Green roofs present many possibilities and opportunities to create more environmentally-friendly environments.

Urbanisation is an undeniable reality. As a result, maximising space and sustainable design have become modern-day design mantras. With the rise of the green roof movement, the sky is the limit, writes Karien Slabbert.

An intelligent approach
Why green roofs? Ecologically intelligent design needs to follow an integrated approach. It should mimic nature to harvest energy from the sun, produce oxygen, minimise carbon emissions, manage water and provide habitat. To achieve this through design, there needs to be an understanding of the unique local climate and how to preserve, support and enhance the sense of place. However, in cities this could prove to be quite a challenge.

Here limited space calls for innovative solutions to create green landscapes. Rooftop gardens present a savvy way to green our urban jungles from the top down. With outdoor space at a minimum in many cities, rooftops are turning into prime space. Whether you want to create a green urban oasis or reduce your carbon footprint, rooftop landscapes provide the means. Dynamic and ergonomic, the key to rooftop landscapes is responsive design, quality engineering and stringent standards.

High design through the ages
Green roofs have developed extensively since the Hanging Gardens of Babylon (600 BC) and traditional Scandinavian sod roofs. In the pre-1960s, Le Corbusier and Frank Lloyd Wright applied green roof practices in their vernacular buildings, while these roofs were regarded as a “green solution” in Northern Europe in the 1960s, according to a presentation by Sidonie Carpenter, president of Green Roofs Australia. In recent years, there has been an unprecedented boom in “living architecture”, such as green roofs and vertical green walls, with a range of ever-evolving technological innovations to support these structures.

According to the Michigan State University’s Green Roof Programme, Germany is widely considered as the leader in green roof research, technology and usage. It is estimated that 12% of all flat roofs in that country are green and the German green roof industry is growing with 10% to 15% per year.

South Africa is also catching on to the trend. The eThekwini Municipality’s Green Roof Project is a striking example of the positive environmental impact of green roofs. The programme was initiated in 2004 and forms part of the Municipal Climate Protection Programme. It initially focused on understanding the city’s vulnerability to the impact of climate. According to www.durban.gov.za, this knowledge is being used to inform the development of appropriate adaptation and mitigation responses and strategies at a municipal and community level.

Benefits of gardens in the sky
According to www.rooftoplandscapes.co.za, green roofs have the following benefits:

Green building credits, carbon credits, tax incentives: The Green Building Council of South Africa’s (GBCSA) Green Star SA rating tools provide the commercial property industry with a basis to measure the effectiveness of green buildings and to recognise environmental leadership in the industry. Installing rooftop landscapes can earn property developers a significant number of green building credits. Governments from North America to Europe and Japan have begun to offer tax incentives and subsidies to encourage green roofs.

Greening urban spaces: In dense urban environments, rooftop landscapes provide more amenity space that reconnects urbanites and office workers with nature.

Biodiversity: Eco-roofs can provide important refuges for wildlife in urban areas.

Reducing stormwater run-off:  A rooftop landscape will reduce the rate and volume of rainfall leaving the roof. This reduces flash floods during storms and minimises storm drain requirements. A green roof can absorb up to 90% of an area’s typical rainfall. Rooftop landscapes also act as a rainwater collection tank, which naturally filters the rainwater. Rainfall can be harvested from green roofs and integrated with grey-water recycling systems.

Thermal regulation: Rooftop landscapes slowly absorb and retain heat from the sun and release it when the ambient air cools down. This reduces the building’s heating and cooling demands. In South Africa, where air-conditioning influences electricity usage, roof gardens can reduce energy consumption considerably.

Due to the high degree of insulation that they provide, green roofs are known for their ability to provide an extremely constant temperature throughout the year. During the winter they keep the heat in, and in the summer they provide a relatively cool environment. Green roofs also have the ability to soften harsh edges of buildings in sensitive environments, making them blend in with the surrounding area.

Noise insulation: The combination of soil, plants and trapped layers of air within the rooftop landscape act as a noise insulation barrier. Sound waves are absorbed or reflected. The growing medium tends to block lower sound frequencies, while the plants block higher frequencies.

Reducing the “urban heat island” effect: Green roofs cool the surrounding air. In fact, by respirating the retained water, they can cool the roof and surrounding air significantly. This microclimate can significantly reduce adverse “urban heat island” weather patterns.

Protecting the roof structure and waterproofing membrane: While installing a green roof requires a larger initial upfront investment than a typical roof, an eco-roof can last three to four times longer than a conventional roof. It can reduce the surface temperature of the roofing waterproofing membrane significantly, while protecting the surface of the roof membrane from exposure to ultraviolet (UV) rays and adverse weather conditions, such as rain and hail.

Improving air quality: Rooftop vegetation filters airborne particles, pollutants, CO2 and toxins from the atmosphere. In return, oxygen is set free, which reduces a building’s carbon footprint.

A waterwise solution: Many rooftop gardens grow solely on rainwater, making them eco-friendly and sustainable. Rooftop landscaping also reduces cooling energy needs of buildings by up to 25% and extends the life of a roof from 10 to 40 years.

Design considerations
The most important considerations when designing turf and green roofs are ensuring that the roof is strong enough to support the weight of the turf or plants (even when they are fully saturated by rainfall) and ensuring that the roof is watertight.
Structural calculations need to be made. With existing buildings, the foundations and roofing timber are likely to require upgrading in order to support the additional weight of a turf or green roof.

Rooftop landscape systems should be designed in accordance with the worldwide industry-leading German FLL (Research Society for Landscape Development and Design) Standard and the South African National Building Codes and Regulations.

Challenges

  • Space is often limited, which calls for careful design.
  • No single type of green roof works for all buildings and climates.
  • Bear in mind that a cubic metre of water weighs about a tonne.
  • The type of roofscape is limited to the load the roof can withstand.

Types of green roofs

Going green: Vegetative rooftop garden
A vegetated roof, living roof or eco-roof is partially or completely covered in plants. It consists of a vegetated roof covering that is planted over an existing roof structure.

There are three types of vegetative rooftop gardens:

  • Intensive rooftop gardens replicate ground-level gardens. They have a growing medium depth of 200mm or more and are normally installed over concrete roof decks.
  • Extensive roof gardens are covered with low-growing plants, with a growing medium depth of less than 200mm.
  • Biodiverse roof gardens recreate pre-existing ground ecosystems. They are designed with specific biodiversity objectives in mind, such as maximising birdlife or providing a habitat for specific plant species.

Creating a living roof
A Johannesburg-based company, Rooftop Systems, gives the following tips for designing a rooftop garden: According to Justin Witten from Rooftop Systems, developing a green roof involves creating “contained” green space.  Green-roof systems may be integrated, with drainage layers, filter cloth, growing plants already prepared in movable, interlocking grids. Or each component can be installed separately.

A well-designed rooftop system must take four main functions into account:

  • It must retain sufficient moisture for plants.
  • Excess water must be allowed to drain freely, so that water does not sit on your roof and oversaturate your soil, drowning the plant roots.
  • It must be as lightweight as possible to minimise stress on the building structure.
  • It must protect the roof structure and waterproofing membrane from aggressive plant roots.

Rooftop engineering
The engineered system comprises the following layers, listed from the bottom to the top:

  1. The waterproofing membrane prevents water from entering the roof structure.
  2. The root-protection barrier protects the waterproofing of the roof from penetration by plant roots.
  3. The drainage layer, in the form of a high-strength, cuspated, high-density polyethylene (HDPE) sheet, ensures that surplus water is safely drained away to gutters.
  4. The filter layer prevents soil particles from entering the drainage layer and obstructing the water flow.
  5. The growing medium ensures that the vegetation is provided with sufficient water and nutrients, while remaining lightweight.
  6. The vegetation layer may range from turf to trees.

Key elements
Drainage and waterproofing: Roof gardens need excellent drainage systems and waterproofing. All water (rain or irrigation) must be able to leave the roof quickly and easily. Ensure that waterproofing and drainage is done properly.

Irrigation: Drip irrigation works well, as roof gardens tend to be breezy. Roof gardens need regular feed applications as roots have limited space. A tap somewhere on the roof is useful for cleaning purposes.

Soil: Soil depth will determine whether you are able to grow trees on the rooftop or merely ground covers. The more soil depth you have (either in containers or over the entire roof), the larger the plants can grow.

Prevailing weather: This is a major consideration. Roof gardens are more exposed to wind, temperature and drought than ground-level gardens.

Practical factors

  • The building and roof must be engineered to withstand an extra load of watered soil and plants.
  • Consider both the construction of the roof and building foundations.
  • Consult an engineer if you are converting an existing space into a roof garden. New structures can be designed with the roof garden in mind.
  • If provision was not originally made for a high-rise garden, the outer walls, rather than the centre of the roof, need to bear the weight of the various structures.
  • Temperature.
  • Wind direction and velocity in summer and winter.
  • Glare from windows and other reflective surfaces.
  • Hard landscaping: Because of weight restrictions and available depth, wooden decking or Astroturf are often the best choices for a rooftop garden, unless the building was originally constructed to cope with thicker, heavier surfaces.
  • Choose hardy, endemic plants: Wind resistance and low water requirements are crucial. Aloes and succulents such as Aloe arborescens, Aloe maculate, Aloe rupestris, Cotyledon orbiculata, Crassula spp., Delosperma rogersii, Kalanchoe spp., Kleinia fulgens, Portulacaria afra and Senecio are ideal.

Systems and types
Rooftop landscapes can be modular or permanent and can be installed over pitched or flat roofs.

Built-in-place systems

  • The various layers of the rooftop landscape are installed one by one on the roof membrane.
  • Plants are planted in the garden at the site. It takes longer to install this type of system than a modular system.
  • They allow more diverse plant selections.
  • Roofs generally require a few months to two years to mature.

Modular systems

  • Interlocking stainless steel trays are pre-assembled with the drainage system, growing medium, and vegetation prior to installation. Thus, the green roof is delivered pre-planted and ready to set in place, allowing instant greening of the roof.
  • The modules can be rotated seasonally, allowing greater design flexibility.
  • Clients have the option of installing the rooftop landscape in sections, offering opportunity for future add-ons and alterations.

Types
Construction methods differ between pitched and flat green roofs. Flat green roofs can be either extensive (have a thin layer of growing material such as sedum matting) or intensive (greater soil depth with shrubs and even trees). The weight requirements for intensive green roofs are such that they are normally installed over concrete roof decks.

Pitched
Waterproofing systems for pitched green roofs tend to be simpler than flat green roofs as water “pooling” has less potential to cause a problem. As with all waterproofing products, the success of the system is dependent upon the quality of installation.

Typically, the studded membrane is laid over suitable exterior grade bitumen-based roofing felt to form a waterproof drainage layer. The turf or sedum layer is then laid directly over the membrane.

An edging board constructed from treated timber is secured around the perimeter of the roof, using special turf roof hooks. This holds the turf in place and prevents it from sliding off the roof. For roofs with a pitch of greater than 23°, it is necessary to provide a framework to retain the turf. This can easily be constructed from treated timber.

Flat
Flat green roofs can either be extensive, with a thin layer of turf or sedum matting, or intensive, with a greater soil depth that can accommodate shrubs and trees. This is often used for roof-top gardens. The build-up of a flat green roof will vary depending upon the intended use.

A standard roofing membrane and insulation layer are  laid onto the underlying roof structure, then a studded, perforated drainage membrane is used in addition to provide a drainage layer with reservoir capability. A filtrating drainage layer is laid down before adding the required soil loading and plantings.

In situations where the roofing membrane does not offer adequate root resistance, a root barrier might need to be included between the roofing membrane and the drainage layer.

Green roof tick box
Does your green roof tick the following boxes?

Site analysis

  • Climate
  • Building
  • Surrounding environment
  • Climate
  • Seasonal constraints
  • Wind
  • Humidity
  • Rainfall
  • Plant hardiness zone

Building

  • Structural load bearing
  • Access and safety
  • Roof slope
  • Integration with other systems
  • Size
  • Building type – residential/commercial
  • Exposure to elements
  • Existing waterproofing
  • Fire
  • Water, access/storage

An integrated approach
Instead of replacing the natural environment with the built environment, we should strive for an intermingling of the two so that each can help to sustain the other, according to www.greenroofs.com. “Building rooftops, usually considered forgotten spaces and deserts in biological terms, now represent valuable opportunities for creating a new life environment,” states the website. The basic philosophy of green roofs applies sustainable “green development” techniques to support our human and natural communities and development while remaining economically viable for owners and users of a site.

Full acknowledgement and thanks are given to William McDonough, “A Field of Dreams” Green Roofs: Ecological Design and Construction, www.safeguardeurope.com, www.rooftoplandscapes.co.za, www.gbsa.org.za and www.greenroofs.com for writing this article.