Wallaard Groen rapidly greens dozens of rooftops |
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Largest sloped green roof project kicks off in Zevenhuizen
With 125 new homes being equipped with a vapor-open sloped green roof, the Swanla Triangle in Zevenhuizen is the largest project of its kind currently being executed by Wallaard Groen.
| The green roofs on the first housing block of the Swanla Triangle project in Zevenhuizen (South Holland) |
Robin Castelijn, Director of Green at Wallaard Groen, isn't fazed by the scale of the project. The company has tackled similar projects before in Uden, Rijswijk, and Arnhem. Last year, 18 CPO homes in Heusden-Oost were fitted with a vapor-open sloped green roof system. "The project in Zevenhuizen is comparable in size to what we are doing for BAM at the 'De Kwekerij' project in Doetinchem."
"We've been working on the first two phases—67 homes—of the Swanla Triangle since May. Once those are complete, two out of four phases will be finished." Construction is moving at a fast pace. Castelijn explains: "The installation fits into a broader trend where entire residential areas are built prefabricated, with an emphasis on sustainable green concepts. We were involved early on by the client, Knaap Bouwbedrijf in Rotterdam, in drafting the design. This allowed us to collaborate closely on working out details for roof windows, penetrations, and dormers."
collaboration
Of course, we wanted to hear from Knaap Bouwbedrijf about how the Swanla project and the collaboration came to be. Kristian van der Knaap, general director: "Synchroon Gebiedsontwikkelaars in Utrecht launched a design competition for the project; it was won by the Rotterdam-based studio Architectuur MAKEN. Next, we looked for market players to realize the design. That's how the ball started rolling. We are the main contractor; Wallaard Groen is the green roof specialist and works together with green roof supplier Sempergreen."
Van der Knaap explains that Synchroon Ontwikkelaars not only provided funding, but also pursued the ambition to be #LParis proof$L. "The sloped green roofs fit this goal, as do the timber cladding and CLT structures we're using for construction." CLT stands for #Lcross-laminated timber$L, referring to massive wooden panels composed of multiple layers of wood glued crosswise, just as the name suggests.
Castelijn is also pleased with the collaboration with Knaap Bouwbedrijf. "They're a client with a family business DNA, just like Wallaard. This results in a practical and efficient approach." As a result, the project is progressing satisfactorily and on schedule.
construction site of the future
Were there any unexpected issues during construction? Van der Knaap: "We did struggle to obtain construction power, so we set up a sustainable construction site. A gigantic battery, wind turbines, and solar panels now supply power for the project. That's how we ended up with a construction site of the future." Castelijn adds: "Wallaard also contributes to the sustainable setup of the site. All our site containers now come standard with a green roof."
installation of sloped green roofs
Sloped green roofs are the future; Wallaard Groen is installing them more and more across the Netherlands. Castelijn: "We use an innovative vapor-open green roof system that ensures the substrate stays in place and the sedum vegetation can root properly. The system can be combined with PV panels—ideally with the south-facing side covered with panels and the north-facing side with Sedum. The green roof trays, which function like roof tiles, are filled with substrate that buffers rainwater and are topped with a reinforced Sedum mat. The roofs come standard with drip irrigation lines to provide water during prolonged dry periods. Naturally, a fall protection system is installed to ensure our employees can work safely—both during installation and for future maintenance."
Since Wallaard Groen was involved from the beginning in the development of the vapor-open sloped green roof system, the company has gained extensive experience and resolved initial teething issues. "We have certified staff with the necessary expertise, which allows us to provide solid and reliable advice. As a result, Wallaard Groen is increasingly being involved at the start of the building process, ensuring optimal coordination during the engineering phase."
 | | Robin Castelijn |
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This article was originally published on July 14, 2025 on the Stad + Groen website.
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