From the Top Down
Showhouse Magazine, July 2009Being green takes on a whole new meaning when it comes to incorporating green or ‘living’ roofs and walls into buildings. By planting these elements with vegetation, house builders are doing more than making a visual statement; they are lessening the environmental impact of the building because the plants can help reduce rainwater run off, absorb pollutants from the atmosphere, provide habitats for wildlife and diminish the ‘urban heat island effect’.
Green roofs are common in some parts of Europe and Scandinavia and are enjoying a rise in popularity in the UK. In principle the green roof concept is straightforward: sedum, a succulent commonly known as stonecrop, or indigenous wildflowers, herbs and meadow grasses are planted into a thin layer of soil or substrate. The complication is that to achieve a satisfactory green roof requires knowledge of both roofing and horticulture with the weight of the plants and substrate, their maintenance and root growth all issues that must be considered.
“The green roof industry is growing very quickly,” says Liz Williams, sales manager for Blackdown Horticultural Consultants, a company specialising in the design, supply, installation and maintenance of lightweight roof planting systems. She warns: “Our advice would be to look very carefully at any system; unless you have the right system the plants may not thrive on the roof in the long term. We work closely with roofing manufactures and we tell them what we require in order to make the thing work.”
Examples of green roofs on residential projects include Thornsett Group’s Charterhouse thesq development in Clerkenwell, London which features 1,500 sq m of living roof. Here Bauder, an award winner within the green roof industry, worked with the London Borough of Islington to ensure the roof fitted the objectives of improving biodiversity and reducing the impact of the buildings on local drainage through storm water attenuation. The Islington Ecology Centre specified the plant mix of wildflower and sedum seeds to support a variety of flora and invertebrates and attract birds seeking a food source.
Crosby Lend Lease has introduced green roofs in Birmingham at two developments: The Orion Building and Sirius. The roofs act as added insulation and help attenuate water runoff while the sedum covering also flowers throughout the year and attracts an array of wildlife.
According to Crosby, The roofs have been well received by residents, many of whom have moved from leafy suburbs into the city environment. Andrew Mann, associate with architects BBLB, who worked with Crosby to integrate the green roofs into both schemes, believes “Green roofs are fast becoming the fifth elevation in terms of design aspects considered by planning authorities when considering planning appliances.”
PRP, the multidisciplinary architectural practice, is using 7,051 m2 of green roof supplied by Lindum at its Haggerston West and Kingsland scheme which involves the regeneration of a 4.3 hectare estate in Hackney for London and Quadrant Housing Trust. The Green Roof Centre has confirmed that the project constitutes the largest area of green roofs for a single master plan in Greater London and the largest project associated with residential mixed development in the UK.
Green roofs vary hugely in their make up. At Blackdown, Williams explains that its system is 105mm deep because it has a 50 or 70mm growing medium. “Some people will put a green roof without any growing medium and the plants will look fine when it is first applied but, overtime, it is not going to look so good. A green roof is low maintenance but it is not maintenance free, if you want a roof to look good over the long term you are going to have to put some effort into maintaining it and we usually say that once a year it needs to be fed.”
Paul Jones, director UK and Eire of Pittsburgh Corning, the manufacturer of Foamglas cellular glass insulation, which utilises 68 per cent recycled glass combined with silica sand and a small element of carbon, emphasises that longevity is partly dependent on the performance of the roof build up below the planting medium.
“A roof deck has to be able to withstand the increased loading to the structure. The built-up layers, the plants and the retained water create a static load and it is critical that the saturated weight of the proposed green roof system is obtained. If the roof deck is not strong enough to support the weight of the system it could result in damage to the roof construction.”
While cellular glass insulation may overcome weight issues, Lytag, the manufacturer of a lightweight aggregate made using a by-product of coal fired power stations, suggests that the choice if aggregate within the planting layer can help increase the sustainability credentials of a project.
When it comes to green walls, or “living facades”, one of the largest currently in the UK is at the 02 Arena. Green walls are also being showcased at the BRE Innovation Park, in Watford, North London, where PRP Landscape is using them within its overall design for the park.
From an aesthetic perspective, Richard Hodgetts, associate director with PRP Landscape, sees them as a means of providing façade greening, creating contemporary, living, vertical architecture in urban environments.
“Green walls assist noise reduction, reduce global warming by absorbing six grams of CO2 sub micron particles per sq m annually, absorb fine dust, create improved air quality and prevent graffiti. The dense foliage encourages biodiversity, this may be further enhanced by the use of bird, bat and insect boxes, which can be completely hidden from public view,” says Hodgetts.
Among the systems employed at the BRE is one by Mobilane which, in the UK, is supplied and installed by Hedera Screens. It uses ivy, rooted in and grown to the height required before installation.
Sean Farrell, of Mobilane, explains that ivy is the only evergreen plant that the company believes can tolerate these types of installations. “Many of the systems need a lot of water and the maintenance on some green walls can be horrendous because dead plants have to be replaced.”
The ivy is supplied in Mobilane’s aluminium planters which are designed to protect the roots from both heat and cold. The plants grow up a galvanised steel frame screen which stands 20cm clear of the front façade of the building.
To control the watering and feeding regime, the system needs a small computer controlled plant room, says Farrell. “We zone areas of the building. Each zone gets a different watering and feeding regime because the top will need more than the bottom, the south will need more than the north. It’s important to minimise the use of water.”
At the BRE Innovation Park, PRP has also specified a green walling system from Cityroofs. This employs a façade structure and a growing board made from recycled plastic. According to the company’s business manager, Matthew Hoddinott, the façade structure will incorporate mainly specialist low growing grasses, herbs, clovers and some brassicas such as watercress.
“The wall is not only attractive but assists with solar gain, by protecting the building fabric from warming up as the plants take the sunlight rather than it transferring heat to the building. The effectively means that, during the summertime, less air-conditioning is used and hence less energy expenditure.
“The system is irrigated through a hydroponic systems which delivers nutrients and water through the drainage growing board. The water is then collected at the base of the wall and returned to a closed loop irrigation system which maintains the levels of nutrients. Maintenance is effectively only undertaken on the irrigation system as the plant choice is designed to be as low growing as possible,” says Hoddinott.
Walls and roofs are not the only elements that can be green. Zack Barrett, marketing services manager at Brett Landscaping, points to the importance of specifying the right hard landscaping.
“The increased use of permeable paving systems has been partly driven by recent legislation which effectively cancels the right to pave over the front garden of a domestic property wih any reasonable sized area of impermeable surfacing material without making allowances for controlling water run-off. Permeable paving is an allowable option which does not require planning permission.
Brett is offering a number of permeable paving solutions utilising a gap formed between the individual blocks to allow water to be channelled into the underlying porous strata for temporary storage or dispersal to the ground or collection.
A more visually green option is Marshell’s Grassguard which is designed for areas where occasional parking space is needed. The permeable grass grid reinforces naturally draining soil and allows coarse rye grass to grow through the blocks while providing an area of hard standing.