There's still a world to gain on the rooftop |
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Flat black roofs are being transformed into green oases
'The Netherlands has an enormous number of unused flat roofs. How wonderful it would be if we could turn them into green oases,' says landscaper Mattijs van Dalen. With his company, Green Revolution, based in Breda, he has already greened 500 roofs and recently created a special rooftop garden on a 100-year-old former carpentry factory in Nijmegen. He used Adezz retaining walls in an original way to form circular planting beds around two skylights, filling them with perennials and herbs. Another notable feature is the use of reclaimed (and thus circular) decking wood for the project.
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Skylights Become the Centerpiece
'The skylights were initially a design challenge but have now become fully integrated with the large borders of plants and trees on the rooftop garden,' explains Van Dalen while putting the finishing touches on the planting beds. The first design featured smaller borders with more seating space, but homeowners Fleur van den Bosch and Sytze Malda wanted to create an even greener oasis. The circular borders now contain an abundance of plants, flowers, waving grasses, and aromatic herbs such as thyme, sage, oregano, anise hyssop, and mint. 'Herbs grow well in rocky soil and can handle rooftop weather conditions, where it can get very hot. It's also important that the plants are wind-resistant, as rooftops are more exposed to the wind.'
In addition, he planted a large-flowered Japanese dogwood and a fig tree—unusual choices for a residential rooftop garden. 'Private rooftops rarely have the structural capacity for trees. In this case, it was possible. Of course, trees require more soil than smaller plants, but they will never grow very large due to limited root space. I used extra substrate to ensure stability: 50 centimeters for the trees and 15 centimeters for the perennials.'
Green Revolution
'As a child, I preferred being outdoors with a wheelbarrow and tools, maintaining the neighborhood. I wasn't very interested in school. My grandfather, who was a gardener — that's what it was called back then — helped me create my first garden,' Van Dalen says. Originally from Nijmegen, he studied horticulture at Helicon (now Yuverta) in Nijmegen and later gained commercial experience and training in business economics. Before starting his own business seven years ago, he worked for a large landscaping firm in Amsterdam, where he specialized in rooftop gardens.
With his company Green Revolution, Van Dalen has already transformed 500 flat roofs in Breda into green spaces. According to him, green roofs not only provide urban oases but also improve indoor climate: 'A green roof offers insulation, making the interior more comfortable.' He mainly installs sedum roofs but also uses perennial plants. He also runs his own urban nursery for sedum.
Organic Shapes
Van Dalen previously worked with the Nijmegen homeowners when he designed their rooftop garden in Arnhem. 'The brief for this project was clear and perfectly suited to my expertise. In the end, this rooftop became even greener than originally planned.'
Malda explains: 'We wanted the interior and exterior to form one cohesive whole. The interior features a lot of wood, reflecting the building's history as a former carpentry factory. When we bought the property, it consisted of little more than four walls and a leaking roof. We aimed for a robust, industrial look, combined with organic shapes and a Moroccan touch.' The latter is reflected in the curved, sand-colored walls of the bathroom and the use of clay plaster in the kitchen. 'The large planter embedded in the concrete floor further emphasizes the organic design." Smiling, he adds: "Actually, the planter was installed before the concrete floor was poured. The concrete workers were surprised to work around a planter — it was a first for them.'
Corten Steel
'We could have concealed the building's steel structure, but we chose to keep it visible to reflect the industrial history of the property. The rooftop garden is the icing on the cake,' says Malda.
Van Dalen opted for corten steel for the rooftop garden: 'Corten steel has a natural look that complements the greenery and creates a connection between the indoor and outdoor spaces, one of the design requirements.' The outer rings around the skylights and the round planting borders were available from Adezz's standard stock, while Van Dalen fabricated the inner rings from corten steel supplied by Adezz. 'The Adezz website makes it easy to check material availability and find technical specifications that are very helpful in rooftop garden design.' Corten steel is considered sustainable because it has no coating and can be easily recycled in the future.
Van Dalen made a conscious decision to avoid purchasing new materials when possible. Although he couldn't find reclaimed corten steel in the right thickness, he sourced many other second-hand materials. 'I've used my network at NL Greenlabel to find reused materials. For rooftops, hardwood is advisable because it withstands extreme weather. However, I prefer not to use tropical hardwood from overseas. For example, I found insulation material for the borders at a demolition company in Breda. When searching for hardwood decking, I discovered Van den Berg Hardhout, which has a wide range of reclaimed hardwood. I purchased wood that had previously served as dock planks in a marina for 30 years. Although FSC-certified tropical hardwood is generally a responsible choice, I prefer reclaimed materials when possible.'
Crushed Roof Tiles as Substrate
'For rooftop substrates, lava is commonly used, but I chose a locally sourced material," Van Dalen says. "The substrate from Greenmakers consists of recycled materials, including crushed roof tiles and compost.' The broken red roof tiles create a striking contrast with the green plants. 'The sustainability of this product is verified by NL Greenlabel's A rating.' On this warm day, Van Dalen advises against watering too soon: 'Plants need to be trained and not spoiled with too much water; otherwise, they won't develop strong roots.'
For Van Dalen, greening the city is a personal mission — something he also shares with his students at HAS Green Academy in Den Bosch, where he teaches the Green Living Environment program. 'No paved gardens or artificial turf for me. I see urban greening as my calling. And there are still millions of square meters of unused rooftop space available in the Netherlands.'
Award-Winning Rooftop Garden
On Friday, September 13 2024, Mattijs van Dalen received the Private Rooftop Garden of the Year award for this project at the Rooftop Symposium. 'The icing on the cake after a fantastic collaboration,' Van Dalen says.
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