Create a green roof

Green roofs are steadily growing in popularity. Helping to improve biodiversity as well as the aesthetic appeal of the building, green roof technology can be employed across a wide variety of commercial applications. David Davis discusses the key points

14 October 2010 

1. Reduce pollution

With towns and cities becoming increasingly crowded, the installation of a green roof is the ideal option to help reduce pollution levels created as a result of the busy environment. High levels of CO2 emitted by cars can be reduced via plants’ natural respiration process. Airborne particles and volatile organic compounds found in the air can also be removed due to the absorption process of a green roof. Excellent water attenuation will also help reduce the risk of flooding. During the summer months for example, a green roof can retain between 70 – 80 per cent of the water run-off.

2. Reduce your energy costs
The environmental benefits of a green roof can bring bonuses for the organisation too. Energy and cost savings can be made; by its nature a green roof is thicker than standard roofing materials and so significantly reduces heating and cooling costs. This results in more even temperatures across the roof surface. As an example, according to the Environment Agency, Paradise Park in Islington has saved 1.6 tonnes of CO2 as a result of its green roof installation. Due to the roof’s thermal mass, an air conditioning unit was not required, leading to a reduction of 3,800kW/hrs of energy with significant cost savings.

3. Increase green space
In a built-up area with little access to outside space, any available ground that can be used as green space is vital in ensuring the health of the urban realm. By increasing exposure to outdoor space and fresh air, employers can help to increase both the physical and mental 
wellbeing of staff, that will, 
in turn, increase staff productivity. Considering this, extensive research undertaken by English Nature (now Natural England) suggests that green roofs can significantly help to improve the concentration and health of staff purely through exposure to oxygen. Aside from these benefits, the introduction of a green roof can also bring aesthetic advantages, improving the overall appearance of the building.

4. Promote biodiversity
By introducing more green space, biodiversity levels can be increased. Attracted to the newly placed green roof, varying flora and fauna will start to use the roof, increasing the wildlife in an area, a point of particular notice in built up towns and cities.
By this measure, the Black Redstart, a rare species of bird, has been found to use green roofs in the capital, a point supported by the inclusion of a plan to incorporate more green roofs in London to attract the species in the England Biodiversity Strategy (Defra, 2002) and the Sustainable Development Report (Defra, 2003).

5. Selection of materials
When selecting a green roof, there are a number of points to consider during the initial stages to determine which material will work best for the roof in question. For large projects, such as an office roof, a layer of sedum cuttings is advised when used with a substrate layer to ensure substantial anchorage of the root system while feeding the plants with an adequate supply of water. To ensure the roof continues to thrive all year round, even during extreme heat, a water retention and feeding layer should be fitted underneath to provide an additional supply of water to maintain the life expectancy of the system. Due to the nature of a roof, with its exposure to the elements, an adequate drainage and filtration system must also be applied.
A membrane designed specifically to filter particles which may have passed through from the substrate will stop the possibility of a blockage occurring, helping to eliminate the risk of flooding.

6. The best solution

During the material selection process, it is also worth considering the differences between systems depending on the intended use of the roof. In essence, there are two basic types of green roof system; ‘intensive’ and ‘extensive’. An intensive green roof is generally designed as a park or garden at rooftop level. Requiring a deep layer of soil to plant shrubs and trees, this style of roof requires a high level of maintenance, which can be expensive, to ensure the plants are kept in good order.
An extensive green roof has a shallow soil depth and so requires little maintenance. Ideal for flat to 40° pitched roofs, an extensive system commonly uses sedum matting which can be laid directly on to the roof membrane and provides an instant green roof.

7. Seek specialist expertise
For FMs considering the installation of such a system, the professional guidance of an expert must be sought to ensure that a suitable system is supplied to ensure weight loading does not compromise the structural integrity of an existing building. It is important to consider the considerable weight difference once the roof has been planted and the water it absorbs as a result.

8. Green roof equals 
good PR
In general, a building installed with a green roof will be seen to be environmentally sound. For the occupying organisation this can prove to generate extremely positive PR that can be harnessed to express their consideration for the environment.

David Davis is national technical manager of Renolit