Green roofs generally fall into two categories: intensive and extensive. Intensive roofs are very heavy with deep substrate levels, allowing for a wide variety of plants to be used. On some intensive roofs the substrate is so deep that they can have trees or bushes planted on them. This allows some intensive green roofs to be enjoyed in the same way as parks. Benches and paths can be installed to turn the roof into a new amenity space. Chicago City Hall is a great example, which utilises the unused space on the roof to give the workers a lush, relaxing area to eat their lunch and take their breaks.
Some green roofs have been converted into bowls lawns, restaurant terraces or in one extreme case, a grass tennis court on the helipad of the Burj Al Arab Hotel in Dubai. This is a very clever way of utilising otherwise unused space and to bring the green space back into the city.
Another ingenious use of space is the rooftop greenhouse. Companies are developing hydroponic greenhouses on top of buildings, such as Lufa Farms in Montreal. They grow fruit and vegetables on top of buildings then sell the fresh, local produce to the community. Not only does it make use of space, it also reduced carbon emissions from transport and allows fruit and vegetables to be sold, having been harvested just the day before. The greenhouse also creates a microclimate that allows produce to be harvested all year round. Some companies are using the waste bio-fuel to heat their greenhouses, giving a completely carbon neutral production unit.
If a building is not suitable for large amounts of weight to be added to the roof, there is the option of extensive green roofs. These are becoming more popular and use thin substrate depths to reduce weight. Sedum is often used as it provides a lush green coverage of the roof, is very resistant to drought and can be grown in very thin soils. Not only do the substrate depths reduce the weight of the system, they also ensure the sedum remains the dominant species. Many weeds and invasive plants cannot grow well in the thin, low nutrient substrates that are used on extensive roofs. They are unable to establish and, if they do, will soon die off.
Other plants like wild flowers and grasses can be added to increase biodiversity and attract a range of animals to the area. Wildflower meadows are the natural habitat of many native British animals such as bees, bats and butterflies. As wild flower habitats are becoming increasingly scarce because of urbanisation, these species are declining in numbers. Wildflower roofs offer a great way to these animals to your home and garden and also help them to survive and try to increase their numbers.
You can increase the biodiversity of a green roof by adding other features like wood piles, rock mounds, bird or bat boxes. The natural wildlife of the area will be drawn to the roof and be given a habitat where they can breed and flourish. This is especially important for some endangered species such as starlings, skylarks, bees and graylings that have fewer and fewer places to live as their habitat gets taken over by urban sprawl.
Brown roofs are another type of living roof. They are often left unseeded and sometimes contoured. This creates niche habitats for the local plant species that are self seeded by wind or birds. Once the natural plants start to develop other wildlife is attracted to the area. This replicates as closely as possible the environment that would develop at ground level. Brownfield sites provide a perfect habitat for Black Redstarts, another British bird that is threatened because of human activity.
Solar panels can also be added to green roofs, which will increase the diversity of the roof and help the efficiency of the PV panels. If you want to read more about solar panels and green roofs, check out our article.