Sky Gardens

Working in the area of green buildings, I often get asked if I install green roofs. Truth be told, I didn't know much about green roofs until recently. However, after delving into the subject over the past few weeks, I am completely sold.

When it comes to green roofs, the benefits extend far beyond just a nice patch of grass. Advantages of green roofs include:

• Stormwater Management: Green roofs act as a natural sponge, absorbing excess stormwater, reducing runoff, and mitigating the risk of flooding.
• Cooling Effect: They contribute to natural cooling, diminishing the urban heat island effect and creating a more comfortable environment.
• Biodiversity Boost: Green roofs become mini-ecosystems, providing a haven for various species and fostering biodiversity amidst the concrete jungle.
• Mental Health Oasis: A touch of greenery amid the urban landscape can do wonders for mental well-being, creating a serene escape.
• Aesthetic Appeal: Beyond their functional benefits, green roofs simply look lovely, transforming urban landscapes.

WA building in New York experienced a pretty impressive transformation after installing a green roof. It not only absorbed 7 million gallons of stormwater but also used 26% less energy, saving $3m in reduced energy costs. And when the grass arrived, so did the rest of nature. The new roof created a thriving ecosystem hosting 25 bird species and over 300,000 bees. 

That was just off a space of around 60,000 m2. There are 30 million buildings in the UK, average roof size is 75m2 equals 2,250,000,000 m2 available space. That’s about 550k acres of land which is equal to an additional 1 and a half Londons dedicated to green space. 

I thought originally that having a green roof sacrificed space needed for solar panels, but the opposite is the case. Green roofs, in fact, enhance the efficiency of solar panels. Solar panels operate optimally when cool, and green roofs provide the perfect cooling effect. Case in point is a study by Lendlease from around 2 years ago in Australia which revealed that, on hot days, the green roof being studied reduced temperatures by up to 20 degrees, significantly boosting solar panel performance.

It’s safe to say that green roofs tick an awful lot of boxes. What’s some of the downsides? 

• Lack of standardisation: The lack of a British Standard is often cited as a real barrier to whole-scale uptake of green roofs. This is reflective of the fact that there is also a lack of skills for installing and maintaining green roofs, and a lack of policy to support and inform wide scale implementation.
• Safety: If installed and maintained incorrectly, green roofs can pose threats to the structural integrity of the building (hence the need for standards). Another big one is dry vegetation, which, in our increasingly hotter summers, is a clear fire hazard. Although there are clear guidelines on what constitutes a safe green roof when it comes to fire, this of course adds to the overall maintenance. Which leads us inevitably to the final point:
• Cost: As with most climate solutions, cost continues to be an inhibitor for not only installing but also maintaining green roofs. While it would be beneficial to find a method to quantify the value of nature and mental health for inclusion in return on investment (ROI) calculations, currently, the only widely accepted measure of value is the cost savings related to energy—specifically, the reduced expenses on heating and cooling facilitated by green roofs. This sometimes doesn’t always make the economical case for green roofs. 

I lifted these drawbacks from a paper published by Greater London Authority in 2008(!). It’s quite depressing that it’s been 15 years and we’ve still not moved forward on green roofs, where the benefits seem to greatly outweigh the drawbacks. True, 2008 was a different world (remember Obama and Britain being in the UK?), and people were still sceptical of anthropogenic climate change, but I think it’s also a classic sign of our collective unwillingness to change until things get really bad. 

Potentially the business case for green roofs just needs to be a lot clearer? With potential subsidies to lend a helping hand? I also think this could link quite nicely to the biodiversity net gain requirement for new building developments. Could people start selling their roof space to unlock more cash for green roofs?

I’d love to know your thoughts as to what’s stopping more green roofs, and if you’ve seen any interesting solutions in the space!