Detention and Retention Roofs against flooding

Posted on Wednesday 14 July, 2021

More and more flooding is occurring around the world. Especially the intensity of rain showers, combined with longer dry periods, cause parts of the world to flood during heavy downpours. The recent floods in places like United Kingdom, the Netherlands, the United States and Australia are a case in point. Green infrastructure such as Detention Roofs and Retention Roofs offer the solution to this problem.

Increase in torrential rain events

As an effect of climate change, there is an increase in the number of extreme rainfall events on a global scale. In many places, the streets are regularly flooded. What's more, the number of peak downpours each year will only increase, resulting in more water than the stormwater drains can handle. This creates an unsustainable situation for sewer systems and will lead to even more water damage in the future due to increasingly frequent rainstorms.

Extreme weather events are rising on a global scale

Measures from municipalities and regional water boards

In order to make our cities future-proof, local governments and water boards are taking measures. Effective stormwater management is needed. More and more municipalities have drawn up a stormwater ordinance or code, which sets requirements for new buildings, conversions and renovations regarding the collection and storage of rainwater. The rainwater ordinance allows municipalities to lay down rules for discharging rainwater runoff. By means of these regulations, a municipality can enforce the realisation of water storage, both in new and existing buildings.

“The City of Amsterdam adopted a stormwater ordinance on 26 April 2021. This ordinance regulates an obligation for new buildings and for existing buildings that are subject to major renovation (...) to store at least 60 litres of rainwater per square metre and to drain this rainwater over the following 60 hours.”

Underground water storage

In order to comply with these rules, architects, property developers and housing corporations can choose from various solutions when constructing or renovating buildings. They can install water storage systems in the ground in the form of bioswales (for temporary storage), infiltration crates or water tanks. In the last two cases, the collected water can be reused, but it costs a lot of underground space and does not bring greenery back to the urban area.

Solutions against flooding: water tank, bioswale, infiltratation crates
From left to right: water tank, bioswale, infiltration crates

Detention and Retention Roofs against flooding

Infographic of a city using Detenion and Retention Roofs

Building green infrastructure in cities increases effective stormwater management, such as water retention and/or detention on the roof. The Sempergreen Detention and Retention Roofs provide the solution by harvesting rainwater on the roof. For example, almost 50% of all Dutch roofs are flat. This amounts to some 600 square kilometres of flat roof. By using this largely unused space, rainwater can be efficiently stored without having to dig a hole in the ground. This is especially useful in urban areas with little space and on existing buildings. In addition, these water retaining and detaining green roofs also provide extra benefits. They include increasing local biodiversity, extending the lifespan of the roof covering and cooling the indoor and outdoor areas. Both Retention and Detention Roofs can meet the requirements and wishes of the municipality or water board.

Sempergreen Detention Roof for existing buildings

The Sempergreen Detention Roof is a proven and innovative green roof concept that, contrary to Retention Roofs, can also be applied to existing buildings. This is because the Detention Roof is a lightweight green roof system that can also be installed on roofs with a slope of up to 2⁰. Thanks to its unique construction layers, the Detention Roof is able to fully buffer new downpours, even when saturated, and to drain them in a controlled manner, thereby minimising the risk of flooding on a project.

Download technical drawings Detention Roof

Sempergreen Retention Roof for water storage on new buildings

The Retention Roof is a popular water storage solution for new-build projects with flat zero fall (dead flat) roofs. Sempergreen offers various Retention Roof systems: in addition to Sedum plants, it is also possible to cover the roof with vegetation such as herbs, wildflowers or host and nectar plants to increase its ecological value. This water-storing green roof system can also be used in combination with solar panels.
Sempergreen Retention Roof systems
From left to right: Retention Roof Sedum, Retention Roof Sedum lightweight, Retention Roof Aroma, Retention Roof Biodiverse

Download technical drawings Retention Roof

So there are plenty of ways to store precipitation. We humans have experience in dealing with water. Let's solve this water problem together!

Added value for your project

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