Detention and Retention Roof
Due to climate change, we increasingly face extreme weather events where a lot of rain falls within a short period of time. However, our sewer systems are not designed to cope with this. As a result, our streets are more often flooded during and right after a downpour. Blue-green roofs, such as Detention and Retention Roofs, are an ideal solution to combat this water problem. Let's collect rainwater where it falls first; on the roof!
The benefits of a blue-green roof
It is possible to capture rainwater in the ground, such as in retention crates, water tanks and bioswales. Something that is difficult to achieve and can also be pricey in dense and urban areas. Roofs can provide the solution! On a flat roof, water retention crates and other water storage layers can be used to capture water. Many flat roofs are unused and have lots of empty space. So there is plenty of room!
A roof specially designed to (temporarily) store and delay water runoff is also called a blue roof. If the roof is also covered with plants, it is called a blue-green roof. With this combination, you benefit not only from the advantages of a water roof, but also from a living and varied view, improvement of biodiversity, and air purification.
Project-specific advice for the right solution
Do you have water regulations to adhere to or interest in a multi-purpose roof? We will be happy to help you find the perfect solution for your project! Request a water calculation or contact our specialists for advice.
Sander Scheper | +31 6 151 634 02 | sander.scheper@sempergreen.com
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Sempergreen’s blue-green roofs
Sempergreen offers two types of water-storing green roof systems for flat roofs and roofs with a low slope: the Detention Roof and Retention Roof. But what is the difference between these two? We explain that below.
Retention Roof – for flat roofs with a high roof load capacity
A Retention Roof is a green roof where a Water Retention Box is installed instead of drainage. The most suitable type of box and the desired storage capacity can depend on the water regulations. We offer various options and provide project-specific advice. A Retention Roof can be covered with a mix of Sedum, a variety of nectar and host plants, or a combination of the two. This green roof system is suitable for roofs with a roof pitch of 0 degrees that can support a lot of weight and is therefore often used in new construction projects. The Retention Roof can also be combined with solar panels and a Sempergreen Biodiversity Package.
Some of the rainwater collected by the green roof evaporates via the plants and substrate; evapotranspiration. As a result, less water ends up in the sewer system. In addition, the roof ensures that the water does not all flow off the building immediately during heavy rains. This is where the drain restrictors on the roof play an important role. We offer both static and dynamic (smart) retention roof drain solutions, which can control the runoff speed and timing.
Please refer to the system build-up documents below for more (technical) information about these green roof systems. Our Sales Department is also happy to provide you with more information.
- System Build-up Retention Roof Sedum
- System Build-up Solar Retention Roof
- System Build-up Retention Roof Biodiverse Lightweight
- System Build-up Retention Roof Biodiverse
Detention Roof – lightweight retention solutions for roofs with a low slope
Due to its low weight and the fact that the Detention Roof can be installed on a roof with a slope of up to 5 degrees, this blue-green roof system is ideal for existing construction and renovation projects! The build-up of the Detention Roof, based on the Purple-Roof Concept, consists of a detention layer, a storage layer, substrate roll and a Sedum-mix blanket. By means of friction, the detention layer ensures that the runoff of the rainwater collected in the storage layer is delayed. The delay can be up to 24 hours.
For an even more multifunctional solution, the Detention Roof can also be combined with Over Easy Solar's lightweight solar units. Check out the system build-up below and read more about this system with vertical solar panels here.
- Download the System Build-up for the Detention Roof here
- Download the technical drawings of the Detention Roof with HC40 here
- Download the technical drawings of the Detention Roof with HC60 here
- Download the System Build-up for the Solar Detention Roof here
How does a project with greenery & water storage on the roof work?
Sempergreen's blue-green roofs are suitable for larger roofs, from 50 m2. For new construction projects the Retention Roof is very suitable. The roofs of existing construction and renovation projects can be converted into water roofs with the Detention Roof system. The installation methods are similar to that of other Sempergreen green roof systems. If the roof is larger than 50 m2, you can request a water calculation with us. Based on this calculation we can determine exactly what the Detention or Retention Roof can do for your project.
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A Detention Roof of almost 600 m2 was installed on the premises of soft drink supplier Vrumona in 2021. Read the article here to find out more about this project!
