Wavin PolderRoof: Water as a resource for a climate-proof city

Heat waves, droughts, but also violent storms and floods are once again making it clear this summer that climate adaptation in metropolitan areas is more relevant than ever. But how can you achieve this in metropolitan areas, where space for refreshing and green areas and water retention is scarce? Wavin, European market leader in PVC water pipe systems and partner supplier of Euro-Mat, presents an innovative solution. The PolderRoof system makes it possible to collect rainwater on the roofs of buildings, reuse it for plants, and thus reduce both heat stress and flooding.

‘We need to move away from the idea that rainwater is a problem that needs to be disposed of as quickly as possible,’ says Claudia Goemans, responsible for Wavin’s blue-green solutions. ‘It is a local resource that can make cities greener, cooler and more climate-proof.’

Wavin has been active in stormwater management for decades, primarily through underground retention tanks and infiltration systems. However, thinking has shifted. ‘Cities are increasingly focusing on integrated water management: from drainage only to retention and reuse,’ explains Goemans. In 2022, Wavin acquired the Dutch start-up Metropolder, a pioneer in the field of smart blue-green roofs. This became the starting point for PolderRoof, Wavin’s smartly controlled rainwater collection system for blue-green roofs.

The principle is simple: rainwater is collected and used to irrigate a roof garden (see insert 1). This reduces the demand for drinking water and helps cool down the environment. Thanks to smart control, linked to weather forecasts, water is drained in a regulated manner before a heavy downpour, ensuring adequate collection capacity. The result: less strain on the sewer system and optimal watering of greenery, both for new and existing buildings.

In principle, the system can be used on any flat roof, provided that the structure can support the weight – up to around two tonnes per m². In practice, it is easiest to use on new buildings. In many Dutch cities, regulations already stipulate that rainwater must be collected on site. Where there is insufficient space for wadis, ponds or underground collection tanks, the roof surface is a logical place. ‘Existing buildings can often be adapted, although this requires more work and coordination,’ says Goemans. ‘We have projects in Amsterdam where existing roofs have been successfully converted.’

The benefits extend beyond water management. A blue-green roof also protects the roof and the building underneath from overheating. ‘On hot days, a traditional bitumen roof can reach 70 degrees Celsius. Underneath the vegetation, it stays around 30 degrees, and the water in the buffer is often even cooler,’ says Goemans. This makes any solar panels on the roof up to 4% more efficient and the temperature in the building under the roof is 2 to 5 degrees cooler on hot days, reducing the amount of energy needed for air conditioning. In addition, a roof – for example, with a garden, terrace, or playground – increases the value of a building and contributes to biodiversity in the city.

A PolderRoof is more expensive than a traditional roof, but subsidies make the business case more attractive. Various European countries have local regulations aimed at rainwater management, greening, and sustainable construction, such as BREEAM or LEED. In addition, the system saves on irrigation costs, especially on high-intensity green roofs.

‘You have to compare it with the costs of an irrigation system and the added value for the building,’ says Goemans. ‘In some cases, it is even a legal requirement to collect rainwater (see insert 2). With PolderRoof, you can put it to good use.’

According to Goemans, interest in PolderRoof is growing rapidly, especially in densely built-up cities where space for water storage at ground level is limited. The Netherlands served as a testing ground, but projects are now underway in Belgium, Austria, England, Germany, France, Scandinavia and even the United States. In Eastern Europe, pilot projects are being conducted in the Czech Republic, Lithuania, and Poland.

According to Goemans, the concept is particularly popular in larger cities that are struggling with heat stress, flood risks and ambitious greening programmes. ‘In compact city centres, the roof is often the only place where something can be done about water storage and greening. That is where the puzzle fits together nicely.’

The successful rollout of PolderRoof relies heavily on a network of partners. Wavin works with specialised installation companies that install and maintain the smart control systems. Specialised landscaping companies take care of the construction and maintenance of the water storage and vegetation on the roofs. Contractors integrate the system into the construction process and coordinate it with other disciplines such as roofing, insulation, and installation technology.

‘No single company can do this alone,’ emphasises Goemans. “The system requires collaboration between disciplines that traditionally have little in common. It is precisely such cross-fertilisation that makes it successful. It offers builders’ merchants an opportunity not only to broaden their range with sustainable, innovative solutions that respond to current regulations and climate challenges, but also to become knowledge partners for contractors and project developers who are working on climate adaptation.

For Goemans, the essence of PolderRoof is simple: rainwater is valuable. ‘The principle of circularity is the same as in construction with reused materials. Why should we use drinking water to water plants when rainwater is readily available?’

The PolderRoof system demonstrates that making the built environment more sustainable involves more than just solar panels or insulation. It also involves intelligently addressing the basic needs of the city – water, green spaces, and cooling – and using every opportunity to bring these together.

The next step is integrating water solutions at the area level, where roofs, squares and underground buffers work together. For example, municipalities could use Digital Twins (3D models of the city) to coordinate all water management parameters in a neighbourhood or district. This requires intensive collaboration and an important level of digitisation, but the potential is enormous.

Goemans concludes: ‘We are already seeing that the need for integrated water management is approaching faster than we ever thought possible. And every new rainfall once again proves that this is the future.’