A British Standard for Green Roof Substrates

What are green roofs?

Green roofs are typically flat rooftops that have been intentionally planted with various hardy, native and wildflower species with the dual aims of enhancing building efficacy and urban biodiversity.  There are many reported benefits of installing green roofs to buildings, including improved energy efficiency, storm water retention, aesthetic value and mitigation of habitat loss in cities; by providing homes for local biodiversity often lost through new building projects.

Green roof on the gift shop at ZSL London Zoo. Image by Chloe Molineux.

Currently, there is no agreed standard for construction of these green roofs in the UK, meaning that they often fail to fulfil the engineering and ecological criteria for which they are designed. There is also the issue of quality, at present there are no go-to standards for products to be comparable in the market place – this can be misleading for clients as different characteristics can be calculated by a variety of methods producing a spectrum of values that have little meaning.

SRI’s research

I have been working on the growing media (or substrate) of green roofs since 2006. My work has focused on the components of these ‘engineered soils’ and how recycled materials could be incorporated into the blends to promote successful plant growth on rooftops.

Green roof substrates are generally comprised of a blend of 70-80% lightweight aggregate (such a crushed bricks, crushed roof tiles, LECA (lightweight expanded clay aggregate) or Lytag) with a small amount of organic material (like a PAS 100 compost or loam) and sometimes sand. My research has also looked at the potential for alternative aggregates to be used, including those produced from limestone quarry wastes, and sewage sludge ash and paper ash – waste bi-products from the treatments of sewage and newspaper recycling respectively.

Example of a substrate using LECA and Limestone waste. Image by Chloe Molineux.

Testing standards

At present, green roof substrates are only able to be tested using British Standard topsoil specifications or the German green roof guideline recommendations (FLL, Forschungsgesellschaft Landschaftsentwicklung Landschaftbau) – which are heavily based on DIN soil standards and landscaping methodologies. These methods are not ideal for green roof substrates due to the predominant aggregate content within blends. This is a big issue for the green roofing industry because it is difficult to correctly determine the growing media characteristics and therefore is almost impossible to fairly compare substrate products in the market place.

The British Standards Institution

Producing a British Standard for green roof substrates has been an ultimate goal for me since completing my PhD in 2010. Fortunately, in 2017 I was finally able to begin working with the British Standards Institution (BSI) to develop such a testing standard. Firstly, a BSI committee panel was selected with experts in different green roof and soil science fields. Members of this committee also included landscape architects and substrate manufacturers/suppliers. In particular, I worked with Dr Tom Young, from the Sports Turf Research Institute (STRI), to produce a set of working green roof substrate testing methods that are required for several material characteristics.

These included:

• Particle Size Distribution (% Mass)

• Loose Bulk Density (Mg/m3)

• Particle Density (g/cm3)

• Porosity

• Water Permeability (mm/min)

• Water Content (% Mass/Vol)

• pH Values

• Salt content (g/l)

• Organics (%)

• Nutrient analysis (mg/l)

• C:N (Carbon:Nitrogen) Ratio

• Visible Contaminants and Phytotoxic Elements
  

In addition to detailing precise testing methods, optimal parameters which the substrates should fall within have also been included. This should allow all green roof stakeholders to have a benchmark for substrate products in the market place, and help inform their choices when green roofs are specified, designed, procured and installed.

BS 8616

The Standard, BS 8616: Specification for performance parameters and test methods for green roof substrates, is currently out for public comment – it can be viewed here – http://standardsdevelopment.bsigroup.com/projects/2018-00765

It is hoped that final publication will be around Summer 2019.

By Dr Chloe Molineux