Green Roofs are one component of a sustainable drainage system. Stormwater management is a common issue we face in our cities. Most of our existing urban drainage systems are currently at capacity, many having been designed so long ago and as a system of combined surface water and foul water.
If a sustainable drainage or low impact design approach is taken as part of a development, it ensures the site is not increasing surface water flood risk or polluting the environment.
There are three key factors in the sustainable drainage approach being:
- Quantity of surface water run off
- Quality of surface water run off
- Amenity benefits
The nature of climate change at a regional level will vary in NZ. Our projections of future climate change indicate that there will be more frequent, short-duration, high intensity rain. NIWA estimates for a mid-range scenario, that a 1 in a 100 year event, could become a 1 in 50 year event by the end of the century (Climate Change Projections for New Zealand, NIWA National Climate Centre, August 2008).
Green Roofs can provide significant biodiversity benefits. Many countries use green roofs as mitigation for the loss of habitat. Switzerland has moved towards introducing living roof systems that mimic natural habitats found locally.
Living Roofs can improve an areas biodiversity by providing much needed green space especially in industrial or commercial areas. They can create new green links/fingers for species to network and move along. They may also provide a mosaic of habitats for endangered plants, invertebrates and birds.
It has been demonstrated that to create an invertebrate rich living roof consider varying substrates, varying depths, different local plants and incorporate dry wood or rocks for habitat.
Written papers on this subject by Dr. Brenneisen, Dr. Kadas and Dusty Gedge are available at:
“There is evidence from Europe that with good design green roofs can benefit a number of ground nesting birds including Skylark, Oystercatcher, Ringed & Little Ringed Plovers, Common Tern and Lapwings."
"Although gardenesque in character such roofs can provide a significant resource for local biodiversity with good planting schemes. These should include some native and certainly include flowering plants that are of known foraging value for bees. Inclusion of water features, dead logs and nest boxes can increase the potential for such roofs to act as a resource for local wildlife.” (Text courtesy of livingroofs.org).
Living roofs have a substantial thermal mass, insulation properties and through evapotranspiration, a cooling effect. Green roofs provide significant insulation and thermal mass benefits to non-insulated buildings and existing flat roof buildings.
“Studies have shown that the membrane temperature beneath a green roof can be significantly lower than where the membrane is exposed.
The National Research Council of Canada noted temperature fluctuations during spring and summer on a conventional roof were of the order of 45ËšC whilst under a green roof the fluctuations were in the order of 6ËšC.
The positive effect on the temperature of the membrane under a green roof not only protects the membrane from the effects of UV, frost and sunlight, but also moderates heat flow through a building by shading, insulation, evapotranspiration and thermal mass.
Although green roofs do provide potential energy savings by improving building insulation characteristics, it is difficult to give them a U- Value rating.
Studies in Germany have provided various estimates. Figures attributed to ZinCo estimate that 2 litres of fuel oil are saved per m2 of green roof per year.
A more recent study of domestic buildings in North East German with flat roofs suggests that there is a 3-10% winter saving on fuel bills. The results of the study over five years suggest that there is a maximum saving of 6.8kWh/m2 [1.5kg/m2 CO2 e tonnes] and a minimum saving of 2.0kWh/m2 [0.44kg/m2 CO2 e tonnes] during the winter.
This correlates with the Ottawa study referred to above, which compared a conventional roof system with a green roof system.
'The average daily energy demand for space conditioning caused by the reference roof system was 20,500 BTU to 25,600 BTU (6 kWh to 8 kWh). However, the green roof system's growing medium and plants modified the heat flow and reduced the average daily energy demand to less than 5,100 BTU (2 kWh) – a reduction of more than 75 percent.'
The City of Toronto has estimated that the direct energy savings citywide, through reduced energy for cooling as a consequence of whole scale greening would be in the order $22 million, equivalent to 4.15kWh/m2 per year [CO2 emission saving of 1.7kg/m2. There would also be a reduction in peak demand in the order of 114.6MW leading to fossil fuel reductions in the region of 56,300 metric tonnes per year.
Studies by Environment Canada have shown that the upper floor of a building with a green roof is likely to save 20% of its energy demand through reduction in cooling needs. This study also calculated the reduction on floors below the upper floor. The summer savings on the top floor of the building would be in the order of 20%. If a building is five storeys or more, the total summer savings would be in the region of 6% and with a two-storey building, the total summer savings would be between 10 – 12%.
An energy study undertaken by the City of Chicago estimated that, with whole scale greening of the cities rooftops, energy to the value of $100M could be saved each year due to the reduced demand for air conditioning. This would to a reduction in peak demand in the order of 720MW.
A 1999 study undertaken by the city of Chicago estimated that the greening of all the cities roofs would save $100 million energy each year, especially due to a reduction in the need for air conditioning - the equivalent energy consumption of several coal-fired generating stations or one small nuclear power plant.”(Text courtesy of livingroofs.org.)
Green roofs can act as a significant barrier for sounds. The components of a green roof system, from the soil, vegetation and drainage layers all act to either absorb, reflect or deflect sound waves. Studies in the UK suggest that a living roof can reduce sound by 8dB compared to a standard roof. Urban areas that suffer from high levels of noise pollution, such as, buildings within flight paths could all benefit from the installation of green roofs.
Urban heat island effect
NIWAs regional climate change model suggests that the number of hot days in Auckland could double or treble. Currently our most populated city experiences about 21 days per year, above 25ºC. By the end of the century, we could face an extra 40 days or more of high temperatures under a low-medium scenario, and more than 60 extra hot days under a medium-high scenario. (Climate Change Projections for New Zealand, NIWA National Climate Centre, August 2008).
Our urban areas have a higher average temperature than our rural areas. The urban heat island effect is the term used to describe the difference in these temperatures. With the effects of climate change taking hold, the number of hot days we experience in our cities will increase and as such our reliance on air conditioning. Green roofs are a proven technique to help mitigate the urban heat island effect.
The two most recognised methods for reducing the urban heat island effect is to:
introduce more vegetation into the urban environment which will provide shading and cooling through evapotranspirationincrease the albedo or reflectiveness of roofs to reflect a higher amount of solar radiation back into the sky, thereby producing less heat
Green roofs are now commonly being used overseas to mitigate the effects of the urban heat island effect.
Improved air quality and water quality
Vegetation and soil have been proven to help filter pollutants and dust from the air and water. There are a number of factors that affects the ability of vegetation to absorb pollutants from weather conditions to the type of vegetation. Wetlands are being trialed on green roofs in the UK, which can filter and treat water.