Achieving Biodiversity Net Gain with External Storage Buildings

Table of Contents:

• How Will BNG be Measured?
• Specifying Green Roofs for External Storage
• What Are the Technical Design Considerations?

An approach to development, and land management and acquisition that aims to deliver measurable more for nature and habitats than what is lost through development, Biodiversity Net Gain (BNG) is set to come into effect in January 2024, following a delay from the initial commencement date of November 2023.

There are 6 Statutory Instruments (SIs) designed to help deliver the new BNG framework, applying to most new major developments under the Town and Country Planning Act (TCPA), and to small sites from April 2024.

How Will BNG be Measured?

The metric will measure biodiversity in standardised biodiversity units, assessing the values of an ecosystem by considering the relative features of a specific habitat to calculate its biodiversity value. A habitat will contain a number of biodiversity units, considers aspects such as its:

• Size
• Quality
• Location
• Type 

The biodiversity metric measures how many units a habitat contains before development, and how many units are needed to replace the units of habitat lost and to achieve the minimum requirement of 10% BNG. Assessments for BNG are required in the planning permission stage, and this ensures that any recent or deliberate harm to the biodiversity value of a site can be taken into account. This is preceded by a preliminary ecological appraisal, involving a baseline ecological impact assessment that identifies any priority habitats as part of the metric.

The biodiversity metric tool can be used by – ecologists undertaking a biodiversity assessment; developers who have commissioned an assessment; planning authorities interpreting outputs as part of a planning application, in addition to communities assessing the impacts of a local development. Further surveys are conducted upon completion of the development, with BNG successful if the score is higher in post-development than in the planning phase.

Specifying Green Roofs for External Storage

Green roofs as building features are currently some of the most effective solutions for delivering the highest BNG on developments. External storage units can be designed to accommodate both extensive or intensive green roof types; available as sedum only, or sedum and wildflower.

Extensive green roofs are typically more common, with a sedum-based turf with grass types and sedum plant species. They provide a lightweight and low-maintenance solution, and are chosen mostly largely for their ecological benefits. Intensive green roofs tend to require more intense landscaping and are suited to units placed prominently in the built environment.

Specifying a green roof system can additionally help secure BREEAM accreditation or Sustainable Housing Code value without the complexity of a bespoke build. When considering whether green roof types for securing BNG, the metric utilises a specific criteria that assesses the distinctiveness, strategic significance, and condition 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.

What Are the Technical Design Considerations?

For a code of best practice, the GRO Code of Best Practice (2021) helps to guide actions relating to green roof design, specification, installation and maintenance. As per National Building Specification (NBS) eurocodes and British Board of Agrement (BBA) certification process, considering technical design elements helps to meet criteria guidelines:

• Structural Design – Complying with all relevant structural design criteria, as per BS EN 1990:2002 Eurocodes. The loading guide for green roofs advises 65 kg/m2 for a lightweight extensive sedum roof; 175 kg/m2 for a biodiverse wildflower roof, and 250-400 kg/m2 for an intensive roof.
• Waterproofing – The primary waterproofing layer is critical to the performance of a green roof system. Its suitability must be assured by the BBA Certification process and needs to offer permanent protection by a membrane which prevents plant roots from growing through it. 
• Substrate – Specifically designed to hold the optimum level of moisture, nutrients and air required for sustainable plant development, the growing medium typically contains a mix of organic and porous inorganic material.
• Vegetation Type – A sedum mat is reliable and is able to be established quickly, while a wildflower mat offers a wider selection of species. Seed sowing tends to be a long and slow process but can enable a bespoke mix to be integrated. As an alternative, pre-cultivated plugs are a reliable bespoke method that are planted directly into the substrate growing medium, colonising the roof area to provide an even vegetative cover over time.
• Fire Resistance – As per BS EN 1990:2002 Eurocodes, green roofs should be designed to provide the necessary resistance to the external spread of fire. The substrate depth should be a minimum of 80mm, with a maximum 20% organic content. Fire breaks typically consist of non-vegetated strips and should be provided with a diameter of 30mm, or 50mm if using green roof pebbles.
• Access and Maintenance – All green roofs require some degree of maintenance. Providing edge protection and a means of safe access by a roof anchorage system or containment system is strongly recommended, alongside a fall protection system to ensure safety for maintenance workers.