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What Are Green Roofs and Walls?

Green roof

Green roofs and walls are an increasingly common feature of urban planning discussions. This is unsurprising as, either in conjunction with other green building options within a sustainable development or by themselves, they have a huge range of benefits. These include economic, practical, health and environmental pluses. Also known as living roofs and walls, both can be installed on single buildings or entire developments. We’ve touched briefly on what they are before, but in this post, we’ll explore them, and their benefits, in more detail.

Green Roofs and Walls: The Basics

Living, or green, roofs and walls are ones that are all or partly covered in vegetation. Green roofs use some sort of substrate or matting to allow plants to grow on them. This soil or matting is placed on top of a waterproof membrane to protect the roof. Soil depth determines the size of plants used with deeper soils supporting larger species. Green roofs with deeper substrates have to be designed as part of the building and cannot be retrofitted. This is because it is essential to plan structurally for the increased weight of soil and plants. Using less soil means that they can sometimes be retrofitted, although a structural engineer should always be consulted first. Thinner substrates also mean only using plants with shallow root systems as those with deeper ones won’t have the support they need. Green roofs are usually a more expensive option than green walls.

Living wall near Kings Cross Station
This living wall is near King’s Cross Station in London

Green walls, meanwhile, support living plants that grow up walls. Some incorporate brackets, matting or cables to support non-climbing plants. Others use trellises to support vines and similar climbing plants as they grow up the building. They can be freestanding or attached to the wall itself. The most sophisticated examples include systems that collect water for use elsewhere in the building. The simplest walls are ‘passive’ ones that just allow plants to grow up them naturally without providing any sort of support system. Green walls can be installed inside buildings, as well as externally. A recent Finnish study even found that green walls inside workplaces can improve workers’ immunity to pathogens and allergens. They also increased the number and range of friendly bacteria on the study subjects’ skin compared to the control group.

Living wall Bristol University
Most living walls are external, like this one at Bristol University

The Benefits of Green Roofs and Walls

Green roofs and walls aren’t a new phenomenon. Humans have understood the benefits of covering their homes with turf and grass since at least the ninth century. The Vikings that settled in Iceland, for example, quickly adapted their traditional wooden longhouses to incorporate turf and keep their homes insulated. Completely earth–covered houses are less common in the developed world these days. But even just greening a roof or wall has a number of benefits.

Green roofs and walls Bristol
This city centre toilet block in Bristol now has a green roof
  • Green roofs and walls provide both insulation in cooler conditions and shade in warmer ones. In fact, a 2021 Plymouth University study found that living walls can reduce heat loss from the underlying wall by more than 30%. This insulation and shading means significantly less energy is needed to heat and cool a building, producing less carbon emissions and saving money. And the hotter the climate, the greater the cooling effect.
  • Because the plants and soil absorb heat from the air and provide shade, green roofs and walls can also reduce the urban heat island (UHI) effect. UHIs are towns and cities where the energy from transport, people and closely packed buildings makes the area much warmer than the surrounding rural areas. As temperatures rise, this could become increasingly important.
  • Vegetation and soil also absorb more rain and slow its runoff. As climate change leads to more extreme weather events, this could play an important role in flood prevention. The more plants and soil there is, as well, the more water is retained. They also filter out waterborne pollutants.
  • Green roofs and walls can also be fitted with systems for collecting water and directing it for use elsewhere including within the building. This means using less water from the local supplier and saves money.
  • As well as filtering rainwater, the vegetation traps airborne pollutants. This includes dangerous heavy metals. Airborne pollution is a major threat to health in our towns and cities and while green roofs and walls aren’t as effective as trees at removing particulate pollutants, they are a useful solution in places where more trees can’t be planted.
  • Living roofs and walls sequester carbon and, depending on the species used, can quite quickly offset the carbon emitted by their construction.
  • They can also increase an area’s biodiversity, attracting a range of birds and invertebrates. Where a number of buildings have green roofs or walls, they create vital green corridors for wildlife to move about. This is especially important where highly developed areas have replaced green spaces at street level.
  • As well as absorbing heat, water and carbon, soil and plants help absorb sound. This means buildings with green roofs and walls increased sound proofing in our noisy urban spaces.
  • Access to green spaces and wildlife improves our physical and mental health. Green roofs and walls can help provide this access in heavily built-up areas. They are also aesthetically pleasing and can even increase a property’s value because they reduce household costs.
  • Although large-scale green roofs have to be installed at the construction phase, they can be fairly easily retrofitted on smaller structures such as toilet blocks and bus shelters. As such, they can help councils and developers meet their sustainability targets.
Living wall trellis green roofs and walls
Eventually, plants will cover this very new living wall support

Green Roof and Wall Pre-installation Considerations

As you can see, there are lots of reasons why individuals, councils and developers should consider including green roofs and walls in their projects. There are some things to investigate first, however, if you are contemplating a green roof or wall yourself. Although there aren’t any specific planning regulations in the UK, either for retrofits or new builds including them, it is always a good idea to double check with your local council first. They might have guidelines or policies that you need to know about in certain circumstances. You will also need to find out if installing a green roof or wall affects your buildings insurance or mortgage.

Most important of all is the structural side of things. You will need to be sure the building can carry the increased load of soil, plants and retained water. Of course this does mean that green roofs can be expensive in the short term, although they save money later on by promoting less energy use. There may also be issues regarding how you remove excess water from a green roof. A good quality waterproof membrane below the soil or matting is likewise essential. More information about structural and legal considerations can be found here.

Green Roofs
Green roof projects can be big or small

You will also need to make sure you use the right plants. Otherwise you risk everything dying within a short space of time and losing all the benefits. Because of the lack of shelter, plants need to be drought and wind tolerant. Stonecrops and other succulents are good choices as they can retain water for long periods during hot spells. They are also often low-lying so can withstand exposure to wind. Grasses and mosses are good, too, as they can form stable, low-lying mats. As mentioned above, thinner layers of soil mean only plants with shallow root systems should be used. It is a good idea to use native species of plant, as well. This provides better habitat for pollinators. They have evolved to access nectar from the flowers of native plants and sometimes can’t do so from non-natives.

White stonecrop
Sedums, such as this white stonecrop, make good green roof plants

Going Green

We are in the middle of a climate and biodiversity crisis. Green roofs and walls are undoubtedly valuable tools to help combat both. With a host of environmental, economic and health benefits, they add a huge amount of value to building and development projects big and small. That more projects are including them is cause for celebration.