There are many benefits of a green roof. These include insulation, reduced surface run off, increased biodiversity and an aesthetically pleasing finish, to name a few.
The growing medium and vegetation on green roofs act as insulators from the cold in the winter and heat in the summer. During winter a green roof can save up to 25% of heat lost in dry conditions compared to a bare roof. Having a layer of vegetation can also decrease the heat lost by cold wind by up to 50% and decrease the amount of money spent on heating in your home. In the summer months the vegetation prevents the roof from overheating, as concrete retains much more heat than vegetation. With companies spending more on air conditioning than heating in a year, this effect can reduce expenditure on electricity bills considerably.
Not only are green roofs good insulators of heat, they are also good insulators of sound. Studies show that the addition of a green roof can reduce audible output by 8dB. This may not sound like much, but with increasing noise levels in modern cities any reduction of noise travelling in or out of buildings will make the environment more pleasing for its citizens.
Urban Heat Island Effect
Green roofs not only help to control the temperature inside buildings, they contribute to the control of external temperatures in cities by reducing the Urban Heat Island Effect (UHIE). The UHIE causes average temperatures in urban areas to be 5-6 degrees higher than in the surrounding rural areas and is expected to become worse as cities expand and areas of natural vegetation become scarcer. This is caused by large, dark, manmade surfaces, such as conventional roofs, absorbing a large amount of infrared radiation from the sun over the course of a day. This energy is then reemitted overnight and doesn’t allow the temperature to drop back to its natural level. The major problem with this is that some wildlife cannot survive in warmer temperatures and have to relocate to ever decreasing spaces. This often results in contact with humans and other animals causing both intraspecific and interspecific competition for space and nutrients. Other wildlife, such as plants and some animals cannot relocate so slowly die as they cannot adapt to the ever changing conditions.
Aside from the effect on wildlife there are also human problems caused by the UHIE. The warming in urban areas can alter local wind patterns or cause fog and cloud to develop where they normally would not. The increased upwards motion of warm, moist air can cause an increase in cloud formation, precipitation and thunderstorm activity in urban areas. As the air rises and cools above the city, it condenses into water droplets and causes rain. This effect can cause a total precipitation increase of 51% compared to comparable rural areas.
Green roofs help to mitigate the UHIE by shading the surface of the roof that would otherwise absorb and reemit the infrared radiation back into the atmosphere. The leaves of the plants also absorb a little radiation and use the energy to convert carbon dioxide and water into glucose in the process of respiration.
Green roofs also remove heat through evapo-transpiration, the process by which water is removed from the land to the atmosphere by evaporation from the ground and transpiration from the leaves. As the water evaporates it uses energy from the plant to change its state from a liquid to a gas. This is known as latent heat and removes energy from the system, allowing the roof to cool. This is the same process in which heat is removed from humans through sweat.
Green roofs offer a clever solution to help to mitigate the increasing problem of climate change. Global warming is caused by a combination of increasing greenhouse gas emissions and reduced ability to absorb these greenhouse gasses. Plants absorb carbon dioxide and convert it into glucose through photosynthesis. As carbon dioxide is a greenhouse gas, the increase of its abundance in the atmosphere will contribute to global warming. Green roofs allow humans to develop and increase urban land use without compromising the area that used to be covered in photosynthetic plants. By adding green roofs to houses and buildings, the amount of green land lost to development is reduced which helps to decrease the effect of global warming.
Urban sprawl and urbanisation have caused more land to be covered in impermeable materials such as concrete and tarmac. This stops the natural processes of infiltration and percolation and increases the amount of overland flow of rainfall. Infiltration is where the water is absorbed into the soil after rainfall event and travels in a process called through flow down the drainage basin. A much faster process of water movement is overland flow; where the water does not make its way into the soil but instead travels over the top of the land where there is less resistance. The increase in speed of the water means it accumulates quicker causing a greater risk of flooding. As tarmac and concrete are impermeable water cannot infiltrate so instead runs over the ground quickly, leading to accumulation and flooding. Green roofs can help to reduce this risk by slowing down the movement of water from the roof to the ground and absorbing water into the green roof system. Some of this water is used in photosynthesis and converted to glucose, the rest is released to the atmosphere through evapo-transpiration, reducing the amount of water that makes it down the basin and reducing the risk of subsequent flooding.