Advantages of a green roof

Nowadays a lot of information can be found about the benefits of green roof systems. It is said that green roofs capture CO2, micro particles, retain water and decrease the ambient temperature. If one is not an expert, it is quite difficult to decide whether is true or is just a rumour.
We would like to present here an overview of the advantages of a green roof and also some links to scientific publications.

Financial benefits

1. Insulation
   Conserves energy, ensures low operating costs for buildings (heating, air conditioning)
2. ECO aspect, combining functionality with naturalness
   This increases the value of the property
3. Obtaining grants and tax breaks for green buildings
   Lower implementing and operating costs
4. Longer life – faster depreciation
   Long retention of value

Advantage: low maintenance and replacement costs

1. Insulation: Energy saving

Researches have shown that extensive roofs with vegetation can thermally insulate both summer and winter. Flat roofs with vegetation can be up to 21° warmer than green roofs.

The cooling effect of buildings during summer:
Using green roof systems, the heat taken from the ambient environment can be reduced with 90% and also the insulating layer effect helps reducing the heat transfer with 99%. According to some German studies the temperature in the substrate under the vegetation never exceeds 25°C, not even in hot summer days, when the outside temperature is 35°C. This cooling is due to the evaporation and shading effects of plants, but also the green roof’s ability to reflect sunlight, the energy consumed by photosynthesis and the heat storage power and also of the moisture embedded in the vegetation substrate.

The warming effect of buildings during winter:
Grass and the vegetation structure behave as a “fur” and contribute to increase the heat insulation effect by forming a thin three-dimensional format in which is embedded some air. This vegetation also prevents upper layers of the roof to be directly hit by the cold wind.
A small but noticeable effect has the thermal insulating power of the special substrate of green roofs, namely the infrared reflection of the vegetation layer and the heat produced by the condensation or frost, which is formed early in the morning on the surface of the vegetation layer (to produce 1 g of condense water, 530 calories of heat are released).

Less air conditioning, lower heating costs:
Green roofs significantly reduce the need for use air conditioners in summer and insulate in winter. The effectiveness of winter insulation depends on the amount of moisture retained by different layers. During summer the insulation is optimal, because layers are dry and reflect heat much better. Using less air conditioning in summer and having lower heating costs in winter, green roofs can perform significant energy savings. As energy price is getting higher and higher, roof gardens are becoming more attractive.

Offices are able to save 4,470 cubic meters of gas per year:
According to the „Energy Saving Monitor” from 2006, offices consume an average of 1,250 MJ per m² and 39% of this amount is spent on heating premises. Cooling with air conditioners, adds another 4% to the energy costs, what leads to 538 MJ per year. Green roof systems can reduce energy consumption by 23% for heating and 75% in case of air conditioning. A roof with vegetation saves about 150 MJ every year. 1 MJ means 0,0316 cubic meters of gas. So the conclusion is that green roofs can help us to reduce gas consumption with 4,470 cubic meters every year! (Source: Groen Boven alles – Energy Saving Monitor).

About 91 Euros savings/year in domestic establishments:
Household energy needs differ from those of offices. Household spendings on energy are about 47% for domestic heating and 10% on cooling equipment, for example refrigerators.

Not all houses have air condition systems and fans, that is why the cooling effect of green roofs on houses is not included in this calculation. A study made in Toronto shows that domestic establishments can save 23% of their heating costs by creating a green roof. In Holland 26,364 MJ of gas energy is used for heating, which costs about 395 euro. Reducing consumption with 23%, means a saving of 91 €/year! (Source: Groen Boven alles – Energy Saving Monitor)

2. The natural look increases the value of the property

Thanks to the pleasant appearance of the building, the energy saving and the trend of the environmental responsibility, buyers are ready to pay more for a house with green roof. So this kind of roof with vegetation raises the value of the building.
A green roof has many functional advantages and also becomes a unique design element in the architecture of the building.

3. Subsidizing green roofs

In our days because of the climate changes, strong torrential rains are more frequent. In many cases rivers can cause unprecedented flooding with serious economic and environmental implications. Investments to increase the sewage system capacity is much higher than building a green roof and these roofs can help decreasing the overloaded sewage systems and also water can be collected, which is more and more important in densely populated areas. That is why authorities welcome the idea of installing roof gardens.

Reducing with 60-70% the overload of sewer systems:
Studies demonstrate that green roof systems can significantly reduce the amount of water discharged into drains during summer. Plants and the substrate of vegetation retain water which then evaporates to the atmosphere. The amount of water retained by a green roof depends on the thickness and the type of substrate, type of drainage and vegetation.

Savings of at least 8-10 €/m²:
The amount of meteoric water collected on a green roof reduces costs of the sewer system. This could be 8-10 €/m².

4. Sunscreen and temperature variations. Longer life for the roof

Roofs are constantly exposed to ultraviolet radiations, rainfalls and extreme temperature variations. Temperature differences in extreme cases can reach up to 100°C during a year. Bituminous and plastic layers are sensitive, they are quickly aging because of these large temperature fluctuations and UV radiations. It has been proven over the years that a green roof can resist three times longer than a simple one, because the vegetation protects against almost all climate factors. The vegetation cover and the special layers protect roof against this harmful effects, increasing their lifetime.

Mechanical protection of the waterproofing layer:
During different technological interventions, repairs or maintenance, the waterproofing layer can be perforated (nails, self-tapping screws, sharp pieces of tin could pierce the waterproofing layer), causing seepage. Green roofs protect the waterproofing stratification of these minor but very expensive and unpleasant incidents.

Long-term viability. Return on the investment in 8 to 21 years:
Usually the costs of a green roof system can be recovered in 8-15 years thanks to the savings resulted from the lower operational costs of the building. While the components of a traditional roof are typically replaced every 20-30 years, a roof with green vegetation has a duration of use 3-5 times higher (60-100 years). Green roofs protect the material underneath, so no need for new funds to replace roof elements, which means quite a rapid return on the investment.

Environmental benefits

1. Low ambient temperature
   Reduces the so called Urban Heat Island
2. Captures CO2
   Cleaner, breathable and richer air in oxygen
3. Captures micro particles
   Clean air
4. Stimulates biodiversity
   Improves/maintains the habitat of birds and insects
5. Retention of storm water
   Spares the sewage systems, reducing the risk of floods
6. Purification of storm water by filtration
   A cleaner environment

Low ambient temperature: reduction of the “Urban Heat Island”
Summertime, the temperature in cities is approx. 5-7°C higher than in rural areas, because buildings and paved streets absorb and then give off the heat. This phenomenon is called the Urban Heat Island.
Studies carried out by the Tyndall Centre for Climate Change show that cities need 10% more green areas in order to be able to counteract the effects of the climate changes. Green reflects heat, so it creates a lower and more pleasant ambient temperature. Through water evaporation, plants can very quickly reduce temperature. Unfortunately parks, gardens and all green areas are very limited in the cities, so these ecological roof systems could offer the best solution to increase green surfaces.

Capturing CO2: much cleaner air
The carbon dioxide is a gas resulted from the burning of fossil fuels. After the industrial revolution, its level in the atmosphere has increased considerably. This is considered to be the main cause of the global warming. It is known that plants are absorbing CO2, but unfortunately the number of plants and trees has not grown in proportion with the amount of CO2 emissions. It is not known what quantity can be absorbed by the sedum, but because of the large number of succulents per m², it is expected to be considerable. Each square of sedum vegetation has about 2,4 m² of useful leaves, but an extensive green roof with herbaceous vegetation may even have 100 m² of leaf carpet. Knowing all these information, we can say that green roof systems help to reduce CO2 in the atmosphere, so the global warming too.

Capture of micro particles: much cleaner air
Micro particles in the air represent a serious danger to public health. They can cause heart diseases and worsening lung diseases.
The Institute of Public and Environmental Health in Holland has calculated that the annual number of premature deaths caused by short-term exposure to micro calculated particles reaches the number of 18,000 people.

Elements of the nature can retain micro particles which won’t flow along the sewage systems drains with the rainwater, but they are filtered and retained in the substrate. An extensive green roof can capture about 1,5 kg/ha. This capacity corresponds to the capture of an adult tree. A green roof with sedum can capture more micro particles than the same area of bitumen, because of the irregularity of the surface structure. Urban areas also suffer from exhaust fumes, therefore green roofs have a positive impact on air quality too. Increasing the number of roof gardens in the cities, this would strongly improve the health of inhabitants.

Stimulating biodiversity: improves/maintains the habitat of birds and insects
Wherever we raise our buildings, microhabitats are usually disturbed. Green roofs help to rebuild them and to restore biological cycle. Roofs with vegetation offer an important refuge for microhabitats.
We can stimulate biodiversity by choosing the proper substrate thickness and by a great variety of plants.

According to some Swedish and English studies, green roofs can even provide shelter for rare insects and endangered butterflies.

Rainwater retention: less required sewage systems and as a result, less reversal
As a result of global warming, torrential rains have become commonplace, so sewage systems are increasingly overloaded. Green roofs allow the retention of great quantities of water, so the overload of drainage and sewerage systems can be avoided. This water evaporation also helps reducing the ambient temperature. Another important effect of green roofs is slowing the drainage of the water, so rainwater systems are protected, because they are embedded in the structure of the green roof. In case of a light rain, green roofs with inclination of 12 degrees and 14 cm of vegetation substrate, the water drainage began 12 hours after the rain has stopped and continued for another 21 hours.

Filtering rainwater: a cleaner environment
Green roofs not only retain large amounts of meteoric water, but they also purify it.
First the rainwater flows on plants, then it is filtrated through different layers. The Kohler&Schmidt study (1990) shows that 95% of lead, copper and cadmium sulphide and 19% of zinc reaching the rooftop as rainwater, remains in the substrate. The amount of nitrates, nitrites detained (from acid rain) is also important, but difficult to quantify.

Practical advantages

1. Soundproofing
   A quieter indoor and outdoor environment
2. Fire resistant
   It’s completely fireproof
3. Natural look
   Beautiful view, green has a calming and relaxing effect
4. No ballast is used to protect the roof
   Replacing an inert material with a showy alive one

Sound insulation: a quieter indoor and outdoor environment
The combination of substrates, plants and air from different layers of a green roof, reduce noise pollution. Sound waves are rather captured and absorbed and not reflected as other building materials do. The substrate blocks lower frequencies and plants block the higher ones. For example the noise of aircrafts and those of thunderstorms. Roof gardens can reduce the perceived noise up to 8 dB. It does not seem significant, but for people this reduction means almost 50% less noise.

Green roofs ensure a quieter office, flat, house and a much pleasant urban environment. Sound insulation depends on the thickness of the green roof system and the degree of swelling with water of the substrate. The more thicker, the insulation is more effective. In 2004, Dunett&Kingsbury measured 5dB of noise reduction in the airport neighbourhood in Frankfurt am Mein, after installing a 10 cm thick green roof. Prof. dr. Gernot Minke shows in his study that a 12 cm thick wet layer can reduce noise with 40 dB, and a 20 cm thick layer with approx. 46 dB.

Resistance on fire
In Germany green roof systems are considered “solid roofing”, because they don’t burn and are completely fireproof if the layer’s thickness is greater than 3 cm.

Protection against electromagnetic radiation:
Studies of prof. Dr. Gernot Minke from the University of Kassel show that green roofs reduce the effect of electromagnetic radiation. So a green roof with 20 cm thick insulation layer, drainage layer and a substrate thicker than 16 cm, and having extensive vegetation of herbaceous plants, can reduce radiation in the 2 Gigahertz (in which operates the majority of mobile telecommunication networks) with 24 dB. This means a reduction of about 99% of electromagnetic radiation. If the roof garden is filled with 20 cm thick lawn tiles, the reducing effect of electromagnetic radiation is about 99,9999%.

Natural look: beautiful, calming and relaxing
Densely populated areas usually do not have enough green spaces. Green roofs can offer a pleasant view, improving the quality of an urban life. Green areas have also a psychological relaxing effect: a walk in a botanical garden reduces blood pressure and lowers heart rate. Another study proves that people surrounded of vegetation, can recover more quickly after stress conditions.

No need for ballast or gravel on the roof: replacing an inert material with an alive one
Some roofs need ballast for fixing. Standard green roofs has a saturated weight of approx. 85 kg/m² (there also exist systems of 35 kg/m²). On the other hand, because of the roots, the system forms an amalgam as a single entity. Building a green roof, ballast or gravel won’t be needed anymore!

Do you need consultancy for green roof projects?
Would you like to benefit from the financial, environmental and practical advantages of a green roof project? Our company is ready to help with turnkey projects and long-term maintenance in the field.
We use sustainable and ecological materials, vegetation rolls and plants specially adapted to these ecological roofs and terraces. Our projects take into account customer’s point of view and ideas and they offer the best efficiency/safety and reliability balance.

Whether it’s an industrial hall’s roof landscaped with extensive green vegetation, an office building with intensive green terraces, or a shopping centre with futuristic shape and pedestrian and vehicle traffic on its roof; everything must start with a thorough design. If you want an intensive or extensive green roof, a landscape garden, a green wall located in an office building, just noise insulation, or to complete the ecological function of the green roof by installing solar panels, we can provide projects, system solutions, estimates and consultancy in the field.