CPD articles

CPD: Biodiverse green roof

3 August 2012

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The centre at Gunpowder Park in Essex incorporated a bramble finish

How does a main contractor deliver value for a biodiversity green roof when the balance between legislation, client’s expectation and budget is a difficult equilibrium to achieve? Nick Ridout, Green Roof Product Manager at Bauder Ltd, talks through the issues faced by main contractors

Green roofs are becoming commonplace within the construction industry, and increasingly contractors are being tasked with delivering a system which meets both the client’s expectations and tight budget constraints. A green roof is normally required to satisfy a variety of standards such as BREEAM and the Code for Sustainable Homes, and the specification needs to be developed by drawing upon the vegetation species list recommended within the ecologist’s report which is required to be generated under BREEAM.

On a significant number of projects it will quickly be determined at budget stage that installing a biodiverse green roof can be both a practical and cost–effective way of securing valuable accreditation points.

So, what are biodiverse roofs and why are they increasingly appearing on specifications?

Biodiversity can be simply described as the degree of variation of all living things and so all plants and animals contribute to our environment. Biodiversity is currently reducing across the entire planet and this is primarily caused by human impacts such as construction and the increased modernisation of our lives. It has been argued that the present rate of extinction is sufficient to eliminate most species on the planet Earth within 100 years[1].

The pressures of urbanisation place an ever-increasing demand on nature and the natural resources in our urban areas. Preventing biodiversity loss is a big challenge and all new construction and development has an impact, both in the short and long term. There are now site designations and numerous pieces of legislation, such as the London Plan and Living Roofs 2008, in place to help conserve and enhance biodiversity, all of which drive the inclusion of biodiverse green roofs into projects and which are complemented by other ecological features to ensure that the space available maximises the scope to increase local wildlife.

Sustainable Construction Strategy sets out that all construction projects over £1m should have biodiversity surveys carried out and the necessary actions instigated. BREEAM, CEEQUAL, LEED and the Code for Sustainable Homes help contractors to measure and report on biodiversity impacts as part of wider environmental assessments. It is these reports that provide the starting point for what needs to be delivered on the roof with regard to plant species selection and the environment that needs to be created to ensure their success.

Green roofs at Perran Sands holiday resort in Cornwall

How does a main contractor deliver value for a biodiversity green roof when the balance between legislation, client’s expectation and budget is a difficult equilibrium to achieve?

One of the big challenges in meeting current legislation and the recommendations set out in the ecological survey is that a significant proportion of the plant species identified on the ground will not survive at roof level. The climatic conditions at an elevated location will always be far harsher than at ground level, as it is more exposed and subject to greater wind speeds, which creates a setting only a limited number of plant species will flourish in.

It is a common misconception that the plant list from the ecological survey needs to be replicated exactly, whereas it is now generally considered acceptable for species from the Local Biodiversity Action Plan (LBAP) to be selected as effective substitutes. So, knowing this, the eventual plant selection can be a combination of what is suitable from the ecology report combined with other local species which will have the same effect and benefit to the local environment. This is further complicated in the case of BREEAM and the Code for Sustainable Homes, where there is usually a general requirement to deliver a further six species in addition to the number recorded in the ecology survey.

Close attention should be given to understanding what the client’s expectation is for the visual finish of the roof, especially if it is one that will be overlooked and will need to be more aesthetically refined. A fully vegetated effect can sometimes be required from the point of practical completion, or it might be that the roof vegetation can establish and mature over a 24-month period following hand-over.

The value engineering of biodiversity green roofs most commonly leads to both a reduction in the density of the planting and a change in the format in which it is supplied, which has led to the initial vegetation cover being reduced to just a scattering of a UK provenance native species seed mix. In this instance the client would be fully justified in disputing the completion of the works and withholding final payment until the roof has achieved the desired level of vegetation and, as all main contractors know, the costs of carrying out remedial works almost always significantly outweigh the initial saving.

A wildflower blanket

Biodiverse Planting Options

There are three options for planting a biodiversity roof, and all will use plant species native to the UK: vegetation blanket, plug plants and seed mixes.

Vegetation Blankets: Similar in concept to lawn turf, a vegetation blanket provides an instant carpet of plant coverage and includes a broad mix of plant species to ensure a wide diversity of foliage and flower. A vegetation blanket differs from turf in that the plants are grown on a carrier with a lightweight growing medium, which is easily harvested and rolled up for delivery.

Bauder, a well-known manufacturer and provider of green roofs, produces a wildflower blanket comprising a carefully selected mix of native wildflowers, herbs, sedums and grasses suited to a rooftop location, which meets most of the biodiversity requirements that project-specific ecology reports are now demanding, providing an instant and easily managed solution to most projects.

A key benefit of a vegetation blanket is its ability to overcome the risk associated with plug plant and seed mix installations of external factors such as unseasonably dry weather or insufficient establishment maintenance from affecting the eventual outcome.

Plug Plants: This option allows for individual immature plants or “plugs” to be planted into the growing medium by hand, which will then grow on to give good cover over the next two growing seasons. Plug-planting allows for the selection of the optimum range of species to suit the location whilst providing the broadest colour scheme and level of biodiversity to meet any project-specific requirement. The plugs can be arranged to create a pleasing design whilst also creating a thriving plant community.

A seeded roof

Seeding: Seeding is a quick and cost-effective method of initially meeting the criteria set for a biodiversity green roof where there is no necessity either for a specific visual appearance or a fully vegetated finish at the time of practical completion. The mix of seeds is broadcast across the growing medium to allow natural germination and plant establishment to develop over at least two complete growing seasons.

A full appreciation of the different planting options ensures that the client will receive the green roof finish they expect, and this is where consulting those with expertise in all the three key elements – waterproofing, green roof components and vegetation – provides the greatest benefit when looking to value engineer a biodiverse green roof system.

Combining a mix of vegetation blanket, plug plants and seed into a design scheme for larger roof areas will very often provide a satisfactory outcome at an affordable cost without detracting significantly from the desired visual appearance. For example, a roof perimeter can be planted with a vegetation blanket to provide instant coverage to a width that visually appears to cover the entire roof when viewed from an adjacent building with a similar elevation, whilst the inner area can be plug-planted or seeded.

The Barclays building at Canary Wharf has the highest biodiversity roof in Europe

Establishing the vegetation

It is essential to identify from the outset that the construction of the green roof is just the first step in the process of the establishment and ongoing development of the vegetation. What is then needed will vary significantly depending on the time of year and the prevailing weather at the time the work is carried out. Whilst hard biodiverse landscape items such as rock mounds, sand berms and log piles can be left to their own devices, the vegetation will need significant help to get established and minimal but nonetheless essential planned maintenance thereafter to keep it in good order.

The key to successful establishment is to ensure that the vegetation receives the correct type and amount of nutrition through the application of an appropriate fertiliser. The plants and growing medium should also be kept moist for a period of at least 28 days, as with traditional landscapes, and sometimes for longer through periods of warm, dry weather before tailing off irrigation to encourage the plants to become self-sufficient before the winter arrives.

It is worth bearing in mind that the maintenance costs increase as the level of maturity of the vegetation installed is reduced, so that a vegetation blanket requires less maintenance and seeding a roof requires considerably more attention. In many cases the eventual total cost may be very similar, so assuming a seed mix is the most cost effective is inaccurate.

Once the vegetation is fully established on the roof, which can take up to two full growing seasons if plug plants or seed mixes are used, the level of maintenance will then depend upon what the client’s visual requirement is, with a starting point of a simple twice-yearly visit to remove surplus and dead vegetation, inspect the drainage and apply a fertiliser.

The way forward

To conclude, the best way to achieve a satisfactory and cost-effective biodiverse green roof installation is to:

As in so many cases within the construction industry, the best way to secure value for money is to ensure that you have spent enough at the outset to avoid the need for return visits to site to put right any shortcomings.

Nick Ridout is Green Roof Product Development Manager for Bauder Ltd, www.bauder.co.uk, and works with main contractors and developers to deliver green roofs.

Reference: 1 Okasha, S. (2010). "Does diversity always grow?". Nature 466 (7304): 318.


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