With a heavy focus on being environmentally sustainable we’re seeing the installation of green roofs – or ‘living’ roofs – springing up. We check out this rising trend and discover the best way to install one.
Green roofs also known as eco-roofs, living roofs, vegetated roofs or planted roofs, employ plants to improve a roof’s performance and enhance visual appeal. They are a recent alternative to saving the planet. Although it has grown in popularity over the past few years, the Scandinavians have been building them for years!
A roof garden on the Group 5 head office in Waterfall City.
Image credit: Waterfall City
Why have a green roof?
Advanced Green Roofing says that green roofs can contribute to landfill diversion by prolonging the life of waterproofing membranes, reducing associated waste, the use of recycled materials in the growing medium, as well as prolonging the service life of heating, ventilation and HVAC systems through decreased use.
Installing a green roof
“Green building isn’t a DIY thing. Hire a professional and many serious things can go wrong,” says Justin Sam of Vertical Landscapes. Installing a green roof isn’t a one size fits all, all specifications and sizes depend on a building’s structure. However, installation must follow sans 10400 guidelines. “A building has lot to do with interpretation. Take precautions to work with engineers, contractors and architects,” says Sam.
Clive Greenstone of Green Roof Designs says there are two methodologies that can be applied to a green roof. One is the growing medium can be placed directly on the specific underlay required for green roofs or alternatively the plants can be grown in modules or trays. The module approach is beneficial for difficult applications such as on corrugated roofs and where you want an instant effect or it needs mobility. A specialised plant layer is required for a well designed and installed roof. Greenstone describes these as drought resistant and heat resistant, plants must be low growing and self-seeding (to replace themselves from seed when stressed from heat and water fluctuations), wind resistant and they must be able to survive in extreme growing conditions.
Advantages and disadvantages of the modular approach
This system can be installed using three different depths. The trays are light weight and flexible, making set up and design easier than the direct application. In terms of maintenance and repair, containers or trays can be moved easily without disturbing plants and the growing medium. The option of containers allows for the installation of green roofs in sections. Therefore, offering the opportunity for future add-ons and alterations. Trays can be pre-planted and so offer quick installation. The container system components can quickly be put in place on the roof in accordance with design. It is also a DIY user-friendly technique. Trays can be installed on basically any existing roof surface in good condition and structural capacity but some plants may struggle as their roots need space to roam.
“Green building isn’t a DIY thing. Hire a professional and many serious things can go wrong,” says Justin Sam of Vertical Landscapes.
Advantages and disadvantages of a direct green roof
For maintenance and repair purposes, layers need to be lifted, rolled until the problem is found and this could potentially disturb the plants. It is often difficult and expensive to change or add on due to edge design requirements as various layers need to be installed prior to planting. Direct systems are often heavy and may require additional roof surface replacement or support and plant roots have a greater space to move and network.
Considerations before undertaking the greening of a roof
Michael Hickman of Ecoman, gives an extensive outline of what a builder should consider before installing a green roof. “Before considering constructing a green roof or to green an existing roof it is wise first to discuss the proposed green roof with a specialist in green roof technology,” he says.
It is essential that the roof is greened and has a durable and reliable water proofing and root protection layer that can’t be penetrated by the roots of the plants to be grown on it. – As it is a very expensive business to lift an established green roof or to have a leaking roof repaired then to replace it. The roof barrier must extend well above the surface of the growing medium and must be well attached or bonded to the structure to prevent water and roots from entering behind it.
Here in South Africa many roofs are water proofed using bitumen based products which do a very excellent job of waterproofing and where vegetation is not allowed or required to grow on them. In Germany, many roofs exist that have successfully been waterproofed and root proofed with bitumen based products that have not experienced problems, however they have mostly plants growing on them such as Sedum which do not have excessively invasive root systems.
Hickman says, “Here in South Africa I do not recommend growing plants directly on roofs which have been waterproofed using bitumen based products, without the addition of a very strong root resisting barrier that is constructed over the bitumen waterproofing. It must be 1mm or thicker and have LDPE plastic sheeting fitted and welded together to prevent roots from damaging the water proofing.
“This is my preferred system which is highly effective and simple to install and it has been perfected in Germany and other parts of Europe over many years specifically where millions of square metres have been installed on green roofs.”
The steps of applying a direct green roof.
Image credit: Teach about US
A moisture triggered polyurethane membrane consists of a glass fibre reinforced liquid applied polyurethane membrane is made up of a seamless cold applied fully bonded, highly elastic, one-component, moisture-triggered polyurethane water and root barrier. The glass fibre matting (chop strand) reinforcement is bedded in the first coat of the liquid moisture cured polyurethane membrane giving a continuous seamless waterproofing and roof barrier. This system has the added advantage that it can be moulded to the most complicated shapes that are created by the various details on roofs in a simple and very effective once-off waterproofing and roof proofing operation.
The root barrier may need to be protected from mechanical damage. Depending on the waterproofed surface of the roof that is to be greened, where LDPE plastic sheeting is to be used, it may be necessary to put a layer of geofabric on the roof as protection for the root barrier. It can also be useful to cover the root barrier with a layer of geofabric for protection against mechanical damage caused during planting and maintenance or when specially designed growing containers are placed on the roof for the plants to grow in.
Where a plastic sheet such as LDPE is laid over the waterproofing as the root barrier it must be well fixed to the structure well above the level of the growing medium using non-corrosive metal or plastic fixing strips in a way to prevent water and roots from entering behind it.
On most roofs, some means of drainage will be required, again the need to keep the weight to a minimum is an important factor, fortunately with modern technology also comes modern light-weight products that are most suited for this.
Conclusion
Applying a green roof comes with benefits in addition to its aesthetic appeal. A professional installation ensures that the structure of the roof won’t be compromised over time. The sudden popularity of green roofs demonstrates how well they work for the environment and buildings overall.
Industry bodies, such as the Green Building Council of South Africa (GBCSA) or the Institute for Timber Construction (ITC-SA), are a few places where you can find qualified and accredited installers in green roofs. Finding a good contractor is one thing, finding one that knows the ins and outs of installing a green roof is another; these industry bodies can help to ease the load.
METHODOLOGIES GREEN ROOFS REQUIRE
The roof must be checked by a specialist structural engineer to ensure that the existing roof can take the additional load of a green roof.
Next a green roof specialist or designer must be consulted before design.
An additional protection layer is applied on top of the existing roof membrane.
A drainage layer is placed on top of the protection layer (not required for module application).
Specialised soil layer is carefully placed on top of the drainage layer.
Then the vegetative layer can be planted.
Written By Ntsako Khosa
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