Green roof design includes the following.
- Understanding the structural stability and load bearing capacity (dead and imposed loads) of roof. The structural loading is a combination of dead loads, all permanently placed parts of the roof, including hardscape, plants, growing medium, features, etc; and the imposed loads, such as people and temporary components.
- Waterproofing of the structure and the green areas.
- Arrangement of surface and subsurface drainage of the roof.
- Soil fill, and planting with limited root zone.
- Location and planting of trees.
- Location and design of pavements, walls, landscape features and structures.
1) Design Guidelines
The guidelines given below shall be followed for the design of green roofs (see Figure above).
- A concrete layer shall be laid on the roof slab to provide slopes for subsurface drainage of vegetated areas, drains.
- Where there is a large green area on the roof, perforated pipes laid in screed shall be used to channelize subsurface water to the adjoining drainage chambers.
- A protective layer shall be laid on the sloped concrete surface to protect the buildings structure and the waterproofing membrane on the building slab.
- Drainage shall comprise of surface drainage and subsurface drainage.
- Subsurface drainage layer shall be very porous to permit water to pass easily through it.
- It should be permanent and continuous over the entire roof surface and strong enough to support the weight of the plant materials and hardscape above it.
- This layer shall be kept free of any materials that could prevent the free flow of water to the drains.
- Drainage cells may be used to increase soil depth.
- Surface drainage shall allow for easy flow of water from the roof surface to the drainage chambers on grade.
- It shall comprise of drains, catch basins.
- They shall be carefully designed to avoid expansion joints on the roof.
- Helps in containing the roots and the growing medium, while allowing for water penetration and prevents clogging of the drainage layer and roof drains.
- The depth of soil shall depend on type of planting, screed thickness and structural allowance for fill above slab.
- Shall be grass, shrubs, ground covers, trees.
- Type of vegetation shall depend on the structural allowance for fill above slab.
- Shall be adequately provided.
- The thin depth and well-drained soil used in green roof construction cannot provide the plantings with the subsurface water normally available to ground level plantings.
- Care shall be exercised to prevent the soil mass from drying out and causing damage to the plant materials.
- As described in by the Consultant shall be integrated with the landscape
- Includes paving, street furniture and water features, walls, fences, screens, pergolas, kerbs, fire paths, roads.
- The landscape elements shall be considered in relationship to the structural limitations of the roof and its supports below.
- The street furniture, including lighting fixtures shall be carefully anchored on rooftops.
2) Green roof system
- Green roofs have the potential to improve the thermal performance of a roofing system through shading, insulation, evapotranspiration and thermal mass, thus reducing energy demand of building for space conditioning.
- The green roof moderates the heat flow through the roofing system and helps in reducing the temperature fluctuations due to changing outside environment.
- If widely used, green roofs can reduce the problem of heat island effect which in turn reduces the energy consumption in urban areas.
- Green roof systems comprise a lightweight growing medium, plants and a root repellent layer in addition to the regular components of a roof.
- The additional components and thickness of the growing medium provides thermal insulation, while the green cover lowers ambient temperatures through evapotranspiration.
- Issues related to dead weight and water proofing should be considered while designing roof.
- There are three types of green roofs, as follows.
Intensive green roof
- It is like a conventional garden or park with almost no limit on the type of available plants including trees and shrubs.
Extensive green roof
- It is designed for little maintenance or human intervention once it is established.
Modular block green roof
- It is designed in modular units which interlock and each module contains drainage systems and plants.
- Use of photovoltaic panels for renewable energy can also be advantageously used for their effect on shading of roof top.