When De Dakdokters first presented their great pitch for their polder roof, both Amsterdam Zuidas and the Green Business Club Zuidas were enthusiastic from the outset. They immediately suggested a potential location: the roof of Old School, located in Gaasterlandstraat. Director Friso Klapwijk: ‘Of course, that was great, but our lack of practical experience meant it was a process of ‘learning by doing’. We’ve now advanced much further than the initial prototype, but it was really good to have the opportunity and we were able to demonstrate the effect in practice. It also sparked interest from other parties.’
What makes polder roofs special is their increased capacity to store, hold and gradually drain rainwater. The rain on the roof is collected in buffers that are controlled by software. Klapwijk: ‘Our computer program ensures that the buffers slowly allow water to seep into the sewer and water the plants. This creates a green oasis that provides its own watering system and also a prevents large puddles developing after heavy rainfall.’
Roof garden walks
The roof garden and polder roof have become a familiar part of Zuidas. There are even regular organised ‘roof garden walks’ to allow everyone to take a look. Klapwijk: ‘We’ve created around ten roof and polder gardens in Zuidas, the best known of which is probably the roof of the Vivaldi office building, in Antonio Vivaldistraat. Zuidas is highly suited to roof gardens and polder roofs. It’s a way of offering your employers some greenery without them having to walk some distance away.’
‘People also occasionally forget that Zuidas is actually a polder, from which the excess groundwater and rainfall have to be drained away through the sewers and pumping systems’, continues Klapwijk. ‘If a lot of rain falls in a short space of time, you’re more likely to have excess water remaining on the roofs and on the streets. Installing a polder roof helps reduce that problem.’ De Dakdokters are also working on polder roofs in other places. They have already installed 25 in the Netherlands and five in other countries. A further 100 polder roofs are in preparation.
Back to the relocation. Although the system itself is not relocating, around 135 sq. m. of the total 5,000 sq. m. surface area of plants is moving to the roof of the food bank in De Pijp. Another section will go to the roof of Xpositron in Amsterdam West. Klapwijk: ‘Unfortunately, neither of the roofs on the food bank or Xpositron is suitable for the polder roof system because the load-bearing capacity is insufficient.’ The good news is that the roofs are suitable for accommodating large numbers of plants. ‘It’s still great that they will now have a second life on a different roof. That’s why, as Roof Doctors, we’re happy to assist with this move.’