All new developments within England will, come January 2024, have an obligation to deliver measurable Biodiversity Net Gain (BNG), which mandates a minimum 10% net gain. Alongside living walls and vegetated drainage systems, green roofs are currently the most effective solutions for delivering BNG on land developments.
External storage systems can be designed to incorporate either extensive or intensive green roof systems, however, as the quantity and variety of green roofs has increased, so too has the diversity of green roofs available. As a result, the differences between an intensive green roof and an extensive green roof are not as distinct as they once were.
Intensive v Extensive
Whether a green roof is described as extensive or intensive depends on certain key aspects, predominantly the substrate depth and plant types used. An extensive green roof usually has a shallow layer of substrate, typically less than 200mm, and covers a large area, while an intensive green roof has deeper layers of substrate, but are confined to smaller areas.
Extensive green roofs are more a common installation and provide a light-weight, low maintenance solution, and they are chosen mostly for their ecological benefits and opportunities to encourage biodiversity. As the name suggests, intensive green roofs tend to require more intense landscaping requirements and are suited to structures with a visible presence in the surrounding area.
Plant Types
Plant types define the green roof type. Alongside the overall intention of the installation, such as whether it is implemented to sustain local biodiversity, or to improve views from surrounding buildings to create a more pleasing environment, they help to determine whether an intensive or extensive roof is best suited to a specific project.
An extensive green roof is a sedum-based turf with grass types and sedum plant species.
Sedum vegetation can withstand harsh, urban conditions and, due to its drought tolerant ability, minimise water loss.
A larger range of plant species are available on intensive green roofs, with the turf containing a mix of sedum, wildflowers, herbs and flowering perennials.
Intensive green roofs are well-suited to locations with a highly visible presence; however, maintenance requirements are higher, with dead growth removal required at the end of the flowering season.
Specifying Green Roofs on External Storage
With the potential to reproduce varied ecosystems in environments eroded by urbanisation, specifying a green roof system on external storage helps to establish a consistency of design, reducing visual impact by enabling the specified structure to blend in and soften the urban landscape.
External storage units can be designed to incorporate either extensive or intensive green roof systems, and are available as sedum only, and sedum and wildflower. The lower height from ground level and uncomplicated construction of these structures help to ensure that both original build and ongoing maintenance costs are reduced.
Specifying a green roof system on external storage can help secure additional BREEAM accreditation or Sustainable Housing Code value, while ensuring BNG targets can be attained without the complexity of design and construction processes commonly associated with a bespoke build. When considering whether intensive or extensive green roof types are best for securing BNG targets, the biodiversity metric utilises a specific criteria that assesses the distinctiveness, condition and strategic significance of a unit.
Distinctiveness: Provides a score that is measured based on the type of habitat and its distinguishing features. The metric automatically assigns a distinctiveness category based on habitat type. Distinct and rare habitats with a dense collection of plant life will score higher than a more common green roof with mostly moss and grass.
Strategic significance: Based on the local significance of the habitat based on its location and habitat type. A high score is granted when the location has been identified within a local plan or strategy as being ecologically important for the specific habitat type, or where the habitat has been identified as being locally ecologically important.
Condition: The score is a measure of the state of a habitat. This is often linked to past and present management and land use, and is a way of measuring variation in the quality of habitat against others of the same habitat type. Using this criteria helps to ensure the habitat meets the correct standard.
More basic sedum roof types can contribute to BNG but tend to score lower in terms of distinctiveness in comparison to intensive green roofs with more diverse plant life. Intensive green roofs are classified as high maintenance and should have a minimum 50% native and 30% non-native wildflowers, with 70% of the roof area consisting of soil and vegetation, including any water features. Achieving a ‘Good’ condition for a green roof will depend on whether it is combined with elements of additional habitat.
Determining the Right Choice
Integrating green roof systems into external storage units is a natural fit, helping support both sustainable development and environmental principles simultaneously. Determining whether to specify an intensive or extensive green roof will largely depend on the green roof’s primary purpose, such as whether to sustain biodiversity, enhance on-site design, or reduce energy consumption.
In terms of working towards BNG, new developments will typically score higher for utilising biodiverse green roofs over simple sedum blankets. While extensive green roofs offer a lightweight, low-maintenance solution with a shallow substrate depth that is suitable for promoting biodiversity, intensive green roofs feature deeper substrates and more diverse native and non-native wildflower species. Combining these with elements of additional habitats is key to achieving a ‘Good’ condition
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