Green roofs - Getting it right
publication date: Jul 8, 2008
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author/source: David Harris
Lightweight green roof systems were introduced into the UK by Bauder over 20 years ago, when forward thinking architects, planners and the more environmentally aware clients were the main people to show interest.
Within the last five years, real intensification has occurred as local authorities and politicians have realised that green roofs will benefit our changing environment through their ability to negate the increasing effects of CO2 emissions, reduce global warming, curtail localised flash flooding, replace habitats for wildlife as well as being able to provide additional recreational space.
Green roofs have now become big business and ‘experts’ are appearing everywhere; with no British Standards covering Green Roofs, to monitor the products and installation, it is important that any supplier is able to prove their credibility and to offer valuable guarantees. The German research organisation, the FLL, provide standards which are recognised across Europe and some suppliers have adopted these to ensure quality. It is very important that all parties involved work together and fully understand what is to be achieved to avoid the risk of failure.
Let’s just look at a couple of the fundamental, but vital points to consider:
• Why is a green roof required and what performance is expected?
• Is the site suitable and will it deliver the desired outcomes?
Why is a green roof required and what performance is expected?
There could be many reasons why a green roof is wanted, it could be to satisfy a planning constraint in which case economic options will be considered; mitigate against storm water where the design will maximise water retention; support a specific species of wildlife for a biodiversity solution; provide recreational space with public access; offer additional energy savings – photovoltaic units are up to 10% more efficient when used on a green roof; or for aesthetic reasons where the building needs to be masked into its surrounding environment. It is crucial that the correct type of green roof is specified to achieve the desired outcome. For example, a sedum roof will only encourage limited biodiversity but will provide a lightweight solution if weight loadings are an issue. Nowadays, many clients are under the impression that any green roof will deliver everything – it won’t. Unless the benefits are prioritised then the outcome won’t be satisfactory.
The growth in development and requirement of the “Brown” or “Bio-Diverse” roof systems, particularly on urban brownfield development sites, over the last 5 years has lead to much debate amongst builders, developers and specifiers as to what constitutes bio-diversity within a vegetated roof and how best to meet the planning requirements for the site. As might be expected, there are no simple answers to these issues.
The initial suggestion that the rubble from a brownfield site could be set aside, crushed and utilised as a growing medium for a “brown” roof has now largely been disregarded, due to problems relating to the amount of free lime, the lack of nutrition and water retention and the amount and type of contamination within the material.
Engineered substrates are now increasingly being used, which are mixed specifically to support the planned vegetation mix considered necessary to attract the aphids and invertebrates which most ecologists now identify as providing bio-diversity on a roof.
This type of system, whilst potentially providing the optimum bio-diverse solution, comes with a number of drawbacks. The depths and types of substrates being recommended on some projects to support the vegetation species specified can lead to a loading of 800kg/m2 in areas, a figure more associated with intensive green roofs.
This means that the structure has to be far more substantial than would otherwise be required, using a greater volume of raw materials, forcing up construction costs and probably extending the construction programme, none of which is desirable.
Is the site suitable and is it possible to achieve the outcomes desired?
There are some fundamental considerations when planning a green roof and many factors which will influence its success. Get it wrong and the plants won’t survive.
Climate factors which take into account rainfall or drought patterns will govern the level of drainage and water retention layers or if irrigation is required, all of which need to match the plants’ requirements. In other words in areas of low rainfall where ‘thirsty’ plants that depend on regular water have been selected, irrigation must be included. Also up for consideration is the plants’ tolerance to wind, frequency of frost or snow.
Now take structural considerations, the slope of the roof and the design loads possible. Whilst sedum blanket will not be right for every application, the use of this type of system where the primary requirement is for greening and rainwater attenuation, is hard to ignore. With a saturated weight loading of 46kg and an annual rainwater attenuation of 40%, it meets many criteria for a green roof, couple this with the low initial installation and ongoing maintenance costs and the whole life costing for the sedum blanket system is probably the lowest commercially available for a proven system.
However, it doesn’t perform so well if the main criteria is for biodiversity. Again you can see that unless a list of priorities for what you are trying to achieve is determined early on, the outcome is once again going to be disappointing.
Looking now at plant or wildlife factors again some vital issues need to be considered and the suitability for the biodiversity species to be supported. For many birds inhabiting the, green roof means that no ground level predators are present so creating a safe haven for their existence. However, there are now other hazards presented, such as strong winds, exposed conditions and the height of the building itself means living on a rooftop will bring its own challenges.
There can be many reasons why a green roof disappoints, and only a couple have been looked at here. Mainly though, they arise because of a result of poor consideration and lack of planning, a blasé approach to getting the cheapest option, believing that a green roof will ‘tick all the boxes’ and a disregard for the expert information that is out there and available to those who take the time to investigate. Careful thought must go into the selection of a type of green roof and everyone must be involved at an early stage so that all factors are taken into account. Only then will the correct green roof be a success to flourish for years and years.
