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Green Roof is a building's roof that is completely or particially covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier, drainage, and irrigation systems. It is commonly divided into two categories namely Extensive and Intensive:
- Extensive Green Roofs, also named as Eco-roofs, are characterized by their low weight, low capital cost and minimal maintenance nature.
- Intensive Green Roofs, also named as Podium Gardens or Sky Garden, are characterized by greater weight, higher capital costs and maintenance requirements and more plant choice. It is geared for recreational use by human beings.
In Hong Kong, Intensive Green Roofs are already well-established in the form of podium gardens. Extensive Green Roofs, on the other hand, are better-suited to retro-fitting projects which have their own technical constraints, and are thus not yet well-established in Hong Kong.
A detail comparsion between the extensive and intensive green roof systems are as follows:
| Type |
Extensive |
Intensive |
| Outlook |
 |
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| Layout |
 |
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| Characteristics |
Extensive application over large area |
Intensive capital and maintenance input for optimal benefits |
| Soil thickness |
50 - 150mm thick (thin soil) |
200 - 2000 mm thick (deep soil) |
| Irrigation |
Little / Not required |
Required |
| Maintenance |
Little |
Frequent |
| Pros |
- Lightweight (80kg/m2-150kg/m2)
- Low maintenance
- Suitable for retrofit projects
- Relatively inexpensive
- Suitable for large areas
- Suitable for root with slope 0-30o
- Vegetation develop spontaneously
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- Diverse utilization of roof
- Greater diversity of plants and habitats
- Greater insulation properties
- Simulate a wildlife garden
- Attractive
- Visually accessible
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| Cons |
- Limited of plant choices
- No access for recreation or other uses
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- Higher cost
- Not suitable for green roof retrofit projects
- Greater weight loading
- Greater needs of energy. Water and materials resources
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