John Lorinc of The Toronto Star writes:

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.

While typical green roofs function like sponges and look like gardens, Rotterdam is working with public and private landlords to develop a “green-blue grid.” Instead of simply fitting out roof areas with plantings, these spaces can also be equipped with reservoirs or tanks to retain excess flow — blue roofs. The tanks, in turn, are equipped with electronic drain valves that can be opened and closed remotely, in some cases via a smart phone app.

Van Roosmalen adds that a green roof can absorb about 15 millimetres of rain per square metre, whereas a roof with a reservoir can retain 10 times as much. The city’s goal is to convert one million square metres of flat roofs to include water retention systems and solar panels. Aggregated across even a portion of the city’s flat roofs, he says, “it’s a tremendous amount of water.”

Read more: Rotterdam is using smart city tech to solve pressing urban problems - Here’s what Canadian cities can learn from the Dutch model