On a sunny roof terrace in the centre of Rotterdam, residents from different cultures are getting to know each other over a special lunch arranged as part of the city’s Rooftop Days festival.

Some linger over the view of a lower roof lush with vegetation which is keeping the space cool on this hot, dry summer day. A few notice another roof, further below, where rainwater is being stored. Others catch a glint of the solar panels on the highest roof, where clean energy is being generated for the entire building.

​High above the streets, this vision of city liveability and sustainability goes almost unnoticed at ground level. But it’s time to look up!

​From the fabled draping flora of the Hanging Gardens of Babylon to the grass roofed Viking houses of the Middle Ages, there’s a long history of making the most of rooftops. And this tradition has never been more needed.

​The last urban frontier

Cities are becoming critically crowded. In just five years’ time, 70% of the world’s population will live in cities. Urbanisation, climate change and social issues are presenting cities with seemingly impossible challenges. Every square metre counts.

Buildings are compounding the problem. They account for 33% of global greenhouse gas emissions and their construction and maintenance for 40% of global primary energy requirements.

So why focus on their roofs? Well, they account for 20-25% of buildings’ overall surface — and most remain unused and unloved. Yet rooftops could potentially play an important — and exciting — role in the kind of solutions cities are searching for.

Cities like Rotterdam, a fast-growing modern metropolis which needs more than 50,000 extra homes by 2040 and a constant supply of new commercial buildings. Buildings that will eat into the city’s already scarce space and cause emissions and energy demand to rise.

Fortunately, the city also has an abundance of flat roofs — more than any other city in the Netherlands in fact thanks to wartime bombardment.

Going green is not enough

Not surprisingly, Rotterdam was an early convert to the concept of green roofs. It has been busy planting grass and foliage on top of buildings for decades, to better manage storm water, indoor temperatures, air quality and biodiversity. So impressive was its pioneering Rotterdam Roofscape project that the city received a C40 Cities Award recognising its impact in 2015.​

Even with this heritage, the city still has 18.5 square kilometres of flat roof space waiting to be put to use and brought to life. Going green is clearly not, on its own, going to get the job done. So how could the city ramp up development of its roofs? What would encourage more organisations to get involved? The answer? Add more colours!

“We started our green roof programme talking about the need to delay run-off from roofs to our sewer system, but this technical message was not very tempting to citizens,” explains Marloes Gout, sustainability consultant in Rotterdam’s urban development department.

“So we started communicating about adding quality to their houses, for example by combining a green roof with a roof terrace or solar panels for cheaper energy. Our residents saw how they could benefit personally. With multifunction solutions you can give unused space proper roles and create more value from the roofs.”

Golden combinations of functions

And so it was that the city added three more colour-coded roof types:

• ​Blue roofs are designed to better manage storm water by temporarily storing rainwater and releasing it slowly when the rain stops
• Yellow roofs enable the generation of sustainable clean energy through the use of solar panels or boilers and windmills
• Red roofs enhance social cohesion and make up for lack of space in the city by providing meeting places, leisure facilities and green and play spaces.

“Our intention is to make ‘golden combinations’ by understanding how to choose the right mix of functions for the right location,” explains Ms Gout. “In the industrial harbour area for instance there’s no need for water storage but a lot of demand for electricity generation. As vegetation ensures a 5-15% higher return on solar panels, the ideal combination here is green and yellow roofs.”

Something else was needed alongside more colour – collaboration. As most of the roofs are not owned by the municipality, it needed to partner with housing corporations, building developers, charities and home owner associations to turn its rainbow roofscape plans into reality.​

Three experimental show-roofs

With the help of these partners, and funding from the EU programme LIFE@urbanroofs, three multifunction roofs are being created in Rotterdam — on buildings due roof maintenance within the next few years.

There’s the De Heuvel building — whose celebratory lunch we dropped in on earlier — where a charity’s mission to promote sustainability and intercultural understanding is reflected in its multi-coloured roofs.

Over at De Peperklip, a complex of 550 homes owned by a housing association, the largest green roof in the Netherlands is under construction. A 7000 square metre area is being planted with sedum and ferns to attract bees and butterflies. Shallow ponds will appeal to birds and old wood and diverse soils will draw insects and fungi.

The third project is taking over a whole street and giving local people and organisations what they ask for — from a healing and wellbeing green roof for a women’s shelter to the opportunity to learn how to implement high-tech water and energy systems for a property developer.

Suppliers will also be able to test product innovations at the project’s experimental roof. Designers, meanwhile, have been given carte blanche to come up with innovative ideas which will be turned into a book to inspire multifunction roof development. So far, these include a rooftop zoo, vegetable gardens and even a hotel.

Tools and ideas for the future

Rotterdam’s roofscape has already become a tourist attraction. And there’s every sign that roofs will continue to go up in the world as spaces for radical solutions.

Commentators on urban regeneration in the age of climate change are agreed that, as urban architect Steffen Lehmann says, “integrated development that concentrates on energy and water management, green infrastructure and the urban microclimate will take a leading role.”

Rotterdam is furthering this cause by developing a social cost benefit analysis tool that will make it easier for authorities and investors to see where value lies in roof development. It is also stimulating more smart ideas whose value is plain for all to see.

Here’s just one potentially life-saving example.

A city collaboration is investigating how drones might be used to bypass busy city traffic and deliver urgent blood, medicines and diagnostic samples to the rooftops of healthcare facilities and patients’ homes faster than ever before.

A brilliant idea, but it does prompt the question — what colour should medical roofs be?