Elissaveta M. Brandon of Fast Company writes:

There’s a saying in Dutch, “Je kan het dak op,” which, translated literally, means “you can go up on the roof.” But what it really means is “get lost.” The origins of the saying are difficult to trace, but for Paul van Roosmalen, it probably has to do with the fact that no one thinks of a flat, grey roof as somewhere you want to be. Van Roosmalen works in the Department of Sustainability for the municipality of Rotterdam, where he leads a program called Multifunctional Rooftops. And, in many ways, his job is to change that preconception.

The Rooftop Days festival is part of a campaign to use more of Rotterdam’s flat roofs to add greenery, harvest solar power, and collect rainwater.

If you think of a traditional Dutch house, you probably picture a gabled roof, but Rotterdam is different. Much of the city center was razed during World War II, leaving behind a flattened landscape rife for reinvention. Today, Rotterdam has over 650,000 inhabitants and more than 150 million square feet of flat roofs. Most of them are unused, but in the past few years, this has been changing.

Rotterdam is opening 20 of its rooftops to the public. Think of it like an open house, but for roofs. Known as the Rotterdam Rooftop Days, the festival aims to highlight the untapped potential of rooftops by organizing an extensive program of tours, concerts, exhibitions, and dinners.

This year, the festival is anchored by two striking rooftop installations, the Rotterdam Rooftop Walk, (co-designed by local architects MVRDV and Rotterdam Rooftop Days) and The Podium (designed by MVRDV). They’re only temporary–one will come down at the end of June, the other in the middle of August—but they’re part of a flourishing network of green rooftops that are helping the city combat the urban heat island effect, absorb rainwater during storms, reduce air pollution, and increase biodiversity.

FROM GREEN ROOFS TO MULTIFUNCTIONAL MACHINES

It all started in 2008, when Rotterdam became the first of three municipalities in The Netherlands to provide a subsidy for green roofs (the other two being Amsterdam and Groningen). Then the program managers realized they could do more than simply greening roofs. About 90% of Rotterdam is below sea level, so the city has become something of a laboratory for clever water management: sponge parks absorbing water, sunken squares doubling as water retention ponds, and parks designed to flood. Except the city doesn’t have enough space for bigger interventions and large water retention ponds. So, what about roofs? “If you can do this on a lot of rooftops, you have a lot of small-scale solutions and that would add up to this one big-scale solution,” says Paul van Roosmalen.

LOOKING TO THE FUTURE

What the city can do, however, is offer subsidies: Qualifying roofs must be able to retain 30 ml of water per square meter. In return, building owners can get up to $530 for every cubic meter of water collected through the green roof. To further encourage building owners to retrofit their roofs–and help them understand the hidden potential—the city has also built an interactive online map tool. Anyone can go on the website, plug in a ZIP code, and see if their roof is fit for an upgrade. You can learn about the load-bearing capacity of the roof, choose between solar and various levels of greening, and see how big a subsidy you would get for the size of your roof. (To green a 750-square roof, for example, the tool estimates it will cost about $4,500, with a municipal subsidy of $1,900.)

Ultimately, the city’s goal is to convert over 10 million square feet of flat roofs into multifunctional roofs with greenery, water retention systems, and solar panels. It’s a slow journey and it may take 25 years, but by the end, maybe that Dutch saying will have lost its meaning–and rooftops will be a place people actually want to be.

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Read more: Inside Rotterdam’s Quest to Green 10 Million Square Feet of Rooftops