Water Roof: The Innovation Against Flooding

As heavy rainstorms become more frequent, cities are searching for smart ways to prevent flooding. One of the most promising solutions? The water roof. This innovative type of roof not only temporarily retains rainwater but also provides cooling, biodiversity, and a more sustainable living environment. A water roof, also known as a blue roof, is specifically designed to store rainwater in a temporary buffer tank on top of the roof. This technique helps prevent flooding during peak rainfall and provides natural cooling for the building below. When combined with a green roof, it becomes a water retention or detention roof—a multifunctional roof with numerous benefits.

Retaining Water on the Roof

The climate is changing. We are increasingly faced with heavy, short rain showers. Urban areas, where there is a lot of paving and little greenery, are especially vulnerable to flooding. Sewer systems cannot always handle the volume, resulting in flooded streets and basements.

That is why it is crucial to retain water where it falls, and the roof is the ideal place for this. Instead of draining rainwater directly into the sewer, water roofs temporarily store it. This relieves pressure on the drainage system and prevents sudden surges during heavy rainfall.

But it goes beyond water buffering alone. Because the water remains on the roof, it also has a cooling effect on the building, especially during warm periods. This effect is enhanced when combined with a green roof. The result: less heat stress and a contribution to urban climate adaptation.

Want concrete solutions on how to turn your garden into a water storage space?

Water Retention Roof: Turn Your Roof Into a Water Roof

A roof no longer only serves to protect against rain and sun. Roofs are increasingly being given an additional function—think solar panels, rooftop gardens, or green terraces. With a water retention roof, you add another crucial role: water storage and delayed drainage.

A retention roof temporarily stores rainwater in a special buffer field. Through a controlled outflow, the water gradually drains away, preventing peak loads on the sewer system. The system can even be operated smartly via sensors so that, when rain is forecasted, space is automatically made for new rainfall.

And the best part? You can combine a water retention roof with a green roof. This creates a roof that contributes to:

• Biodiversity: more flowers, bees, and birds
  
• Air purification: plants filter fine dust and CO2 from the air
  
• Noise reduction: less ambient noise, a more comfortable indoor climate
  
• Sustainable use of space: the roof becomes a fully functional surface
  

In short, turn your roof into a climate-active space.

How Does Roof Water Buffering Work?

The technology behind a water roof is surprisingly smart. Depending on the system and type of building, different solutions are available. Some common systems include:

1. Passive Water Buffering
   A simple variant where water is temporarily retained in crates or special storage layers. This system works with a delayed outflow to the sewer.
2. Active Water Buffering
   Smart systems with sensors and valves that monitor weather forecasts. Is rain coming? The system automatically creates space to collect new rainfall.
3. Combined Systems
   A water retention roof combined with a green roof. In this setup, water is not only stored but also used to irrigate the green roof, promoting better plant growth and extra cooling.

Each situation requires a different approach. For existing buildings, load-bearing capacity, drainage capability, and maintenance are important considerations. For new constructions, specifications can be factored in from the start. Therefore, it is always wise to work with a specialist.

Water Roofs in Practice

More and more buildings are choosing water roofs because they contribute to climate adaptation, energy savings, and sustainable use of space. The following project examples show how theory is being put into practice—each one an inspiring example of smart water buffering combined with green and energy-generating roofs.

KBF Warehouse, Amsterdam – Multifunctional Water Roof Without Slope

Challenge: High demands for sustainability, aesthetics, and functionality

The redevelopment of the former Chewing Gum Factory into the creative KBF Warehouse posed high demands for sustainability, aesthetics, and functionality. The biggest technical challenge? Creating a fully functional water retention roof on a completely flat surface without slope, while also meeting the strict requirements of the Amsterdam Rainwater Ordinance (at least 60 liters/m² of water storage).

Solution: Innovative Retention Roof System

In collaboration with Daktec, Sempergreen, and Optigrün, an innovative retention roof system was installed that retains 37,200 liters of rainwater on the roof. Thanks to a controlled drainage capacity of 1 liter per m² per hour and an emptying time of 60 hours, peak loads on the sewer system are greatly reduced. The vegetation layer, consisting of Sempergreen’s Sedum mix mats, is combined with retention crates and integrated solar panels. This creates a multifunctional roof that combines water buffering, vegetation, and sustainable energy generation.

Result: A Future-Proof Water Roof

The result is a future-proof water roof that meets all water storage and multifunctional requirements. It actively contributes to water retention during heavy rainfall, reduces heat stress, and promotes biodiversity. In addition, it is accessible to employees as a usable space. Thanks to the smart combination of blue (retention), green (vegetation), yellow (solar panels), and red (use), the roof was nominated for Roof of the Year 2024. The project demonstrates how innovative water roofs play a key role in climate-adaptive urban redevelopment.

Vrumona Headquarters, Bunnik – The First Detention Roof in the Netherlands

Challenge: Increased risk of flooding due to paved surroundings

At Vrumona’s business site in Bunnik, there was an increasing risk of flooding due to extreme rainfall and hardened surfaces. The climate adaptation project group of Green Business Club Bunnik, together with the regional water authority De Stichtse Rijnlanden, mapped out vulnerable areas. The conclusion: the roof surface of the Vrumona building offers great potential for buffering water and relieving the sewer system.

Solution: Innovative Detention Roof

To address this challenge, an innovative detention roof of nearly 600 m² was installed on the Vrumona headquarters. This smart water retention roof buffers rainwater on the roof and drains it slowly, even when the system is fully saturated. This greatly reduces peak sewer loads during heavy rainfall. Sempergreen supplied the green roof, which was installed in collaboration with Van Doorn Roofing Specialist and De Dakdokters. Within two days, both roof surfaces were equipped with a climate-adaptive solution.

Result: A Water Roof That Reduces Peak Discharge by 97%

The water roof on the Vrumona building reduces peak discharge by as much as 97%. The peak is also delayed by 25 minutes, while the roof takes a total of 38 hours and 24 minutes to drain the rainwater in a controlled manner. This means the project meets municipal and water authority standards. The result is not only a technically advanced water retention roof but also an inspiring example of how companies can actively contribute to water retention, climate resilience, and a more sustainable living environment.

Water Roofs: The Roofs of the Future

Cities face enormous challenges: drought and flooding, heat stress and lack of space. Solutions must be multifunctional, and roofs offer the perfect opportunity.

Whether it is a water roof on its own or combined with a green roof or solar panels, the trend is clear. Roofs are being actively used for climate adaptation, energy generation, and urban greening.

With a water retention roof, you give your building an edge in sustainability and comfort. You make a visible contribution to the city of tomorrow and save on energy costs.