Car park of Gravenloo apartment complex fitted with polder rooftop


A polder rooftop has been constructed on the car park of the Gravenloo apartment complex. This rooftop is capable of storing rainwater during torrential rain. Its storage capacity can be controlled remotely. In addition to water storage, the Gravenloo home-owners association and the municipality of Leidschendam-Voorburg intend for this roof to provide cooling and room for biodiversity.

Polder rooftop

The polder rooftop covers a surface area of 1,200 square metres. It comprises a remote-control peak water storage facility topped by a green roof. The rooftop is fitted with measuring and control equipment. Prior to torrential rain, the rooftop can be emptied by opening a remote-control sluice valve. This frees up some 50 cubic metres of peak water storage and reduces the risk of waterlogging.

Image: the construction provides 50 cubic metres of water storage

The rooftop is situated in a densely paved area which retains a lot of heat during summer. The green roof has a cooling effect, because its plants evaporate water from the water storage.

The polder rooftop comprises two sections: the peak water storage facility and the green roof on top. The water storage facility is owned and maintained by the municipality. The green roof is owned and maintained by the home-owners association.

Image: green roof

Its eye-level situation makes the roof easy to view. A viewing platform is provided for children. An information sign for residents and other interested parties explains how the polder rooftop works. This creates awareness.

Lessons to be learned from the project

  • In the municipality of Leidschendam-Voorburg, this is one of the first projects to combine a public water storage facility with a privately owned green roof. It has demonstrated that residents are prepared to help implement climate-adaptive solutions, even on their own premises, if such solutions involve a win-win situation. The municipal authorities are realising water storage, and the home-owners associations are extending their roofs’ life spans by constructing green roofs.

  • The polder rooftop is co-funded by the municipality and the district water board. Safeguarding the continued operation of the water storage facility called for agreements on the use of the rooftop. For that reason, the initial intention was to vest “building and planting rights”. However, this is complicated and costly for larger home-owners associations: the notary deed must be signed individually by all the home-owners. This is not practical. That is why it was decided to conclude instead a 15-year loan agreement for the use of the rooftop. The home-owners associations will receive a once-only compensation for the loan.

Contact person

Simon van Damme (gemeente LeVo) en Klaartje van Etten (Delfland)
Simon: Scl.van.damme@lv.nl  
Simon: 070-3009089


Project type
Completed measure
Participant
Gemeente Leidschendam-Voorburg, Vereniging van Eigenaren ’s Gravenloo & Hoogheemraadschap van Delfland
Scale
District / neighborhood
Theme
Drought, Heat, Waterlogging