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Green Roofs
Introduction
A green roof is a roofing assembly consisting of a waterproof membrane
and additional component layers - including growing media, drainage, and
root protection - allowing for the propagation of vegetation across all
or part of a roof surface. Green roof can be used in many applications,
including industrial facilities, residences, offices and other commercial
property.
Benefits of green roofs
The green roofs can bring a number of benefits including environmental
and ecological aspects. The benefits of green roofs include:
| (a) |
Mitigating urban heat island effect
Urban heat island effect is the difference in temperature between
a city and the surrounding countryside. It is mainly due to the large
area of hard and reflective surfaces such as roofs, which absorb solar
radiation and re-radiate it as heat. Reduction of the urban heat island
effect can reduce the distribution of dust and particulate matter
throughout the city and the production of smog. Covering conventional
roofs with green roofs can significantly reduce the temperature above
the roof by transpiration and photosynthesis of the vegetation. According
to a Hong Kong case study, green roof can reduce the roof surface
temperature by 10 to 30°C compared with conventional roof. [1] If
sufficient roof surface are covered with green plants, the cooling
effect of green roof can significantly reduce the adverse effects
of urban heat island. |
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| (b) |
Reducing heat gain from roof
Green roofs insulate buildings by preventing heat from transmitting
through the building roof. The reduction of heat gain from the roof
will reduce the cooling energy consumption of air-conditioning system.
The insulation properties can be maximized by using a growing medium
with a low soil density and a high moisture content. The benefit of
solar heat gain reduction will be realized most fully in warm climates
like Hong Kong, where energy expenditures on air conditioning are
high. However, this benefit will be less significant in multi-story
buildings because of the low ratio of roof area to the total exposed
building facade. Since green roofs are more complex than simple insulators,
project-specific building envelope analysis is required to predict
energy effectiveness under specific project conditions. |
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| (c) |
Prolonging the life of roofing materials
Green roofs can provide protection to waterproofing materials of the
roof structure. Their value in roof protection has demonstrated by
more than thirty years of experience in overseas countries. The multiple
layers of the green roof protect the underlying roof materials from
the elements in three ways: |
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i) protecting from mechanical damage from humans, animals
and debris etc.;
ii) shielding from ultraviolet radiation;
iii) buffering temperature changes, minimizing damage due to the daily
expansion and contraction of the roof materials. |
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| (d) |
Reducing sound transmission
The multiple layers of the green roof can be used for insulation of
sound. Sound waves that produced by machinery, traffic or airplanes
can be absorbed, reflected or deflected. The substrate tends to block
lower sound frequencies and the plants block higher frequencies. |
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| (e) |
Controlling storm water runoff
Compare with conventional storm water runoff, vegetated roof covers
are unobtrusive, low maintenance, and reliable. During rainstorms,
green roofs act as a sponge absorbing much of the water that would
otherwise run off.
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| (f) |
Others
In addition to the above benefits, adopting green roofs can create
wildlife habitat and improve the aesthetic environment.
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Structure of green roofs
There are two main categories of green roofs, namely extensive green
roofs and intensive green roofs. Extensive green roofs usually require
150mm or less of soil depth. They are suitable for lightweight buildings.
This type of green roofs capable of including many kinds of vegetative
ground cover and grasses that need low maintenance and can be self-generative.
While intensive green roofs require more soil depth of minimum 300mm usually.
They are common roof garden, which accommodate large trees, shrubs, and
need regular garden maintenance.
Owing to smaller structural loading and maintenance burden, extensive
green roofs are widely adopted in many countries. A typical green roof
comprises various layers which perform different functions in the system:
providing plant nourishment and support, protecting underlying waterproofing
systems and allowing drainage. The figure below illustrates the typical
structure of extensive green roof.
Design considerations
The application potential of green roofs is large because of their versatile
functions. Green roofs can be installed on new buildings and incorporated
in the existing buildings and structures. During the design of green roofs,
a number of factors have to be taken into account. Here are some examples
of them:
- Climate, especially temperature and rainfall patterns
- Strength of the supporting structure
- Size, height and orientation of the roof
- Type of underlying waterproofing
- Visibility, compatibility with architecture
Example Projects
1. EMSD Headquarters, Kowloon Bay
A roof garden provides thermal insulation and thus cut down cooling load.
(Source: http://www.archsd.gov.hk/ExhGallery_ViewPage.asp?lang=1&ViewProject=EMSD)
2. Veterinary Laboratory at Tai Lung, Sheung Shui, Hong Kong
Green garden is available extensively (green portion in the figure below)
in Veterinary Laboratory to enhance thermal insulation.
(Source: http://www.hku.hk/mech/sbe/case_study/case/hk/vet/fea_gn_roof.htm)
Reference
1. Hui, S. C. M., Benefits and potential applications of green roof systems
in Hong Kong, In Proceedings of the 2nd Megacities International Conference
2006, 1-2 December 2006, Guangzhou, China, pp. 351-360.
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