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Put a wetland on the roof!

Posted by Maisie Kelly —

Wetlands are magical. Not only are they teeming with biodiversity, but they also provide essential protection against natural disasters and are the most effective carbon sink on the planet. A wetland is an area where water covers the soil or is present either at or near the surface of the soil, all year round or seasonally.

Typically, wetlands are located on the ground. But does that have to be the case? What if we told you to look up…

At Axter, we believe in the full potential of flat roofs. There is an entire world above our heads that often goes completely unnoticed and unutilised. We are here to change that.

Our team spends a lot of time thinking about how much better off our communities and environment would be, if only we used all that flat space above our heads. Between us, we have an endless list of ideas on how we would love to transform the roofscape within our communities. But for the purpose of this article, let us talk you through just one of those ideas: a wetland on the roof.

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Project overview

At Axter, we support six partner charities that are bringing about positive changes within our communities and environment. One of those charities is Wildfowl & Wetlands Trust (WWT), who campaign to develop, conserve and protect wetlands.

When the opportunity presented itself for us to help create a wetland area on the roof of WWT’s new building at Welney, we jumped at the chance. As part of our Social Responsibility programme we are donating our technical and design services to WWT and we are currently in the planning stage of this project. It has been fascinating to work through the many benefits and challenges presented by this unique project and we are excited to see it develop.

What is so great about wetlands?

Wetlands offer a whole host of benefits. Here are just some of them:

  • Improved water quality: the natural diversity of plants and soil found in wetlands acts as a filtering system being able to capture, process and store nutrients whilst filtering contaminants.
  • Management of urban flooding: wetlands soak up and store floodwater. They play a vital role in SuDS (Sustainable Drainage Systems), where the attenuation and disposal of rainwater can manage stormwater.
  • Increase of habitats: because of their semi-aquatic features, wetlands provide the most productive and diverse of wildlife habitats.
  • Erosion prevention: due to the binding of soil with roots in wetlands, they help to reduce erosion.
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What are the threats to wetlands?

Unfortunately, the world's wetlands are under threat, from multiple angles. Over the last 300 years, we have lost a shocking 87% of our wetlands.

  • A large amount of wetland space has been drained to make way for unsustainable developments, including housing, industry, and agriculture.
  • Of the wetlands that do remain, sadly a lot of them are contaminated with pollution. 80% of global wastewater is released into wetlands untreated.
  • The changes in temperature, caused by climate change, also cause wetlands to dry out. This creates a vicious cycle, because, when we lose wetlands, we are losing the most effective carbon sink on the planet, thus further adding to the effects of global warming.

When wetlands are the very thing that can provide us with the solution to climate change, it follows that they should be protected at all costs.

Why put a wetland on a roof?

For us, utilising the roof space is a wonderful way to ensure that buildings have not taken away from the land they are built on. We like to imagine that the roof is just the ground lifted up a few levels.

As WWT’s new building at Welney is built on a wetland, it makes perfect sense to recreate a wetland on the roof of their building. As well as the benefits that wetlands provide in general, having a wetland on the roof also comes with additional advantages for the building itself.

Evapotranspiration is the process by which water moves from the earth to the air through evaporation and transpiration. This process happens in wetlands naturally, so, when a wetland is put on a roof, it is an effective way of consuming heat without warming up the surrounding surfaces. Therefore, it acts as an insulator and temperature buffer for the indoor environment. For the outdoor environment, roofs of this nature can also help mitigate the urban heat island effect.

As well as aiding the temperature control of the building, a wetland on the roof would also help to reduce rainwater runoff. Like with the wetlands themselves, a wetland on the roof would aid in storing and disposing of rainwater.

The fact that our Axter waterproofing membranes on the roof would be covered by the wetland and not exposed to UV radiation, has the added benefit that they will last longer.

A key win for us, is the fact that a wetland on the roof will lead to an increase in biodiversity, something that we would love to see prioritised in more roofing projects.

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What challenges do we face in this project?

Water Attenuation

Usually when designing a roof, we must think about how rainwater is managed and taken off the roof in a controlled way. However, with a wetland on the roof, the aim is the complete opposite. In this situation, we are trying to keep water on the roof. Careful detailing and design are essential when it comes to the success of this project.

Thermal Performance

Another challenge is that of thermal performance. Wetlands can act as a thermal barrier. However, as with blue roofs, no formal testing has been done for a wetland on a roof. It is difficult to determine the thermal performance of a roof that has constant water on it.


We must also think about the threat weather could pose to the roof. In the winter months, how would a layer of ice affect the thermal performance of the roof? How would the weight of it affect the structural loading of the building? And in the summer, how will the roof and building be affected during drought periods?

What are Axter’s proposed solutions?

The solution that we have come up with is to install several multi-functional green roofs on differing levels. The green roofs at a higher elevation will drain into the lower wetland roof. When the wetland rainwater storage reaches the overflow outlets, the water would then drain into the reservoir wetland ponds at ground level. From here, the water would travel back into the wetland system. As in nature, we will be creating a full circle rainwater management ecosystem.

What are the next steps for the project?

We are now awaiting confirmation from the architects for the planning to be approved.

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