Green roofs have proven benefits but care must be taken when choosing from the different types of systems available, says ZinCo director Fabian Kaiser.
From creating green havens in concrete-dominated cities to safeguarding the waterproofing system of a building, the benefits of green roofs have been widely recorded.
In terms of their ecological benefits, green roofs can cool and humidify the surroundings, significantly reducing the so-called urban heat island effect in urban areas. Moreover, landscaped roofs compensate for green space lost during construction and can offer additional recreational space in densely populated areas.
While green roofs provide natural habitats for wildlife, they can also absorb fine particles and enhance acoustic and thermal insulation to improve the building’s energy performance. These systems are economically viable to install as they can reduce maintenance and running costs by protecting the waterproofing from extremes of climate, and mechanical damage.
Another key benefit is green roofs’ moisture-retention quality, which allows for time-delayed run-off of excess water, greatly reducing drainage flow rates. This quality can be of particular importance to towns and cities facing the challenges of increasing amounts of heavy rainfall, which can result in flooding or even cause overloading of the public sewer system.
Hardly any other roof shape allows for such a variety of roof greening as does a flat roof. However, even sloped roofs with a pitch of up to approximately 35º can be ‘greened.’
There are two types of green roofs: extensive and intensive, offering different benefits. An extensive green roof may cover a large area, but can be installed and maintained at a relatively low cost. The layer depth is approximately 50-150 mm and the weight is low at between 60-150 kg/m², while the plant communities used – including renewable succulents (sedums), mosses, grasses and herbage – can survive with little upkeep.
Nevertheless, the roofs have to be accessed once or twice a year in order to inspect drainage and ventilation equipment and, where necessary, remove wild growth.
In addition to their conventional use, extensive green roofs can produce synergy effects when used in combination with solar energy; this can be achieved as the systems’ comparatively low ambient temperature can improve the efficiency of photovoltaic modules.
Intensive green roofs, also referred to as landscaped roofs, can provide the same range of landscape design available at ground level. Lawn, perennials, shrubs and trees grow in layer depths of between 150 and 1000 mm, very often in combination with pathways, terraces, patios, play areas and water features. Even driveways such as that over an underground car park can be created with the appropriate structural support.
Between the layers
A green roof system consists of a number of layers and each has a unique function. The base layer is always a waterproofing membrane. This layer must be root-resistant, which must be achieved through the installation of a separate root barrier. The first layer of the build-up is a high-grade protection mat protecting the waterproof membrane from mechanical damage.
This is followed by drainage elements that retain rainwater in their water retention cells and drain surplus water through the underlying channel system to the roof drains. Depending on roof structure and green roof requirements, the drainage elements will vary in terms of height, weight and material.
Where the drainage elements are installed across the whole area to be greened, this allows for drainage of the entire area, even beneath pathways and terraces.
The drainage elements can also serve as a permanent formwork for anchoring stonewalls or play equipment.
On top of the drainage layer, a filter mat is installed retaining fine particles from the overlying substrate layer. This substrate layer is composed of mineral and organic materials specifically adapted to suit the planned vegetation. The plants provide the final layer.
Attention to pitched roofs
Pitched green roofs require extra scrutiny from installers due to a number of requirements. Shear forces have to be transferred into stable eaves edges and additional shear barriers may have to be installed to prevent the green roof build-up from sliding. In addition, specialised drainage boards need to be used to help retain the soil substrate, substrate layers need to be increased and additional irrigation might be necessary. Then there is the question of maintenance – the right selection of plants has to be taken into account from the beginning.
To meet these requirements for pitched green roofs, professional planning and execution is essential while the use of tested and proven pitched green roof system build-ups is indispensable.
Advanced water retention
As mentioned previously, water retention is one of the major benefits of a green roof. However, water storage in a standard green roof build-up cannot be increased arbitrarily, since more water could result in changes to the vegetation.
For this reason, and to counteract the increase in heavy rain events as a result of climate change and the consistently high level of soil sealing, so called storm water management roofs have been introduced to the market.
Due to an extra layer of spacer elements underneath the standard green roof build-up, the storm water retention capacity can be increased by a multiple. This means water can be stored for a prolonged period of time, from 24 hours up to a few days, and reused in intelligent ways.
Fabian Kaiser is director of rooftop landscaping specialist ZinCo Green Roof Systems