Rotterdam has 18.5 km2 of – mostly empty – flat roofs. Why has the city only made use of a limited number of roofs, when there is so much potential in doing so? Rotterdam has 18.5 km2 of – mostly empty – flat roofs. Why has the city only made use of a limited number of roofs, when there is so much potential in doing so? To address this topic, the City of Rotterdam commissioned MVRDV and Rotterdam Rooftop Days to produce a catalogue of 130 innovative ideas to make use of the city’s roofs, giving rise to a fantastic new phase of development in Rotterdam.