1: Visual: Public amenity
a: Public Amenity
Rooftops are generally an underutilised space, often reserved for exausts, air conditioning units and other plant equipment. If de- signed effectively, green roofs can provide for public amenity replacing the increasing amount of lost public open space due to urban development. With the addition of roof top structures, these spaces can provide an additional shaded recreational space for the building occupants and/or the public.
Many cities around the world such as Toronto, Portland, Chicago and even London, are planning to introduce more public recrea- tional space on green roofs where access can be controlled making them safer environments to inhabit. Green roofs can become excellent spaces to hold meetings, have a quiet lunch (well above the noisy street) or to go and read a book.
It is not possible to provide amenity to every green roof. In some instances the structural capabilities of the roof deck are not designed to support the extra live load associated with having people walking on its surface or the added weight of materials associ- ated with paving.
b: Visual Advantage
Green roofs are also more pleasant to experience or view from other buildings. Traditional roof structures are quite bleak and ugly to look at, often cluttered with HVAC equipment, exhausts and a bitumen or gravel protective layer. They are also particularly hot areas to be in because of the dark heat absorbing materials.
Green roofs on the contrary are pleasant to be in because they are cooler and offer shading from rooftop structures and taller veg- etation. They are also more attractive to view from other buildings because of their soft foliage and vibrant colours.
2: Economic: Prolonged life span of roof membrane
a: Prolonged Lifespan of Roof Membrane
The economic advantages of green roofs are widely known to be one of the major reasons for building a green roof. Real savings can be accounted for immediately after a green roof is installed. The main economic advantages include;
- Prolonged lifespan of the roof membrane
- The production of food
- Reducing heating and cooling costs
An exposed roof deck without a layer of vegetation protecting it, is exposed to extreme heat, wind and ultra violet radiation. All of these elements deploy varying degrees of mechanical and chemical degradation, which places a large amount of stress on the roof material. Ultimately, the roof deck wears down and ages at a much faster rate than a protected roof.
A green roof, on the other hand, provides direct shading and protection to the roof membrane and therefore greatly reduces the amount of exposure to harsh environmental conditions. Not only does this keep the membrane cool irrespective of the outside temperature, but it also keeps the temperature constant, avoiding maMor Àuctuations which can impose stress on the membrane. This means that the expected lifespan of the roof is much longer than a traditional roof deck, which amounts to real long-term sav- ings. It has been suggested that a green roof can last 20 years longer than a traditional roof.
b :Food Production
On specifically designed green roofs there is the opportunity to develop rooftop farms, which promote the growth of food produce. This is not a new concept but one that is yet to see wide spread application. Depending on the size of the devoted rooftop farm, this type of green roof can sell fresh produce to local businesses and restaurants. Alternatively, on smaller projects the rooftop farm can satisfy the needs of the building occupants.
The advantages of growing food on rooftops are widespread. Produce can be grown at a lower cost because of the reduction in transportation costs when transporting the goods from outside the city. There are also savings in emissions brought about by the elimination or reduction of transportation. The produce on green roofs can have a greater level of control with regard to fertilisation and pesticides meaning that consumers can be assured of better quality, organic produce. This could make the green roof a viably profitable and lettable space, which, no doubt is attractive for developers.
The Fairmont Hotel, Vancouver, is a good example of a rooftop farm that supplies its restaurant with fresh produce that has been harversted(195m2). It is estimated that this saves the company up to $30,000 per year on fresh produce. Earth Pledge (see image above) is a smaller venture that operates in a similar fashion by providing the restaurant below with fresh produce.
c :Reducing Heating and Cooling Costs
In the short-term, savings can be achieved in heating and cooling costs because the layer of vegetation provides excellent insula- tion to the building. The green roof acts as thermal mass keeping the internal temperature of the building relatively constant. It keeps the building cool in the hotter months and warm in the cooler months minimizing the dependence on HVAC systems.
3: Environmental: Increased biodiversity
a :Increased Biodiversity
Green roofs replace habitats that have been lost due to development. It is therefore important to choose the variety of vegetation that suits the local environment to maximize the effect of the green roof on biodiversity. Green roofs offer a safe place for birds, insect and other plants to grow.
b :Improved Efficiency of Photovoltaic Cells
In recent times, Europeans have been testing the efficiency of photovoltaic cells in combination with green roofs and have found that due to the significantly lower temperatures on green roofs, photovoltaic cells run up to 25% more efficiently. These types of projects are increasing in popularity as they reduce the reliance on dirty energy, which results in dollar savings.
c :Water Runoff Management
Storm water management is a key environmental advantage of having a green roof and benefits the environment at both the local
and regional scale.
When water hits a traditional roof it is often distributed across the roof surface, picking up sediment as it does and being flushed down public drains and into the storm water system. In times of heavy rainfall, this system can over flow, mixing with raw sewer- age and become very polluted. It then finds its way into the waterways such as our rivers or oceans.
A green roof significantly reduces the rate of flow from the roof to the storm water system because the water is absorbed into the substrate. This reduces the chance of storm water flooding. This water can also be harvested, stored and used for irrigation or non-potable use.
A second advantage is that the substrate actually filters the water as it passes through, therefore distributing much cleaner water to
our waterways than water that has run directly off a bitumen surface.
d :Improving Air Quality
Vegetation also has the ability to improve the air quality in the local environment. Predominantly larger types of vegetation such as shrubs and trees can capture air borne particles on their foliage, which eventually get washed into the substrate. However, any plant converts carbon dioxide into oxygen through a process of photosynthesis therefore improving the quality of air in the local environment.
e :Reduction in Urban Heat Island Effect
As was mentioned earlier, the greater incidence of green roofs the greater the environmental effect. This is of particular importance with respect to the urban heat island effect. This phenomenon occurs because of the density of hard, heat absorbent materials in urban areas, which cause an increase in the average urban temperature.
Green roofs replace heat absorbent and reflective materials and therefore lower the temperature on top of roofs. If this is dupli- cated over many rooftops it can reduce the urban heat island effect.
f :Reduction in Noise Pollution
Green roofs can markedly reduce noise pollution in urban environments. The layer of substrate and the layer of vegetation work in combination to minimize lower and higher frequencies respectively.
g :Reduction in the Risk of Fire
Green roofs in some circumstance can slow the rate that a fire will spread through a building so long as the substrate has a level of moisture in it. A saturated substrate will have more of an effect in slowing the rate in which a fire spreads. However if a green roof is particularly dry with dead foliage, this advantage will be lost.
