Urban stormwater management is another benefit of green
roofs. To decrease flooding and stormwater pollution, low impact
development (LID) practices have been suggested (USEPA, 2000),
one of which is the use of green roofs. UK Sustainable Drainage
Systems (SUDS) endorses green roofs as a SUDS technology and
has also produced guidance specifically related to the use of green
roofs and living walls on buildings (CIRIA, 2007a,b). Green roofs
utilize existing space and require no additional land area, which is
attractive in cities.
In Taiwan, the movement toward a low-carbon society and
more sustainable cities, as well as initiatives for greening of buildings
and campuses, has increased the number of green roofs. For
example, the Ministry of Education launched a national Sustainable
Campus Program in 2002 and invited proposals for reconstructing
campuses in a sustainable way. Use of green roofs was one
of the strategies for developing sustainable campuses. The program
is similar to the Green School Project in South Korea, which
uses eco-friendly techniques, including green roofs, to repair and
renovate elementary and middle schools (KEDI, 2009; Hong et al.,
2012). The New Taipei City government is also dedicated to developing
a low-carbon city. One of its strategies is establishing more
green roofs, because they have high energy-saving functionality
and can also beautify the city. To mitigate urban flooding, the
Taipei City government passed an autonomous regulation in 2011
to increase the use of facilities capable of retaining and detaining
water, including green roofs. Additionally, green buildings emphasize
large green areas in and around buildings (Ministry of Interior,
2008).
Many green roofs have been constructed in Taiwan since 2000,
but most do not follow a standard design. After promotion of green
roofs by the New Taipei City government, standardized regulations
following the German FLL guidelines (The German Landscape
Research, Development and Construction Society) (FFL, 2002) were
implemented. However, few studies have been undertaken to monitor
green roof performance in Taiwan. Taiwan is located in a
tropical and subtropical region with an annual average precipitation
of 2500 mm, most of which falls during the rainy season
(typically called the plum rain season in East Asia) and the typhoon
season. During the plum rain season, when the winter and summer
monsoons meet and create a stationary front, a large amount of
rain falls in May and June with average rainfall of 230 and 300 mm,
respectively. From July to September, abundant rainfall is associated
with typhoons. Fig. 1 shows the monthly average rainfall
in northern, central, and southern Taiwanese cities. Thus, intensive
rainfall is the major source of precipitation in Taiwan. This
precipitation pattern is unlike that of Europe and North America.
One obvious difference is that there is no snowmelt in Taiwanese
cities. For example, the lowest temperature in winter in Taipei
Fig. 1. Map of Taiwan and monthly average rainfall in Taipei, Taichung, and
Kaohsiung City, located in northern, central, and southern Taiwan, respectively.
Data for 1981–2010 were obtained from the Central Weather Bureau database
(http://www.cwb.gov.tw/).
City, which is located in northern Taiwan, is usually 10 ◦C and is
never lower than 0 ◦C. These climatic differences may result in
differences in green roof performance, particularly in stormwater
quality and quantity and thermal improvements. Williams et al.
(2010) addressed similar green roof challenges in Australia. They
found that predicting green roof success relying only on European
and North American experiences without local evaluation
is problematic. The applicability of green roofs is highly dependent
on local climate conditions, and unfortunately, there have
been few green roof investigations in Taiwan. Studies of thermal
improvements have been the main focus (Hsieh and Ou, 2005;
Liang and Huang, 2011; Lin and Lin, 2011; Sun, 2011), as these
effects are relatively easy to monitor and show significant results
in areas with high temperatures. Hsiao and Chen (2012) sampled
runoff water from green roofs and bare roofs for comparison. No
other environmental studies of green roofs in Taiwan have been
performed.
This paper reviews the existing literature on the performance
of green roofs in Taiwan and compares the results to those in other
countries, particularly in East Asia. Some selected published studies
related to green roofs in East Asia are summarized in Table 1. In
addition to environmental performance, the economic characteristics
of building and maintaining green roofs are discussed. This
study provides unique observations that facilitate an understanding
of green roof performance in tropical and subtropical climatic
zones.