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Friday, 24 October 2014

Raising the 'green' roof

By Helen Roberts


We currently have a real shortage of housing in the UK and the estate agency Savills has estimated that there will be a shortfall of 160,000 homes in the next five years unless local authorities act. With this in mind, I started thinking of the building industry and how sustain­­able building design has become increasingly important over the last few decades. Not only does the industry consider the sustainability of the materials themselves, but designs aim to reduce consumption of non-renewable resources and minimize waste during and after the life of the building, while creating a healthy and comfortable environment for the occupants.


Within the field of sustainable building design is the subject of green roofs. This is an area of design that holds great interest to me, as I am a landscape architect with previous training in plant sciences. Green roofs play a pivotal role in urban environments by reducing rainwater runoff, reducing energy consumption for heating and cooling, heat island mitigation, creating valuable wildlife habitats and also making an aesthetically pleasant landscape for people to escape from the urban environment. 

What is a green roof?
Green roof on Chicago City Hall. Photo credit: TonyTheTiger
[CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)
via Wikimedia Commons

A green roof is a platform or roof on which vegetation is grown or wildlife habitats are created. The basic elements include a waterproofing membrane covered with a growing medium and vegetation. The design, ecology and aesthetics of a green roof can vary considerably, however, and can be adapted specifically to suit a particular location or design brief. Plants in containers on a roof top are not considered to be a true green roof.

The term green roof, however, can also be used to describe roofs that incorporate green technology, such as solar thermal collectors or photovoltaic (solar) panels.

The history of green roofs

Green roofs are not a new concept. Dwellings of the Neolithic period, such as the Neolithic village of Skara Brae in Orkney, are thought to have had turf roofs. The Hanging Gardens of Babylon, one of the wonders of the Ancient World, were extravagant green roof gardens, thought to be irrigated by about 35,000 litres of water brought in through aqueducts and canals.
The houses at Skara Brae, Orkney were thought to have
had turf roofs. Photo credit: Antony Slegg
[CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)],
via Wikimedia Commons

Turf or sod roofs were common centuries ago in Scandinavia and can still be seen in places like the Faroe Islands. I visited Lund, Sweden recently and saw beautiful turf roofed farmhouses in the museum of cultural history. The turf helped keep dwellings cool in the summer and warm in the winter. However, these structures would most likely have leaked and also would have had the inconvenience of burrowing wildlife!

Modern green roofs didn't develop until the 1970s in Germany, when legislation was passed to encourage the introduction of green roofs. Unlike the historical turf roofs, modern green roof designs include drainage and root protection measures, as well as lightweight growing media.

The UK is somewhat behind continental Europe in terms of using government policy to implement green roof technology. But things are changing and there has been an increase in the use of green roof technology over the past decade. In fact, Bristol’s development policy (Bristol Development Framework Core Strategy; adopted in June 2011) encourages the incorporation of green roofs as a way of enhancing the biodiversity value of new building developments and views green roofs as an essential asset of the strategic green infrastructure network.Bristol  

Green roofs can be extensive, intensive or semi-intensive

Green roofs vary in ‘intensity’ in terms of the depth of substrate used and the level of maintenance needed, which affects the type of vegetation that can then be grown. A typical green roof will have, on top of the roof itself, a layer of waterproofing, a root barrier, protection/moisture retention matting, a drainage layer, a filter sheet, the growing substrate and then the plants. Green technology, such as solar panels, may also be incorporated into the design of the vegetated roof.

Green roofs are classified as extensive, intensive or semi-intensive in nature. Extensive green roofs are less than 100 mm deep and are relatively low maintenance. Their shallow depth means they are lighter but that they can support fewer vegetation types. This means they generally have lower biodiversity value and limited water holding capacity. Most people will be familiar with sedum matting as a common form of extensive green roof.
Construction layers of a green roof.
Photo credit: thingermejig (flickr.com)
[CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)],
via Wikimedia Commons

Semi intensive green roofs have substrate depths of about 100 mm to 200 mm, require moderate maintenance, can support a greater range of plant species and have the ability for rainwater attenuation.

Intensive green roofs have deeper substrates (over 200 mm) and therefore require more substantial structural support. The deep substrate can sustain more elaborate plantings, including many different tree and shrub species, which offers a more garden-like space for users. Intensive green roofs require more maintenance and complex drainage and irrigation systems, but can offer rainwater attenuation and a greater degree of species biodiversity.

The aim of the green roof will ultimately influence its design. If, for example, the aim is simply to have an insulating effect on the building, a low-maintenance extensive green roof with low-lying vegetation would probably be sufficient. If, however, the aim is to attract and enhance wildlife, an intensive design is likely required to support a diversity of plant species that can provide a variety of structure and microhabitats. I will discuss biodiversity and wildlife green roofs in more detail in my next blog post.


The benefits of green roofs:

Green roofs help improve the urban environment in many ways, from creating a natural space for office workers to enjoy to helping mitigate the urban heat island effect. Here are some of the benefits of green roofs:

Creating a biodiverse space and a relaxing place

Green roofs can increase biodiversity in urban areas where ground level space has been developed and valuable green corridors lost. Sky-high gardens can be important stepping stones for wildlife and can create habitat and forage for a variety of species, which wouldn't exist with conventional roofs.

These places can also provide a haven for people to visit or to just view and offer a respite from a hard urban setting. 

Green roofs slow down runoff and help reduce flooding

There is a requirement now in the UK (under the Flood and Water Management Act 2010) that new developments mitigate storm water runoff and include appropriate water management systems. An established green roof can significantly reduce the peak flow rates and total volume of water runoff. Water is stored by the plants and substrate and is released back slowly into the atmosphere by evapotranspiration and evaporation. The plants also help filter out pollutants in the rainfall.

Many features of Sustainable Urban Drainage Systems (SUDS), such as permeable surfaces and swales, are not easily incorporated into a hard urban and so green roofs are considered a good solution to reducing storm water runoff. Interestingly, it has been found that in the summer 70-80% of rainfall can be retained in a green roof and in winter 10-35% (due to differences in evapotranspiration in summer and winter). 

The cool down effect of green roofs

Urban areas that are hotter than nearby rural areas are described as heat islands. The additional heat means more energy is used in summer for cooling (air conditioning and refrigeration), there are more incidents of heat-related illness and mortality and there are implications for air and water quality. Green roofs help improve local air quality and cool the urban environment by reflecting more of the sun’s rays compared with conventional roofs. The plants shade and insulate the underlying roof and have a cooling effect as water is released through evapotranspiration and evaporation – the building equivalent of sweating.

Green roofs reduce energy consumption

The thermal insulation properties of green roofs reduce the need for air conditioning in summer and heating in winter, decreasing associated emissions and dependence on non-renewable resources. 

Green roof allotments

There is increasing interest in the use of green roofs for food production and this ties in closely with the provision of amenity space. There is limited green space that can be used at ground level for food production in urban areas, so the logical step is to go up!. Roof-top allotments reduce food transportation and help increase the supply where the demand exists. For the individual household, it can help reduce food costs and provide many benefits associated with growing your own food. For a community, rooftop gardens can become a centre for social cohesion.

Though there are examples of agri-roofs, mainly in Asia, the use of roofs for food production is relatively unexplored and will provide ‘food for thought’ in the design of future green roofs.

Raise the roof on green roofs

With their many benefits, green roofs are likely to become a vital component of building designs in the future. New developments are imminent in the face of a housing shortage and green roofs offer an opportunity to improve the urban landscape, providing habitat for essential species, such as pollinators, and potentially helping respond to challenges with food security. Green space that is lost on the ground needs to be created up above with the transformation of featureless barren roofs into beautiful diverse green places.