Topic 3
Many interactive factors must be considered for balancing optimal performance of green roofs, including:
- climate (temperature and rainfall patterns) and microclimates on roof top,
- strength of the supporting structure,
- size, slope, height and directional orientation of roof,
- type of underlying waterproofing and drainage elements, such as drains, scuppers and drainage conduits,
- accessibility and intended use,
- visibility, fit with architecture, and owner’s aesthetic preferences,
- whether fit with other ‘green’ systems, such as solar panels,
- materials and labour costs.
In this particular section, we consider some factors that are likely to affect construction and effectiveness in more detail…
While green roofs vary greatly in weight, it is important when measuring to consider the ‘wet’ weight, that is when fully saturated fabrics and plants are at their heaviest.
- For example, a lightweight green roof of a 3-inch deep system weights 6.75 pounds/square foot when not wet and 20 pounds/square foot when saturated with plant cover.
- Intensive green roofs require concrete supporting system.
Careful of advertised lightweight media, as they might be inconsistent with the industry standard and could cause structural and compression problems later.
In the U.S.A., the latest fashion has been the introduction of modular or tray green roofs, small projects creating green roof space. Modules have however the following issues:
- erratic and generally poor long-term strength for plants,
- insufficient protection of the underlying waterproofing system,
- poor stormwater management performance.
When plants are installed in thin exposed soils, they can draw strength and durability from the contiguous environment of soil and plants. The classic green roof can provide a lush planted surface that is simply not attainable with a collection of independent containers.
Attaching all elements of the green roof to the underlying structure is not essential, as roots bind layers of media and fabrics in a unified cover and create enough surface wind turbulence to foil potential uplift.
In case the green roof will be located in an unusually high wind area (high rise building or coastal area), there are appropriate wind stabilization measures, such a concealed design.
The maximum slope recommended for a green roof is 30 degrees. This can be accomplished through meshes, slope stabilisation panels, cribbing or battens.
It is important to highlight that roofs with slope greater that 30 degrees can be greened but special techniques are required.
The climate where the building is located can influence the effectiveness of a green roof.
While they can be built anywhere, green roofs require special techniques in semi-arid, tropical and even windy coastal areas.
Fabrics are not good water storage components, as water is heavy and fabrics light. Any water captured in fabrics will be consumed rapidly and will drain out the fabrics over time.
You can use fabrics to distribute (not store) moisture evenly across vegetative cover, to protect underlying waterproofing and to prevent migration of soil materials.
With appropriate engineering and horticultural selection, a permanent irrigation system is not required.
At the same time, when irrigation is essential due to climate, media or plant selection, the water should be delivered deep under the surface – where the roots will seek it and where it will not be wasted to evaporation. Surface irrigation systems are wasteful and require more maintenance.
The most common reason for green roof failure is not the lack of moisture but rather too much moisture! Roof designers often focus on capturing and retaining moisture as much as possible, neglecting good drainage and aerobic conditions in the growing media.
Proper drainage ensures that the growing medium will be maintained in an aerated condition suited to healthy plant growth. Basal drainage must also be designed with large rainfall events in mind.
Vijayaraghavan (2016) adds that some further issues to consider in the construction and effectiveness of green roofs include:
- The selection of vegetation should be performed according to the local climatic conditions and nutrient availability. Native species are the best as they have already adapted to local weather conditions.
- It is advisable to design green roof substrate using local waste material, making them cheaper to install. Additionally, the weight of the substrate should be kept as low as possible.
- The application of a water-proof layer is always advisable, while a root barrier is mandatory for intensive roofs but optional for extensive.
- There are also ways to avoid weeds on green roofs:
- avoid light at the substrate surface by using ground-cover or tall plants,
- use high plant diversity,
- remove parent plants before seed which are physiologically capable of germination (Nagase et al. 2013)