In Beverwijk, the Netherlands, a new construction project is not allowed to discharge rainwater via the sewer system. The apartment building on De Brink with 94 sustainable, gasless homes will therefore be equipped with a Rockflow water buffer under the building and a water-buffering roof with stone wool elements under a sedum cover. The Rockflow buffer beneath the ground not only collects the water from the open plot surface, but also any overflow from the green-blue roof.

Flexible solution

“A fairly unique situation, in which stone wool is applied to both the roof and underground to meet the water storage assignment," notes Daan Los, technical advisor at Rockflow. “Originally the plan was to start working with a crate system, but during the design it became clear that Rockflow is a better solution. Due to the high groundwater level of approximately 85 cm below ground level, there is relatively little room to install an infiltration buffer for rainwater. In such cases, the flexibility of Rockflow is a major advantage. You are not tied to a fixed height for the elements and little cover is required. ” This resulted in a buffer, located under the indoor parking layer, between the columns of the building. The buffer has a total capacity of almost 100 m3.

Load-bearing capacity

When Rockflow was presented as a solution to the space problem, the construction team had questions about the system’s load-bearing capacity, "says Marco Wessels, work planner at Sturm Zaandam (infra/GWW). “I had gained experience with Rockflow in previous projects and I knew it would be an ideal solution for this project. However, the load-bearing capacity of the system was very important here. The Rockflow system forms the foundation for the parking layer of the project. It must be capable of being filled with cars without the risk of subsidence. Marco: “But load-bearing capacity was also a question because the complex is built above the parking layer. The floor of the first floor had to be supported with props during its construction. These props are therefore placed on the water buffer. So you want to make absolutely sure that the system can handle it.”

The material does the talking

Builders are familiar with the soft stone wool that is widely used as an insulation material in construction. Unlike insulation, permeable stone wool elements are produced for Rockflow. Because of the high groundwater level in this project, the buffer here is 33cm high. Marco: “During the presentation by the Rockflow people, extensive explanations were given, but the most convincing moment is when you actually feel a sample of the material. Then it really starts to do the talking. For that matter, it had also happened to me earlier in my introduction to the system. Since then, we have already installed Rockflow on several projects, so we know how strong it is and how easy it is to fit into all kinds of existing situations. The advantage over plastic crates is not only the flexibility of the system – we look at the overall picture. From price per cubic metre to ease of handling and sustainability aspects. But flexibility is indeed important to us. Sometimes you encounter things in the ground that are in the way. If so, it is an advantage if you can easily work around it.“

“You can see how flexible Rockflow is and how easy it is to lay it and run pipes through it. It is lightweight and stackable. If necessary, you can adjust the elements during installation using simple tools.“

Marco Wessels

Sturm Zaandam (infra/GWW)

Free to determine the shape

The installation plan, which translates the requirements into definite elements and connections for the pipework, is the basis of every Rockflow solution. Daan Los: “In this case, determining the shape on site was the biggest challenge. There are columns that hold the building up and the buffer is placed between them. It is a great advantage then if you are free to determine the shape. In this case, it became a 33 cm high buffer between the columns. Stone wool elements can be set against each other until they form the shape you have designed. There’s no need to wrap fabric around it, and you don't need to glue or clamp anything. Sand around it and over it, and you’re done.”

Rockflow - Beverwijk - Brink - Case - Stedelijke klimaatadaptatie

Buffering on the roof

Daan Los: “In more and more municipalities, water storage is mandatory within a building permit. In the event of a new development, water that falls on your building or pavement on your own premises must be collected in order to be infiltrated, or allowed to flow into the sewer or water course at a delayed rate. In this project, rainwater falling on the roof will also be buffered immediately on the roof. ” The 'SpongeTop' roof holds up to 80 litres of water and was designed and installed by Roosendaal Landscaping. Director Dirk Roosendaal: “The beauty of this solution lies in its simplicity. A water buffer made of stone wool that can be installed on any flat or sloping roof. Gravity regulates it further. The simplicity of a natural solution that works with natural forces. In this case, it has become a green-blue roof, water-buffering and with covering sedum. In the summer, water evaporates, cooling the environment and the sedum cover absorbs water from the Rockflow elements. The system is connected to the Rockflow system below the building, ensuring capacity in extreme rainfall.”


Stone wool on the roof and underground contributes to the complex's sustainability score. Stone wool is made from natural stone and is fully recyclable at the end of its life cycle. Marco Wessels: “Rockflow is also chosen for sustainability reasons, and sustainability was an important requirement for this project. Let's just say that overall it’s a hot item and we’re happy to work with it. Working with Rockflow is therefore a good choice.”

More information about stone wool for central attenuation and drainage systems:

Related Rockflow projects:

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