We've all heard of slate roofs, tiled roofs even thatched roofs. But green roofs? They are more than just a trend. One of the first green roofs in Europe was created in Switzerland in 1914 and they are now recognised as being of great benefit to the environment. Thousands of square metres of otherwise redundant space can be planted up to absorb carbon dioxide, release oxygen, insulate our buildings and more.
The world's largest green roof is over 10 and a half acres. It covers the entire roof of the Ford truck factory in Dearborn, Michigan. Ford created the roof for environmental reasons, first and foremost to offer stormwater retention. The water maintains the plants and is evaporated back into the atmosphere. The green roof also cools the factory beneath, protecting the building and workers from too much heat.
The City of Toronto is big on green roofs - there are over 100 already and more to come. It is believed that if 8% of roofing in the city is green then the temperature can be reduced by up to 2 degrees. That significantly cuts the effects of global warming and city "heat islands".
And there are other reasons why they are a great response to climate change. Green roofs are good insulators - reducing the need to use energy heating and cooling buildings. Of course plants also absorb carbon dioxide, emissions of which are increasing, and release oxygen, providing a cleaner, healthier atmosphere.
There has been research to suggest that people in cities are less stressed and more productive in a "green" environment. Some have suggested that humans need greenery as much as food and water for their wellbeing. Concrete jungles don't make anyone feel at one with the world.
In Switzerland and Germany any roof over a certain size has to be green by law. In fact the benefits of green roofs are now so apparent that some of the world's local governments are providing builders with incentives to install them. In the US, Chicago and Atlanta are encouraging their use.
The London Mayor and architect Richard Rogers (of Paris Pompidou Centre fame) are actively encouraging green roof proliferation in London. The Corporation of London, the local governing body of the City of London, has recently granted a number of commercial planning applications on condition that the builders incorporate green roofs. This follows the success of the Canary Wharf commercial district which includes 6,000 square metres of green roof - some of it 160 metres high, the highest such garden in the world designed for biodiversity.
Other UK examples are the roof of the Library at Nottingham University and at the Horniman Museum in South London. There is even a school in North London, North Haringey Primary, which has seen parents, teachers and pupils get together and transform a flat roof which used to be a teachers' secret smoking area!
There are countless examples of successful green roof projects around Europe, Scandinavia and in North America. In Japan there is a particularly beautiful example of a tiered roof garden over a residential block. And Australia is currently doing a lot of research into how it can promote green roofs even in its arid climate.
So there does seem to be a green roof revolution taking place around the world, and for all the right reasons. As urban sprawl reduces the area of land available for parks and gardens, local authorities and national governments are looking up to the skies to introduce greenery for aesthetic and therapeutic reasons. But also to combat the effects of climate change and to maintain or even increase biodiversity in our cities.