Green Roof Systems on External Storage: Securing BNG through Sustainable Urban Solutions

Table of Contents:

• The Impact of Urbanisation
• Sustainable Solutions in Urban Environments
• Green Roofs on External Storage
• Ensuring Urban Sustainability with Green Roofs

Under the National Planning Policy Framework and the Environmental Bill 2021, all new developments within England now have an obligation to deliver measurable Biodiversity Net Gain (BNG), which mandates a minimum 10% net gain. Measured using the Department of Food and Rural Affairs (DEFRA) biodiversity metric, habitats need to be secured for a minimum 30 year period. These changes are set to be enforced in November 2023.

This article explores the contribution towards BNG that green roof systems on external storage can make, first exploring the impact of urbanisation and the relevance of green roofs as part of requirements for sustainable solutions in modern urban environments.

The Impact of Urbanisation

One of the major concerns surrounding urbanisation is that it is helping to accelerate one of the most globally-pertinent issues today, environmental degradation. Intensive urban growth can ultimately create a situation where local governments are unable to provide services for their whole population, while the ever-increasing density and demand of urban populations has collectively to poor air and water quality (the UK consistently ranks among the worst countries in Europe for our quality of water management), concentrated, high-energy consumption, and waste-management issues which can create multiple health hazards.

Urban heat islands, the result of replacing natural land with dense concentrations of surfaces that absorb and retain large quantities of heat, leading to an increase in regional temperatures and higher energy costs garnered through the use of air conditioning, in turn causing air pollution. Paved concrete has also been shown to increase water runoff and decrease soil quality through erosion.

Urbanisation typically results in habitat loss, leading to decreased levels of biodiversity. Despite active legislation intended to protect biodiversity and wildlife in the UK, there has been a 13% decline in the average abundance of wildlife since the 1970s, with 133 of 8431 species already extinct from Great Britain alone. The biggest drivers of these troubling statistics are urbanisation, climate change, agricultural management, pollution and urbanisation. It’s worth noting that 1600 miles of road were constructed in Great Britain between 2006 – 2018.

Sustainable Solutions in Urban Environments

What arises from the impact of urbanisation is the need for sustainable solutions to create healthier urban environments. Air pollution can be reduced by ensuring less reliance on fossil fuel consumption by fully embracing sustainable energy generators like solar, wind, and electric, while the impact of excessive car travel can be reduced by encouraging alternative urban transport systems, such as e-bikes.

Ultimately, urban planning in the years ahead must be more centralised around sustainability and the increasing development of city green spaces, as well as promoting the growth and care of urban tree cover. Singapore is one city-state setting a leading example of how this can be successfully achieved, in particular with their Gardens by the Bay attraction.

As a sustainable urban solution that additionally helps to enhance green spaces, green roof systems can play an integral role in enhancing biodiversity in urban environments. Establishing a habitat for flora, fauna, insects and birds, green roof systems help to reduce the amount of Co2 in the air. Just 1m2 of green roof is able to absorb an annual 5 kg of Co2 which, as a perspective, is around the same quantity of Co2 that a regular car emits on a 80km drive.

Green Roofs on External Storage

For residential & commercial developments in the UK, integrating a green roof system to external storage can secure additional BREEAM or Sustainable Housing Code value, as well as being a valuable resource for achieving Biodiversity Net Gain (BNG) targets. BNG is the UK Government’s sustainability strategy towards land developments that it hopes will provide an essential pathway towards ensuring the maintenance and recovery of nature and biodiversity in the UK. BNG aims to mitigate habitat losses by ensuring new developments give distinct consideration to the creation of essential new habitats, as well as enhancing those that already exist.

External storage systems are readily adaptable to support two types of green roof systems; sedum only and sedum and wildflower, ensuring BNG targets can be attained without the complexity of design and construction processes commonly associated with a bespoke build. Ideal for specification on bin stores, bike stores and storage units, their lower height from ground level and uncomplicated construction of these structures help to ensure original build and ongoing maintenance costs are reduced.

Specifying a green roof system on external storage provides a largely undisturbed space for plants and wildlife, significantly reducing water run-off, and reducing air pollution, while enabling valuable biodiversity to be introduced into urban environments. They help to provide a consistency of design, helping external stores reduce their visual impact to both blend in and soften the urban landscape. More general design considerations will cover aspects such as use, location and installation costs, however considering a number of technical design elements is necessary to ensure optimum output from a green roof system and 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.

In relation to BNG, the biodiversity metric assesses the values of an ecosystem by considering the relative features of a specific habitat in order to calculate its biodiversity value. The metric can be asserted by a range of parties, and considers 5 key aspects: Type, Distinctiveness, Condition, Connectivity, and Size.

External storage systems can be designed to incorporate either extensive or intensive green roof systems. Alongside living walls and vegetated drainage systems, green roofs are currently the most effective solutions for delivering high levels of BNG on land developments, helping to highlight the need for sustainable solutions in urban environments.

Ensuring Urban Sustainability with Green Roofs

In conclusion, the integration of green roof systems to external storage systems, achievable without complication, not only aligns with the BNG requirements of the Environment Bill 2021, but also contributes significantly to creating healthier and more sustainable urban environments.

Green roof systems combat urbanisation challenges, enhance biodiversity, and offer practical solutions for reducing environmental impact. Ultimately, embracing and integrating green roof systems as part of sustainable developments, can provide crucial pathways towards a greener and more resilient future.