This article is a sample of green roofs mapping analysis, a method that provides data, which can be transferred into information about benefits and disadvantages of green roofs. E.g. how do they improve water retention? How does it affect local water management? How do green roofs reduce real estates energy demand? How many tones of air pollution do they reduce? What are their other benefits in specific to administrative area? What is the potential to build green roofs and benefits they might bring?
What is green roofs mapping and why do we need it?
Green roofs mapping gives us information about existing green roofs with division to their types and what is the potential of creating new ones. This tool is the beginning of complex analysis which answer questions like:
- how much water do existing green roofs retain?
- what are the advantages of existing green roofs in terms of energy savings?
- how much tons of air pollution does green roofs phytoremediate?
- what is the potential to build green roofs on existing buildings?
- what advantages would citizens and city get if green roofs potential would be used in just 20%?
As you can see green roofs mapping provides information about many aspects of contemporary city challenges and many of them might be converted to specific values.
Warsaw – context and overview
Three areas were taken for first analysis of Warsaw’s green roofs. Graphic below shows theirs location:
1. centre of Warsaw, 1,62 km²
2. left riverbank of Vistula, 0,44 km²
3. mostly industrial, 1,59 km²
During II World War, Warsaw suffered worst damage of all European capitals. Warsaw was over 1 million people metropoly and was destroyed in almost 90%. In 1945 left riverbank of Vistula in Warsaw was dead debris [Majewski, Markiewicz 2012]. The development of reinforced concrete technology in 50’s speed up the rebuilding process. Till 1964, 62% of residential buildings were based on reinforced concrete [Szolginia 1967]. In 70’s almost 90% of investments were based on this technology [Szafer 1989]. Before the computer era, reinforced concrete was often oversized, it was better to put some more material than to underestimate it. This fact provides opportunities for creating new green spaces in Warsaw – on the roofs.
In XXI century awareness about sustainable, healthy cities has been spreading all over the world. We are facing problems with dense, built-in urban areas, which leads us to inter alia: surface flooding, urban heat island effect, increasing of air pollution, decreasing of biodiversity – to make long story short: lack of green spaces in cities. These problems will intensify, e.g. between 2001 and 2010 number of days in Warsaw, when it was necessary to turn on air-conditioning was about 32 a year. Prediction shows, that till 2030 it will be 43 days, in 2060 – 64 days and in 2100 – 76 days a year – almost 2.5 times more that in the beginning of the century! [Błażejczyk and others 2014].
Nowadays many cities all over the world struggle with similar problems resulting from urbanisation and increase in of paved surfaces. In the European Union over 75% of people live in the cities. In the Netherlands it is 90% of citizens [Jeleński 2010]. Urbanisation and increasing density of paved surfaces result in number of consequences: Urban Heat Island Effect, surface flooding, problems with water management, air pollution and many more. The answer to support and improve those challenges are green roofs.
Flat roofs were divided as follows:
- with existing green roofs (divided on extensive and intensive)
- without potential to build green roof
- with potential to build green roof on (division below)
Flat roofs with potential to create green roof on them, were divided into 5 categories:
- one star * very light roof, extra loading c.a. 0,8 KN/m², substrate depth 5cm + vegetation mat 2cm
- two stars ** light roof, extra loading c.a. 1,0 KN/m², substrate depth 7 cm + vegetation mat 2 cm
- three stars *** roof’s infrastructure might limit the potential, extra loading c.a. 1,2 KN/m², substrate depth 12 cm
- four stars **** roof covered with gravel or paving, extra loading c.a. 1,5 KN/m², substrate depth 15 cm
- five stars ***** roof with potential to create high class semi-intensive or intensive greenery, extra loading c.a. 2,0 KN/m², minimum substrate depth 18 cm
On analysed areas there were almost 350 000 m² of flat roofs, among them 5% were covered with greenery. This level is higher than average for Warsaw, because there are two big green roofs taken into analysis – one is the garden on Warsaw University’s Library, the second is Copernicus Science Centre.
What are the benefits from existing and potential green roofs?
On analysed areas, existing green roofs cover 17 251 m², among them 5 740 m² are extensive and 11 511 m² are intensive.
To estimate annual water retention of existing green roofs, the assumption was, that average substrate depth for extensive green roofs is 6 cm and for intensive 18 cm. Annual water retention of these green roofs1 is 1 711 l for extensive and 5 615 l for intensive green roofs. It total it is over 7 300 l. Economic benefits might be estimated e.g. by avoided costs of modernization of sewage system [Tomalty, Komorowski 2010], [Kleniewska & others 2010]. The result is over 152 714 EUR2.
Green roofs on analysed areas save 19 880 kWh (intensive) and 8 214 kWh (extensive) in terms of reducing demand on air conditioning [Beck and others 2010, Green Roof Energy Calculator v 2.0]. To sum up we have saved 28 094 kWh, which is about 2 500 EUR of savings.
Another measurable value is the reduction of CO2 emission. Producing 1 kWh of electric energy is connected with releasing into atmosphere c.a. 1000 gr of CO2, 10 gr of SO2, 4 gr of NOx [Metodyka wyliczania carbon footprint 2009]. It means that reduction of 28 094 kWh is reduces emission as follows:
- CO2 – 28 094 kg
- SO2 – 281 kg
- NOx – 112 kg
1 m² of green roof phytoremediate annually 0,22 kg of pollution. For 17 251 m² it is 3800 kg! It is estimated, that 1 kg of pollution less is equal to 2,5 EUR of savings on medical services [Bureau of Environmental Services City of Portland 2008]. So we can calculate amount of avoided healthcare costs on 9 500 EUR annually.
Prolonging the period of functioning hydroisolation well is the another measurable benefit. Assumption for investment cost of hydroisolation on Polish market is 100 PLN/m² (c.a. 24 EUR/m²) with lifecycle of 20 years. Green roof prolongs good condition of hydroisolation by factor of two. It means EUR 414 000 savings on hydroisolation due to our existing green roofs.
Only 18% of flat roofs were assigned to ‘no green roof potential’ category. It means, that there is huge 77% potential of building green roofs. Most of them are assigned to one and two-stars categories. It means, that c.a. 20% of flat roofs might have high class green roof on it.
What would happen if we would green just 20% of potential green roofs?
Let’s assume we are taking into consideration only roofs with the biggest potential – our best 20%. Than green roofs might be potentially build on almost 69 000 m² in analysed areas. Assuming creation of high class semi-intensive and intensive roofs, the results would be as follow:
Annual water retention would be 26 036 l. This amount of water would fit into London’s Big Ben (15.2m x 15.2m x 96.3m) if it was 20 m higher. Economic benefits for the city, estimated by avoided costs of sewage system’s modernization would be EUR 550 000.
Potential of energy savings in terms or air conditioning is 103 150 kWh. This is amount of energy that electric car could drive around the Earth 20 times! Saving of this amount of kWh is also reducing air pollution by:
- CO2 – 103 150 kg
- SO2 – 1032 kg
- NOx – 413 kg
69 000 m² of green roofs would phytoremediate 15 180 kg of air pollution annually. The value of avoided costs of healthcare would be 37 950 EUR annually.
Expanding green spaces enriches cities landscape and carries measurable monetary values. Many of the benefits are dedicated to municipalities. This is why they should create incentives for citizens to build green roofs and information actions. When we will have in Poland direct and indirect incentives, development of green roofs market will speed up and the effects of the scale will cause reduction of costs for building green roof.
Enjoy Warsaw Green Roofs map created for this analysis ➡️ here ⬅️
1 Annual precipitation in Warsaw 542 mm/m² [Kleniewska 2010]
2 Exchange rate 1 EUR = 4,1963 PLN, 7th of June 2017, NBP http://www.nbp.pl/home.aspx?f=/kursy/kursya.html
About the author:
Marta Żaryn – Python Developer and green roofs enthusiast. Board member of Polish Green Roof Association.
She learned green roofs mapping technique from Dusty Gedge, the president of European Federation of Green Roofs Associations.
Marta’s ‘pet project’ is mapping green roofs in Warsaw, which she develops in her free time. Database for Warsaw already covers c.a. 50% of the city.
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