Lee Evans looks at common problems with sedum roofs and discusses research into a Swiss-inspired alternative.
Often when clients, architects or specifiers think about green roofs they think of sedum, small succulents which are naturally occurring around the northern hemisphere. So much so that sedum roofs dominate UK green roof specifications, comprising perhaps as much as 85% by area of a £100M annual market. Sedum is popular because it’s claimed to be low cost (less growing medium), low weight (less implied cost to strengthen the structure below) and considered ‘fit and forget’, i.e. low or no maintenance (O&M). The argument goes that because they can be found in relatively thin soil and often in exposed locations, they are ideal plants to put in the extreme conditions of a green roof. In continental Europe they tend to be sown using cuttings or seeds but in the UK, laying-on of pre-grown rolls is more frequently undertaken.
Yet, if you look at the small print, you’ll find that there is a stipulation for permanent irrigation and annual feeding. Lets take these two points separately. Most of us will have seen red and scruffy looking ‘green roofs’. Although some species of sedum plants go a lovely deep red colour during the year, most are green when healthy. The red colour indicates stress, normally due to inadequate water over an extended period and happens when the plant ‘switches off’ its chlorophyll – it literally stops processing sunlight. Instead of the 60mm of substrate plus whatever soil is in the blanket (as per the industry guidelines), we very often see 20 or 30mm at best. Some major suppliers actually sell systems with no substrate at all.
Similarly the ‘fit and forget’ claim that is made for most sedum isn’t really true in most cases. Not only do they need regular watering in most locations to ensure they survive, but the horticultural inputs – slow release organic fertiliser applied every spring – actually create more favourable conditions for windblown, unwanted plants (those with aggressive roots). And this means that the roof can be simultaneously covered in dying red sedum, while unsightly taller plants like thistles and couch grass take hold and make the roof look scruffy and unloved.
Now for all of you out there thinking green roofs sound like a load of hassle, it’s worth remembering why they are such an important part of our efforts to make cities and buildings more conducive to the kind of climate and places we’ll increasingly be living in (or enduring depending on your perspective!)
Some of the benefits are public goods that it’s hard for individuals to justify investing in altruistically – halving water run-off in storms, reducing urban heat island effect, creating space for endangered pollinators for example. But there are very definite, measurable benefits which DO make the economic case for green roofs: trebling lifespan of the roofing membrane, thereby reducing lifecycle costs; moderating heat gain on the top floor of buildings, especially important for commercial premises; improving output of photovoltaics, where combined; increasing wellbeing and tenant/resident satisfaction.
At Organic Roofs, we’ve been working for the past year on an EU-supported R&D project, where we’ve been doing BBA-type product testing on a lightweight alternative to sedum where wildflower meadows – better for pollinators but also more aesthetically forgiving of windblown plants – are achieved with much less of the heavy substrate than normal. The approach replaces around 50% of the substrate with agricultural waste, and although it’s been around for 20 years in Switzerland, where I trained, its not really been properly evaluated. Along with colleagues Duncan Baker-Brown (Faculty of Architecture) and Dr Anja Rott (Biology) at University of Brighton, we’ve found data that confirms what we’ve seen on our own sites around London and the South-East and in Switzerland: that using abundantly available waste makes lighter, cheaper, ecologically richer green roofs, with much greater longevity, than thin sedum.
One of the benefits of the collaboration was also that our full-time researcher surveyed hundreds of stakeholders, including architects, developers, housing associations, and local authorities to identify potential obstacles to uptake. One of the key findings clearly showed us the need for innovative new approaches to our market and its supply chain: more than three quarters of installs are sedum, the main drivers are visual appearance and biodiversity (perhaps surprisingly), but that over two thirds had had significant problems post-installation. It is clear that there needs to be more information about what products can achieve, which project KPIs, what the success rate of the product is, and what costs are involved if things go wrong.
Hopefully, as green roofs become more popular, the quality of products in the market will continue to rise and the amount of accurate information that enables stakeholders to compare like with like, will also increase.
© 2017: Lee Evans and the AECB (for Terms and Conditions click here)
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