1.1 Introduction

Green roof is a new roofing technique to Hong Kong, but it has been applied throughout Europe. In the early 1960's green roof technology was developed in Switzerland and enhanced in many countries, particularly Germany. In the 1970's, a significant amount of technical research was carried out on root repelling agents, waterproof membranes, drainage, and lightweight growing media and plants. Green roof is different from conventional roof top garden, which is bulky and makes huge loading to the building. Modern green roof usually contains following functional layers:

Vegetated cover: xeric plants like sedum-species, shrubs, bushes and even trees

Growing media: a roof planting soil which is adapted to the plants

Filter sheet: prevents fine particles from being washed out of the substrate soil, thereby ensuring the efficiency of the drainage layer.

Drainage layer: profiled drainage elements retain rainwater for dry periods in troughs on the upper side. The surplus-water is drained off through the channel system on the underside. Special holes ensure evaporation and the necessary ventilation.

Moisture retention / protection mat: provides mechanical protection and retains moisture and nutrients.

Root barrier: prevents roots from affecting the efficiency of the waterproofing in case there is not a root-resistant one.

Green roofs can be categorized as 'Intensive' or 'Extensive' systems depending on the plant material and planned usage for the roof area. Both intensive and extensive green roof are either accessible or inaccessible.

Intensive green roof is similar to conventional roof gardens, provides additional recreation spaces and encourages interaction between people and nature. Wide variety of plant is adapted and other scenes such as waterfalls and ponds may also found in some intensive green roof. As a result, growing media depths, the roof loads and maintenance requirement are increased.

As different types of plant and different scenes are adapted, the energy performance of intensive green roof is difficult to quantify.

Extensive green roof is usually not accessible to public. The variety of plants is small and used to be xeric, can be sustained in a shallow substrate layer. Sedums and other succulents, flowering herbs, and certain grasses and mosses are commonly used. The depth of the growing medium id depend on plants, varies from 5-15cm and the total the thickness of roof layers can be less than 20cm (exclude the vegetated cover). Hence the roof is lighter, less expensive and requires little maintenance.

1.2 Thermal Benefits

Thermal insulation

Green roof can act as a good thermal insulator. Its conductivity varied with compositions of growth media, types of vegetation, and day-to-day varying water content. The heat transfer coefficients varied from 6 to 46W/m2K. For building without insulation, the saving is up to 48% in simulation approach.

Reduce thermal fluctuation

The thermal mass of green roof is larger than bare roof. This not only due to increase the thickness of the roof, but also of the latent thermal capacity of damp soil and the almost still air layer around the canopy. This would create a buffer effect and prevent thermal fluctuation.

Cooling effect

Green roof is different from the conventional thermal insulator. It does not only act as insulator in summer, the evaporation of soil moisture and transpiration of vegetation also bringing cooling effect to the roof.

Shading effect

The canopy layer of a planted roof can absorbs radiant energy and prevents it from reaching the surface of the growth media. This can effectively reduce the heat flux to space inside the building.

1.3 Environmental Performance

1.3.1 Impovement of microclimate

Absorption of solar energy

Great quantities of solar energy are absorbed for the growth of plants through their biological functions. Among the total solar radiation absorbed by the planted roof, 27% is reflected, 60% is absorbed by the plants and the soil through evaporation, and 13% is transmitted into the soil. This mean that the thermal energy absorbed by the building is largely reduced.

Evapotranspiration

Evapotranspiration is the combination of evaporation and transpiration. In a green roof system, the water content of soil and vegetation would be evaporated. At the same time, transpiration occurs inside plants. The process of evapotranspiration in plants includes three phenomena: water evaporation inside leaves (stomata cavity), vapour diffusion to the leaves surface, and convective vapour transport to the air. In both processes, water would take away heat energy from the system for changing from liquid to gaseous stage.

Lower surface temperature

The direct effects of planted roof are their thermal benefits in reducing surface temperatures of roofs. The maximum surface temperature of a bare roof could reach 57 deg Cwhen solar radiation was around 1400W/sq m during afternoon. But the maximum surface temperature of bare soil and vegetation in the same condition were only 42 deg cand 26.5deg c. The reason was green plants irradiated and reflected less solar heat to the surround environment. Mean while hard surfaces absorb solar heat and reradiated back to the surroundings. Less long-wave radiation emitted from planted roof was confirmed through comparisons of global temperatures and mean radiant temperatures measured on site.

1.3.2 Provide natural habitat for animals and plants

Part of green surfaces which were sacrificed to building developments can be compensated by landscaped roofs, thereby reducing the quantity of sealed surfaces and encourage limited wildlife development.

1.3.3 Improve air quality

Greenery can improve the air quality in different aspects. First, plants absorb carbon dioxides and emit oxygen. Second, vegetation can filer particles by their leaves and branches. Third, green roof reduce surface temperature and hence reduce thermal air movements. This can minimize stirring up of dust particles found on ground. Fourth, high surface temperature of roof contribute to the chemical reaction that create low atmospheric ozone, therefore green roof can reduce the formation of those chemicals.

1.3.4 Reduce noise level The spaces and voids inside vegetation layer and soil layer act as sound absorbers and barriers and can largely reduce noise levels.

1.4 Other Benefits

1.4.1 Storm water run-off Planted area can effectively retain storm water, reduces the water run-off 10 to 50%. It also can diversify the peak run-off period. These can reduce the construction and maintenance costs of storm water drainage system .

1.4.2 Extent life span of roofing materials The roofing materials shielded from direct exposure of solar radiation and not subjected to extreme temperature fluctuations would be more durable. This reduces the requirements of maintenance and replacement, and decreases the life cycle cost of the building.

1.4.3 Facilitate recreational and leisure activities As intensive green roof is accessible and can function as a garden, it provides urban residents additional space for outdoor recreational pursuits.

1.4.4 Therapeutic effect Visual and physical contact with plants can result in direct health benefit. Plants can generate restorative effects leading to decreased stress, improve patient recovery rates and provide higher resistance to illness.

1.4.5 Increase property value Rooftop garden can provides outdoor amenity space and improve the appearance of a building. These can directly increase the value and marketability of a property. Besides, the ecologically friendly nature of green roof may attract public interest and increase marketability.

1.5 Disadvantage

There are certain barrier to the development of the green roof system which might slower the adaptation of the green strategy in the area.

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