Frequently Asked Questions
about Green Roofs
Rain water
9. Is it possible to combine rainwater use with green roofs?
Most definitely, and it is a great way to save on additional costs.
With up to 50-90% of precipitation being retained by the green roof build-up any excess water can be collected and stored in a modern rain management system or cistern.
Another benefit is the vegetation and substrate layer acting as a water filter; these layers hold back particles, dust, solute pollutants, and even heavy metals.
The excess rainwater can then be used to irrigate the roof, used as toilet water, or any other available purposes. Water coming from the roof can have a light brown tint as a result of the humic particles in the substrate layer; therefore, it is not suitable to be used as wash water. If run-off water is used for toilets it is best to have a sign indicating ‘natural cloudiness’ in order to avoid confusion.
Calculating storage capcacity: It should be noted that during the summer months most rainwater is used by the green roof itself and this should be incorporated into the calculation for the cistern. To make the final determination for the necessary storage capacity a professional for modern rainwater management systems (or similar professional) can be consulted.
10. What is the urban heat island effect?
With global warming, increasing sealed surfaces and radiation from buildings, industry, and traffic, the temperatures within cities are constantly rising. In the summer temperature differences can reach almost 45° F between the more heated city center and the cooler adjoining suburbs; thus, an island effect of temperature is created.
Natural ‘air conditioners’ such as green areas and parks provide soil humidity and the vegetation absorbs up to 80% of the received energy. In densely populated districts green space is especially rare. The missing cooling effect during the night in cities drastically reduces the quality of life and impairs the health of the inhabitants. In regards to this issue, among many others, green roofs can offer a solution. Green roofs can decrease the ‘urban heat island effect’ by helping to reduce temperatures and humidify the air. A more comfortable environment is then created for inhabitants, adjacent offices, and apartments alike.
11. How does a green roof help in terms of energy efficiency and insulation?
Temperatures on the roof surface have an effect on the indoor building temperature; in turn, this contributes to the amount of energy required to heat or cool the building.
The additional vegetation layer over the roof acts as a buffer between indoor and outdoor temperatures. As well, during summer months Plants will transform heat and soil moisture into humidity which in turn cools the surrounding environment, including the building. During cooler months, the buffering capacity of the green roof provides insulation against heat loss through the building roof. Accredited thermally insulating drainage layers add to the insulating capacity of the green roof and therefore, help maximize insulation benefits.*
By reducing temperature fluctuations within the building, less energy and less cost is required for temperature regulation.
12. How does a green roof help in terms of sound insulation?
It has been proven that green roofs can insulate up to 3dB for internal sound insulation, and up to 8dB for reflective sound. The layers of the green roof build-up, such as, substrate, plants, and trapped air, help to provide insulation of sound waves. Expressed in figures this might not be very impressive, but a noise reduction of 10dB appears to the human ear as a noise reduction of 50%. This can be especially advantageous for buildings near airports, or perennial machine activity as well as for noisy discotheques.
In addition, high frequency electromagnetic radiation from mobile stations can also be effectively shielded.
13. How does a green roof help in terms of stormwater runoff?
The natural cycles of the environment are interrupted when changes to the landscape are made. The development and expansion of cities significantly, if not completely, changes the natural environment. Constructions such as roadways and buildings create a seal over the ground and prevent rainwater from being absorbed. As a result, rainwater runs along the sealed surfaces picking up contamination in its path before reaching the sewer system and eventually being dumped directly into water bodies. Sewer systems are often stressed or flooded as a result of such large quantities of water arriving in a short period of time.
Green roofs help restore aspects of the natural water cycle by absorbing and recycling rainwater. Runoff water can be reduced from 10-90%, depending on the green roof build-up. Plants can then use the retained water for natural processes and consequently, acclimatize the surrounding environment.
Excess water can be collected into cisterns or other collection devices. Even if it is not collected, runoff water is less of an issue for sewer systems because it is delayed by percolating through the green roof build-up. This means less water is reaching the sewer system at one time, and it is less likely for the systems to overflow.
14. What do green roofs offer regarding use of space?
There are numerous applications which can be executed on green roofs. Low maintenance Green Roofs, recreational roof gardens, roof cafés, and sporting areas are all possible. Depending on the structural engineering, there are virtually no limits for architectural designs including perennials, small trees, terraces, or planters. At the same time the owner saves costs by not having to purchase additional land. A panoramic view, fresh air and privacy are also included into the price. Roof gardens not only markedly increase the quality of living and working environments, but also offer the possibility for natural and recreational areas to be enjoyed without additional travel.
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