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:
- Distinctiveness
- Condition
- 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.”
