Let's take the Witte Huis; a height of 43m is definitely not going to impress visitors from a high-rise city such as Singapore, but it was the first skyscraper in Europe. The roof gives you a different view of the city, and precisely because of its lack of height you can still see people walking. Just standing on such a roof and looking at other flat roofs from a distance gets us thinking about the use of roofs.

Another example is the roof of the Boijmans Van Beuningen Depot, a special building storing more than 150,000 works of art which are now accessible to the general public. Where the building now stands there was previously a park with trees. The trees are now simply growing on the roof; a 'raised park', as it were. Both residents and visitors to the city can enjoy the green landscape and the view – free of charge – every week. And this rooftop park is home to another inventive feature; we know that the underground root structure of any tree is sometimes just as large as what we see above ground, yet the roots of the trees on this roof do not grow all that deep. The experts have succeeded in promoting flatter root growth, more like the shape of a hamburger. Would you have thought of that?