Going blue and green can help reduce flood risk

Green roofs are now common in the UK

We need more "blue roofs" for flood risk alleviation, says Radmat’s Mark Harris, who is part of a new working group looking at the issue.

While it may not score many points on the political stage, the reality is there is no quick fix solution to flood risk. Building Regulations can help to ensure that the 2% of the built environment that new build construction represents annually performs in a manner to help alleviate flood risk. However, without aligning various regulations on building fabric and landscape design, we will continue to miss a trick.

Another lost opportunity will arise if we ignore the opportunities to use existing buildings to contribute to flood risk alleviation: clients, developers and the construction industry itself need to be more open to the opportunities to do things better, and to adopt solutions proven in other countries. Unfortunately, this is where the “English disease” impacts and the choruses of “it wouldn’t work over here” come to the fore. 

Around 15 years ago, this chorus was heard loud and clear with regard to green roofing, or putting plants on roof space – a design principle at the time already in wide use in western Europe with the principal aim of contributing to sustainable drainage.

In Germany, government buildings were required to have a percentage of their roofs greened, private developers gained additional footprint and insurers reduced premiums. In Basel, Switzerland, it’s not possible to build without having a green roof.

Since then, over the past 16 years, green roofs have become a common part of the UK construction landscape, especially in London, where the London Plan has driven the requirement. London now has more green roofs than any other city in the world.

Here, rainwater attenuation has been one of the key drivers, together with biodiversity. But green roofs can only provide a limited contribution to flood risk alleviation, because once they are saturated they drain just like a normal roof.

But the technology that can provide a far greater contribution is the blue roof. Jonathan Ward, an associate director at Arup, has defined a blue roof as “a flat roof allows stormwater to be temporarily stored and drain away slowly over a few hours, thereby reducing flood risks”. 

Consisting of a water storage medium, typically a cellular geocomposite drainage/retention board, and a discharge control mechanism, blue roofs attenuate rainwater rather than drain it as quickly as possible.

Correctly designing the blue roof releases attenuated water at the required discharge rate, providing the integral source control and attenuation required by a Sustainable Drainage System [SuDS] system without the requirement for land-consuming ponds and retention basins or below-ground geocellular storage tanks.

What’s more, the roof area doesn’t have to be significant: a roof of less than 300 sq m can attenuate 94% of the rainfall volume of a 1-in-100-year, six-hour storm event, draining over a 27-hour period.

Incorporated in an inverted roof construction the blue roof elements are installed above the insulation and thermal sheet, and can be finished with a green roof (extensive, wildflower, biodiverse or intensive), paving or gravel ballast.

Blue roofs are not a new idea. They have been around in one form or another for well over 60 years, in the UK, Europe and the US. They can be used for attenuation, storage, grey water harvesting or a combination thereof. 

So why aren’t we building more blue roofs in the UK? The answer to that probably lies in scepticism, outdated concerns over flat roofing materials, and a lack of guidance documents and legislation.

To breach these boundaries, a pan-industry working group has been established by the National Federation of Roofing Contractors Joint Flat Roofing Technical Committee. Consisting of representatives of the NHBC, BBA and a number of waterproofing trade bodies, waterproofing manufacturers and drainage manufacturers, the committee intends to tackle the issues of certification and codes of best practice. 

With these instruments in hand, the committee will move on to pressing government and the Environment Agency to look more closely at the positive contributions to flood risk management that blue roofs can have, to both new builds and existing buildings.

The BRE will be another target for discussion, as the benefits of a blue roof should have a positive contribution to its BREEAM assessment process.

Mark Harris is head of technical services at Radmat Building Products