. 1, distinctiveness refers to the relative uniqueness of the habitat. For example, a scarce and rare habitat will score higher than a more common one. This alongside the condition are important when considering green roof types.
The Biodiversity Metric describes sedum blankets as: “A 60mm deep green roof (20mm sedum 40mmm substrate); and are of limited value as an offset for loss of biodiversity.”
Although basic sedum roof types can potentially contribute to BNG, they would score very low in terms of distinctiveness, especially in comparison to Biodiverse Green Roofs and to a lesser extent, intensive green roof types.
This could ultimately lead developers towards Biodiverse Green Roof designs over lower distinctiveness wildflower and sedum types. As we will come on to, these designs require deeper soil, greater plant variation, and different environments for supporting varied wildlife.
Intensive Green Roofs are classified as high maintenance green roofs designed as a park or ‘garden’ and includes shrubs, trees, perennials and grasses (including water features).
Biodiverse Green Roofs are designed specifically for biodiversity, and this is reflected in the distinctiveness rating. It is important to note that only by meeting the specific condition requirements can they be classified as biodiverse roofs.
The condition score rates the biodiversity value of the habitat against others of a similar type. So when used in conjunction with green roofs, this will ensure that both intensive and biodiverse roofs meet the correct standard.
For intensive green roofs: “A minimum of 50% native and 30% non-native wildflowers – 70% of the roof area is soil and vegetation (including water features)”
For Biodiverse green roofs: “A varied depth of 80 – 150mm at least 50% is at 150mm and is planted and seeded with wildflowers and sedums or is pre-prepared with sedums and wildflowers. To achieve Good condition some additional habitat, such as sand piles, logs etc should be present.”
Image credit: A green sedum roof for Glasgow Science Centre
A Shift Towards Quality, Not Quantity?
In terms of working towards BNG, new developments will likely score better for Biodiverse Green Roofs over sedum blankets.
This could ultimately lead developers towards using biodiverse designs, as it will greatly help and reduce the need for purchasing more BNG units. This, in turn, should help improve the overall quality of green roofs in our cities.
With 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 is a natural fit, helping support both active travel and the environment simultaneously.
We support all green roof designs within our Cubic cycle shelters, which can handle the deeper substrate necessary for biodiverse green roofs.