A lot of this is due to the generous subsidies springing up in the big cities like mushrooms. But money isn't the only reason: the "greening" of buildings can also make cities climate-resistant.
Since January this year, residents of Groningen have been eligible for a subsidy of at least 30 euros per square metre if they construct a "green" roof. That's roughly 60 percent of the total cost of a new roof. Rotterdam is being equally generous.
Elsewhere, the city council in Amsterdam and The Hague have agreed to establish their own "green roof" subsidy.
Worldwide
The sudden growth in such schemes isn't just a Dutch phenomenon. Neighbouring Germany is installing 14 million square metres of green roofs a year.
In the US city of Chicago, Mayor Richard Daley recently approved the construction of a park on the roof of the city hall and says he wants to make Chicago the greenest city in the US. In Canada, the city of Montreal already offers striking examples of existing green projects and has far-reaching plans for more.
Environmental advantages
It's not that politicians around the world have more money than they know what to do with. Green roofs offer so many environmental bonuses that a city administrator would have to have a deep-seated hatred of ecology not to open up the public coffers and take advantage of them. Mark Ottelee, a postgraduate at the Technical University in Delft, is busy putting the finishing touches to a doctoral thesis on the "greening" of buildings. Standing on the university library's green roof, he sets out the benefits of such schemes.
" "Greening" a roof helps regulate temperatures. Vegetation grows in a thick layer of artificial earth and produces warmth - "evo-pespiration" we call it - which then cools the air above the buildings as well as in them. By itself the earth acts as a natural insulator, but if you then factor in the natural evaporative and cooling qualities of the plants, the reductions in temperature are spectacular. In summer, a traditional black tile roof can reach temperatures of 80 degrees Celsius. A green roof doesn't go above 35 Celsius."
Houseboat with grass on the roof
Greenhouse gas
That difference in temperature has a big effect on the amount of energy used to cool a building, which in turn leads to reduced CO2 emissions. But that thick layer of earth on the roof also works just as effectively as insulation in winter, and provides yet more reduction in emissions. But there are, according to Mark Ottelee, even more advantages. For example, what does he think about the increasingly heavier downpours we're experiencing?
"Green roofs can slow down the rain hitting the roof. The layer of earth acts like a sort of sponge, so that after a dry period it can release the water it's trapped into the vegetation. That means less water being wasted by running into the drains."
Green roofs are also an important addition to a city's biodiversity, since the plants growing on them can suck up the fine particles of dust that clog the city's air, trap them and gradually transform them into more earth for the roof. Photo above: The green roof of Delft University-Library
A missed opportunity
All in all, the advantages are so many and varied that Mark Ottelee would like to see greenery growing on more of a building than just its roof. The fact that people have forgotten that walls can act as a "bed" in which plants can grow he sees as a crying shame.
"Buildings have far more wall than roof!" he points out. That's why his research has investigated the possibility of transforming walls into places where vegetation can grow.
He sees panels made of soft, porous cement that can absorb moisture and nutrients as one idea worth investigating, especially if they could be used to cover the façade of a building. And he's optimistic about seeing such walls on our city streets:
"Well, I hope it will happen very quickly. There's definitely interest from the architectural world, especially with regard to house facades. I'm confident that when my thesis comes out more people are going to become enthusiastic about the use of green facades." Photo above left: A green facade: walls should not be forgotten
It may well be that Ottelee is right: just look at the generous subsidies the big Dutch cities are currently distributing. It also helps, of course, that green roofs and green walls look a lot prettier than bricks and tiles. There's definitely an opportunity for the local government official who wants to polish his image as a "Greener of the City".
Thijs Westerbeek van Eerten
Radio Netherlands
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Roofs with a layer of plants are suddenly "hotter than hot" in the Netherlands.
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