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October 20, 2022

Nature policy on the rooftops in Rotterdam





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Mathis van der Voordt

Programme Support Intern

Rotterdam offers many alternative ways for its inhabitants to experience the city. Those who find walking on the sidewalk a boring activity can plan their next routes on the city's skyline. Adventurous strollers had the possibility to walk from roof to roof and across the Coolsingel. These walkways are placed on high scaffolding at various locations in the city. During this “Rooftop Walk,” you will come across sustainable, innovative, and creative ways to make roofs more inclusive in improving the quality of life in a city. Many examples are also given to show what a city like Rotterdam could look like if the 18,5 km2 surface area of flat rooftops was put to good use.

One of the possibilities for the layout of rooftops is the creation of green rooftops and façades. These green rooftops and façades are intended to make the city greener, improve biodiversity, cool the city, retain water, and make the inhabitants happier by bringing them closer to nature.

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“Making use of the space on rooftops in large cities is of great importance to keeping these growing cities liveable and sustainable.”

Green rooftops

Dealing with limited space in a large city is one of the most important issues for city governments and municipalities. Therefore, every free surface should be used to make a city more liveable and sustainable. In cities, green space is needed to cool down the city, produce clean air, provide a relaxing space for its residents, and an optimal environment where birds and insects can flourish. Rooftops without greenery reportedly reach temperatures up to 85 degrees Celsius in the summer. Temperatures on green rooftops only reach about 35 degrees Celsius. A green roof keeps the space up to 5 degrees cooler. So, you won't need air conditioning, which saves energy. Green rooftops also last up to two times longer than rooftops without greenery, which saves maintenance costs.

LOLA Landscape Architects came up with an idea called: Het Wolkenwoud. This project is now on the rooftop of the Bijenkorf where it slowly spreads through the gravel with the help of wood pellet veins and takes over the roof. The intention is for Het Wolkenwoud to become a permanent forest and help reduce heat stress, produce clean air, and enhance biodiversity. This solution is needed because research shows that nine million people die each year due to air pollution in cities. LOLA’s solution will prove to be very effective in the long term.

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Blue rooftops

Draining water presents another significant issue in large cities, with which Rotterdam already has considerable experience. Due to the high degree of petrification, the sewers have a hard time coping, and infiltration hardly takes place at all. The solution for this problem is a type of rooftop called a “Blue rooftop” which can store between 60-150 litres per m2. Some of the Blue rooftops, called “Static Blue rooftops”, are designed to catch, hold and evaporate rainwater. Between 45%-70% of the rainwater they catch is evaporated which results in a 50% reduced chance of sewer overflow.

The company Cloiq also came up with a great solution: “De Slimme Regenton”. A smart rain barrel 3D-printed from recycled plastic that regulates the water level itself can prevent flooding. Old plastic is recycled, and the vegetation on the rooftops gets clean and well-regulated water. This is just one initiative to use rooftops to store excess water.

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Innovative solutions

One of the biggest obstacles to using rooftops for greening and sustainability is their weight. In Rotterdam, the peat has been subject to a great deal of subsidence, which has consequences for the maximum weight and stability of the buildings. Trees, bushes, other vegetation, and water storage systems add a lot of weight, which can cause structural problems.

However, The Urban Jungle Project found an innovative solution for this challenge. With their lightweight “Bomenveren”, they remove the excess weight of the vegetation. The roots of the “Bomenveren” are not in the ground and, therefore, the trees only weigh approximately 350 kg. Moreover, these Bomenveren can be placed on rooftops, in the street and on city squares, thus reducing the weight of heavy greenery.

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An increasing part of the population lives in cities, which brings many problems regarding living space, greenery, transportation, water, and energy. Using the space on flat rooftops to make these growing cities liveable and sustainable is necessary. This solution naturally also has problems, but by working together on new smart innovations, amazing solutions are engineered. Such as: “Het Wolkenwoud” by LOLA Landscape Architects, “De Slimme Regenton” by Cloiq or “De Bomenveren” by the Urban Jungle Project, to name a few innovative solutions. Through good cooperation between the government, municipalities, and the private sector, all these different projects can be brought together, and their combined impact on Rotterdam and other cities will be even greater.

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