Beeld: Green Roof Initiative
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If you were to plant greenery on all of the city’s flat roofs, you would be able to look out onto a maze of aerial gardens right from your apartment in one of Rotterdam’s many residential tower blocks. That was the idea of Eddy Kaijser and his partners when they started their Green Roof Initiative.

This start-up is located in the heart of Rotterdam and looks out onto a desert of barren roofs. It designs compartments with raised edges in the shape of window frames made out of recycled plastic. The frame’s compartments have room for soil, an irrigation system, and plants. You can stack the diamond-shaped frames next to each other and create figures such as a star or a flower in the middle of a patch of grass or sedum, etc.

Torrential rain

“It’s all dependent on the load-bearing capacity of a roof,” Eddy Kaijser states. A single layer of frames with greenery is suitable for a roof that is not able to support very much. On a roof that can take more weight, you could put two or three on top of each other. The lower container can then store water when there is a downpour.”

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Eddy Kaijser with a model of the Green Roof Initiative modules

The aims of the Green Roof Initiative are not only aesthetic in nature. It should also solve urban problems of heat stress and flooding. Summers are becoming hotter, flat roofs absorb that heat. This puts pressure on living conditions. Torrential rain is also becoming increasingly frequent and the machinery that has to pump this water away can no longer cope. This can cause flooding. Planting vegetation on roofs helps to cool them down and simultaneously absorbs water whenever it rains heavily.

This in itself is not a new idea. The first large private green roof in Rotterdam was installed back in 2010. This is a large sedum roof (sedum is the name of a kind of succulent plant) atop the block of buildings on the Nieuwe Binnenweg, originally built in the 1930s by Dutchman Kraaijvanger, the architect of De Doelen theater. That green roof was made possible with the support of the municipality.

New nature reserve up in the air

What is different about this rooftop system developed by Kaijser and his partners is that it is a concept that can be rolled out over a string of roofs throughout the city. Undoubtedly, this is useful in preventing heat and water damage. But it is even more interesting because it offers the opportunity of establishing a new nature reserve for birds and insects. The ground for this, where insects and some birds traditionally look for food, lay eggs, and breed, is getting smaller and smaller. So if a vast biotope arises in the air, it may mean that species that are now under pressure will be able to survive, or perhaps even new varieties of species may arrive.

It depends on the vegetation as to what kind of insects and birds you attract, Kaijser explains. He is collaborating with the ecologists of the Bureau Stadsnatuur in Rotterdam for this reason. “We hope to be able to advise the Green Roof Initiative in that area,” says ecologist André de Baerdemaeker. ” At present, the various green roofs in the city are islands where a new kind of nature reserve is being created. If the area where insects can live were to expand, the chances of survival on those roofs will also increase for those species.”

They will die out if a particular spot is very dry. But if the connected areas up in the air becomes greener, the chances are higher for a more humid spot on one of the roofs where birds and insects can frolic or fly around.

Brooding oystercatchers and sickle-bearing bush-crickets from the Mediterranean

Bureau Stadsnatuur had seen a new kind of parasitic wasp on the green roof of the Erasmus Medical Center. On other green roofs, the ecologists also spotted Mediterranean sickle-bearing bush-crickets, flies, and spiders. “Young spiders in particular. They hurl themselves up with one of the threads that they’ve spun and then the wind blows them up onto the roof.” The fact that they end up there is essentially a coincidence. “As they can’t actually see if there is any greenery on a roof.”

Those insects found amongst the roof vegetation also serve as food for birds. Sometimes birds find a suitable nesting place in those roof gardens too. Meadow birds like the oystercatcher, for instance,” says De Baerdemaeker. There are fewer and fewer meadows where they can nest. New areas are therefore more than welcome. More and more often you can see birds building nests on these green roofs. Once their eggs have hatched, they fly off the roof to snatch earthworms so that they can feed their young. Once their young are older, they slowly but surely venture beyond the roofs.”

Could the roof garden also be a godsend for other meadow birds such as the black-tailed godwit and the lapwing? “They can also fly to Africa. Altitude isn’t a problem for them.” De Baerdemaeker thinks it may be an advantage. “For one thing, there aren’t any foxes walking around on those roofs.”

But as there is no precedent of such development, De Baerdemaeker says this is not very probable. ,We have not seen it up untill today.”

Seeking funding to build a prototype

After summer, Kaijser will be seeking funding to build an initial prototype of the star-shaped planters. They will be four meters long and four meters wide. Kaijser puts the cost of this first prototype at around €100,000. Recycled plastic for the modules will be sourced from the municipality of Rotterdam. A project developer and the municipality have already promised that they will make roofs available for a trial installation.

The landscaping company that Kaijser has hired, Van der Spek Hoveniers, has experience in planting vegetation on roofs for major housing corporations and other large real estate owners.

Thanks to the entrepreneurial award Kaijser received last week from Desiree Verdaasdonk (director of Rabobank Rotterdam, Mai Elmar (chair of VNO-NCW West), Barbara Kathmann (Rotterdam municipal councilor), Pieter van Klaveren (MKB Rotterdam) and Paul van den Bosch (regional editor in chief at the Dutch AD newspaper), he has managed to get an influential network to back him up. Kaijser adds: “That should help us to realize our plans.”