Specifying the right substrate is key to allow a green roof to deliver its full potential
The roll out of green roofs across UK towns and cities has created a market that’s expanding at a rate of 17% each year.
Effective installations can tackle air pollution, reduce urban heat and rainwater runoff, or even cut energy costs, but failures occur – often as a result of a poorly performing substrate, or growing media.
Lacking proper specification information on substrates, UK architects have historically reverted to ill-fitting German standards. But the release of British Standard BSI 8616 earlier this year has changed that.
Developed by green roof testing laboratory STRI Group, in partnership with the University of East London’s SRI Group, BSI and other partners, the standard provides detailed guidelines on substrates and related test methods for green roofs and extensive green roofs.
Dr Tom Young, technology manager at STRI, told RIBAJ: ‘This gives architects and landscape architects something to quote against and a standard to ensure the substrate has been tested to specific parameters. At the moment there's a lot of miscommunication, varied testing methods, and people lack knowledge of what a good green roof substrate should look like. That’s why you often see failures.’
Substrates have multiple tasks to perform and these can differ according to depth, vegetation type, climate and even slope. The new standard considers particle size, organic matter content, physical and chemical properties, and analysis of pH. Test methods cover sampling, procedure, and results.
Recommended performance ranges for different substrates are provided, rather than strict targets, to allow for different use cases and to encourage innovation.
Choosing the most suitable growing media typically involves a trade off between various factors, says Young: ‘The weight loading on the roof is something the structural engineer is always is trying to keep to a minimum, but you need enough organic matter in the substrate to ensure that plants grow healthily. Bigger plants with more leaf area increase ‘green roof services’, in terms of improvements to air quality, but substrates that promote excessive growth can create a burden for ongoing maintenance and may produce weak plants that die in a summer drought.’
Young hopes the standard will overcome a tendency for some designers to specify a green roof without considering what it will entail. British wildflower roofs are becoming more popular in the UK, but while native varieties can thrive in the spring and summer, they die back in winter and turn brown. ‘It’s important to gauge expectations for a green roof during the design process,’ Young concludes. ‘The substrate will play a big part in ensuring they are met.’