Green roofs can be a key tool in onsite water management.

Any facility manager looking into how to improve the performance of a building in water management may be interested in using green roof technology to help with this goal. The advantage of using a green roof to manage water is that this technology will also create a variety of other types of value for the building, thereby improving performance in several spheres at once.

Here is how it works: water enters green roof gardens – gardens on roofs and balconies – in the form of rain. In most cases, water will also enter a green roof from an irrigation system. The irrigation system may use mains water, harvested rain water, grey water, or a combination of these.

The water that enters a green roof is captured, absorbed and slowly released. Some of the water released drains into the storm water drains. Green roofs are supported by a series of layers, and beneath the substrate and support layers lie the waterproofing membrane and drainage. The drainage ensures that the run-off from the green roof shifts into the building drains, as water does on any roof.

The point of difference on a green roof as opposed to a conventional roof is that the water which enters the building drains accounts for only 20 to 40 per cent of the water that enters the green roof. The remainder of the water is used by the plants or evaporates from the soil (or replacement substrate).

Whereas 100 per cent of the water that falls onto a conventional roof or balcony enters the storm water drains, on green roofs, only 60 to 80 per cent of water goes into the storm water drains.

Through your choice of soil or substrate and plant species, you can manipulate and reduce the volume of water that enters the storm water drains – and eventually the local waterways.

These techniques for reducing the volume of water that enters the storm water drains may be of interest to any facility manager interested in improving a building’s impact on the surrounding waterways and the way a building sits in the landscape.

You can further reduce the volume of water that enters the storm water drains by diverting the water drained from the green roof to irrigate other gardens or flush toilets. Using it for these purposes has the added advantage of decreasing your water bill.

You can also manipulate the quality of the water that enters the storm water drains through your choice of plant species and substrate composition. Some plant species and substrate components provide greater water filtration than others. Some will filter trace minerals and other pollutants. This has two consequences:

  • It improves the quality of the stormwater which leaves your site. This in turn improves the way your building sits in the landscape, in particular its impact on the surrounding waterways.
  • Water which has passed through a green roof is more valuable to other gardens since the green roof will assist in filtering out trace minerals and other pollutants. This means the gardens you water with its run-off receive better quality water than had you simply used grey water or harvested rain water directly from conventional roofs. When irrigated with harvested water filtered by a green roof, your other gardens will perform better and require less care by your maintenance staff, reducing costs while achieving better outcomes.

Side benefits and additional ROI

The presence of gardens on your roof or balcony offers a host of other forms of value to the building, such as increased staff productivity, health and well-being. Green roofs can also increase real estate value since tenants place higher value on buildings that have more vegetation and natural beauty. Green roofs create these forms of value on account of increasing staff access or even merely increasing staff views of nature.

You can manipulate how much value green roofs add to your building in these areas through your choice of plant species and garden layout.

The evaporation of water from a green roof’s substrate and the transpiration of water from the plants mean that if you have a green roof, more water will enter the atmosphere above your roof or balcony than it would otherwise. This is to your advantage, since an increase in water entering the atmosphere means the air above your roof or balcony grows cooler. Cooler air above the roof or balcony in turn helps to lessen the demand for air conditioning in the building and increases the efficiency of the performance of any photovoltaic cells in solar panels on the roof. In effect, the garden provides external air-conditioning for the building.

You can manipulate how much external air-conditioning your green roof provides your building through your choice of plant species and substrate composition.

It is not just by improving the quality of the water that leaves your property and enters the waterways in the surrounding landscape that you are managing water responsibly on and offsite. Your green roof also delays the departure of water from your property. In sudden downpours, this delay makes an important contribution to protecting the local waterways and bays from damage. It takes time for rain to seep into the substrate on a green roof. It takes time before the substrate is saturated, and in the meantime, some of the water is trapped by the stems and leaves and taken up by the roots of plants. Only once the substrate is saturated does the diminished volume of remaining water begin to enter the green roof’s drainage system.

Here again, you can manipulate the impact of your green roof through your choice of plant species and substrate composition.

Integrated water management systems and green infrastructure

Green roofs can act alone or form part of an integrated water management system. They can be combined with infrastructure on the roof, wall or ground, including rainwater harvesting and grey water systems, green walls or rain gardens.

Design considerations

Unlocking the full potential of your green roof requires an understanding of both the design and the key purpose you wish your green roof to achieve. Water is an essential component on any green roof, and the management and maintenance requirements of green roof vary according to a variety of factors.

For people who manage green roofs in the long term, the key point to remember is that the design of the green roof is critical to its capacity to continue to provide water management. If the purpose of your green roof is to filter, harvest, retain or manage water across the site, there are a number of key factors you need to consider:

  1. Substrate depth, composition and porosity – this will directly affect water retention and movement across the site
  2. Nutrient loads – when managing water responsibly, managing the nutrient load is key
  3. Plant selection – the rate at which different plants take up water varies significantly
  4. Drainage – how and where water is directed
  5. Linkages with any additional water management systems – storage for irrigation, connections with green walls or rain gardens, or reuse

Management and maintenance considerations

If you are responsible for managing a green roof, this means that you must:

  1. Make sure your maintenance team understands which plants provide the right type of water management service, so that when they replace plants, they select plants that will provide this service.
  2. Make sure your maintenance team checks the substrate regularly to ensure it is the correct depth and composition.
  3. Monitor the output of water from your green roof so you are able to recognise any deterioration in the performance of your roof and respond accordingly.
  4. Make sure members of your maintenance team understand and are fully briefed on the correct nutrient and fertiliser application across the roof.

With the right design and maintenance, green roofs provide sustainable onsite water management as well as many other forms of value.