During the past decade, thousands of Rotterdam building owners installed green roofs on their dwellings — about 330,000 square metres in total, almost two per cent of the city’s 18.5 square kilometres of flat roof space. But where some cities have promoted such projects to improve energy efficiency and absorb carbon dioxide, Rotterdam’s green roof infrastructure is all about water, and keeping as much rainwater run-off as possible out of aging, overtaxed sewers in order to prevent flooding.
About four-fifths of the Dutch port is below sea level. As Paul van Roosmalen, the city official overseeing sustainable public real estate, puts it: “The water comes from all sides” — the sea, the sky, the river and ground water. “It’s always been a threat.” But he also sees an opportunity to use a marriage of technology and green design to elevate the role of rooftops in managing Rotterdam’s water pressures.