He is co-founder of livingroofs.org, a non-for-profit green roof organisation that educates and advises on green roof development.
So what actually is a green roof?
Basically putting vegetation on buildings. Twenty to thirty per cent of green roofs are intensive, places like parks or gardens where people can go. The rest are extensive which means they are not for amenity and are more like wild spaces.
Isn’t it just a load of rubble with a few seeds thrown on top?
[laughs] No, the three basic principles of a flat roof is that it needs to drain, can’t collapse or leak. Putting on rubble, concrete and brick doesn’t do this.
We use blended substrates with generally very, very little nutrients in it.
Don’t the plants need nutrients? Compost?
The most beautiful, wild and biodiverse places you see around the country grow in soil with very little nutrients.
Most of the plants and flowers you want on a roof need a ‘stressed’ environment.
We don’t want to want to encourage plants to grow to fast or big. Being stressed means they have a higher resiliency when more stress occurs such as heavy rain fall or extreme temperatures.
So why are they being used in London and other major cities?
Mainly, climate change adaptation. In 2004, Ken Livingstone, who was London mayor at the time, started talking about ‘cool poverty not fuel poverty’.
The three key things in climate change adaptation urban heat island effect, reduction in flash floods and biodiversity.
Policy today says that the mayor ‘expects’ developers – unless they have a really good excuse – to put a green rooftop on major developments. So when I see a crane around London, I know a green rooftop is also going up.
Can they go onto old buildings?
Retrofitting around 30% of the central London roof area would come to a staggering ten million square meters. This is getting to the scale of a national park.
But the main problem at the moment is where do you get the money from? Indicatively, it costs around £50-100 per square meter to retrofit a roof.
However, retrofitting over the next 25 years will become increasing important to manage the urban heat island effect. One green roof isn’t going to do it but ten million square meters will.