What is a Green Roof?
No, it is not just a really good emerald paint job. A green roof refers to a building roof that is covered with vegetation – and covered intentionally, not just a mossy growth! An idea that has been around for a while, green roofs are definitely not a new thing. They were very common among sod houses on the American prairie, and green roofs can still be found on many buildings in northern Europe. In fact, in Germany, Switzerland, and Austria living roofs are required by law on roofs of suitable pitch and in France, a new law has just been passed that all rooftops on new buildings built in commercial zones must either be partially covered in plants or solar panels.
The popularity of green, living roofs is rapidly growing as more people are turning towards natural solutions and sustainable living. In recent decades as the technology behind the sustainability of green roofs has improved, architects, builders, and city planners all across the planet have begun turning to green roofs not just for their beauty but also for their practicality and their ability to lessen the environmental problems caused by conventional roofs.
Waterproof membranes now make it easier to design green-roof systems that capture water for irrigation, allow drainage, support the growth, and resist the invasion of roots. There are many, many positives and barely any negatives to having a green roof policy whether it be on a housing estate, office blocks, hospitals or even your own home.
Okay, but Why Do We Need Green Roofs?
For a great number of reasons – the average urban roofscape is a lifeless place of bituminous, chemical surfaces, violent temperature differences, strong winds and an antipathy to water.
Blurgh. Green roofs take these dead zones and turn them into areas of growth that not only help the local environment and the local wildlife but also help reduce pollution, lower your heating bill, are far more pleasant to the eye and increase the durability of your roof.
Simply put, if you were looking out of your high-rise apartment window, what would you rather see around you – concrete, tar, empty roofs shimmering in the heat, the odd bored looking pigeon? Or vibrant, green rooftop landscape, alive with flowers and colour and wildlife?
Global warming is causing a climate change that is increasing the frequency and intensity of rainfall each year (Atkins et al 1999, DOE 1996, UKCIP 2001). Traditional drainage systems cannot cope with such instant and constant changes in flow rate and volume. This leads to the flooding of the drainage system. When rain falls on a conventional roof, it sheets off and floods down into these overworked storm drains. This rain water is unabsorbed and unfiltered, carrying nutrients, silts and hydrocarbons, chlorinated organics and heavy metals from the dead zone surfaces of buildings directly into watercourses.
A living roof works in much the same way a meadow does, by absorbing water, filtering it, slowing it down and even storing some of it for later use. Green roofs store rainwater in the plants and substrate and release it back into the atmosphere through a process called evapotranspiration. That ultimately helps reduce the threat of sewer overflows, and returns cleaner water to the surrounding watershed.
Once established, a green roof will not only retain rainwater, but also moderate the temperature of the water and act as a natural filter for any of the water that happens to run off.
Urban Heat Island and Pollution Reduction
The urban heat island effect is the temperature difference between urbanised areas and the surrounding rural areas. Because urban landscapes tend to have a much higher proportion of dense, dark impermeable surfaces which have a low reflectivity, it means they absorb the heat of the day. This stored heat is then released at night warming the city more than the surrounding countryside. This can make city centres up to 7ºc higher than the surrounding countryside (USEPA 1992).
What’s so bad about that? Well, in the UK over 24,000 people die every year from air pollution (GLA, 2001b) largely due to the air quality in urban areas. The urban heat island effect makes the ground level ozone production, (formed by a natural reaction caused by heat and sunlight between volatile organic compounds and nitrous oxides) worse.
The plants and vegetation on green roofs absorb the light and reflect this heat, dramatically reducing the impact of the urban heat island effect.
Green roofs can also help reduce the circulation of dust and airborne pollutants throughout the city, as well as filter noxious gases and lessen the production of smog. This in turn plays a role in reducing greenhouse gas emissions.
But That Is Not All
Whilst green roofs have many social and economic benefits, are an easy and effective way to beautify any build environment and increase any investment opportunity, the major bonus to them is an environmental one. Green roofs are habitable.
They can obviously not directly replace ground-based habitats but green roofs can become a sort of ‘stepping stone’ for wildlife, and, if planned properly can cater for a variety of flora and fauna to attract a whole range of wildlife species large and small from ants, spiders and beetles to lapwings, plovers and crows. Essentially, a chain of rooftop islands that connect with each other and the countryside at large.
Different types of green roofs can support different habitats and species depending on the type of vegetation they contain. An excellent example of the biodiversity of plant life which can be achieved on a green roof is Sharrow Primary School, Sheffield which has recently been declared a Local Nature reserve.
The Schools green roof covers 2000 m2 and uses a variety of wildlife habitats including limestone grassland, pioneer woodland, urban brownfield meadows and a wetland area with a shallow pond.
Cost of Green Roofs
Developers of living roofs seem to have met most, if not all, of the technical challenges involved in joining a biological layer onto the top of practically anything, from a bus stop to a factory.
While the average cost of installing a green roof can run two or three times more than a conventional roof, it is likely to be cheaper for you in the long run, thanks largely to energy savings. The insulation offered by green roofs can reduce the amount of heating needed in a building, as roofs are the cause of the greatest heat loss in the winter and the hottest temperatures in the summer. Green roofs also shield the roof itself from ultraviolet radiation thus extending its life.
Green roof areas can also add a great deal of value to a building. Not only do the improved views make buildings easier to let or sell, but they can act as an incentive to those interested in the multiple benefits offered by green roofs. You can always have accessible roofs designed too, which allow people to access, relax, attend events or participate in gardening on them, making a real difference to how people use and enjoy the roof area.
Nothing is 100% Perfect…
While green roofs are the future, with a multitude of benefits, the one major downside is the maintenance. Like a garden it needs to be tended to. Even if all you have is grass, it needs to be mowed, watered in the dry months, reseeded in the spring etc.
The one way to avoid this is to install artificial grass on your roof. A good, low maintenance artificial grass installation will give you all the benefits of a living lawn without any of the hassle. Artificial grass will still provide your green roof with the same water maintaining systems, ultraviolet protection and pollution reducing properties as a living lawn without any of the bother of having to maintain it.
Evergreens UK recently installed an artificial green roof to a hospital flat roof so that its residents could enjoy and benefit from the fresh, green look of it through their windows as they recuperated and the hospital did not have to worry about having someone to keep it looking green all year around.
Speak to our helpful installers and they will be able to help you with any questions you may have!