30 Nov 2011

Roof gardens and green roofing

Posted by bcsands

Increasing urban density has led to some innovative ways of inviting plants and vegetation into our environment, one of which is the concept of the ‘green roof’.

A ‘green roof’ (or ‘living roof’) is one that adds or incorporates plants into a roof structure. There is actually nothing new about the idea – many ancient peoples, including the Vikings and Celts, used turf or grass roofs – but its revival in this century helps to compensate for the way in which our need for buildings and streetscapes eats up the natural environment. In fact, the benefits of green roofs have led to some European cities mandating their use on new buildings with roof areas that are over a certain size.

As well as beautifying the environment, a green roof enhances the quality of life both inside and around the building. The roof acts as insulation, reducing the energy use in heating and cooling the building, and also cuts down on urban noise. Outside, the building, the vegetation filters the air and absorbs pollution particles, provides additional storm water absorption, reduces the air temperature and creates a habitat for insects and birds – and any other wildlife that can access it.

All types of green roof require a minimum of a waterproofing membrane and a growing medium, as well as the plants themselves. And all green roofs have an inevitable structural consideration – they add weight to a building.

An ‘extensive’ green roof is the simplest and most lightweight concept and one that lends itself well to pitched roofs. The angle means that excess water easily runs off, while flat roofs need drainage to be installed to prevent waterlogging.

‘Extensive’ green roofs use a relatively shallow growing media, which means that plants with smaller root structures are best suited to this purpose. Hardy grasses and ground cover plants are ideal, in particular plants that can survive long periods without rain. With little or no public access usually intended for these roofs, a simple planting that requires little or no weeding and perhaps an annual application of fertiliser is the best choice.

More ambitious or ‘intensive’ green roofs are considerably heavier and might include trees, shrubbery, elements of hard landscaping and even irrigation systems. The practical demands of these roofs are quite different, including strong structural support, plus constant care and maintenance. They are usually established on flat surfaces with planned public access often part of the design, so they may also include pathways and benches to encourage people to spend time in these urban green oases.

All the benefits of a green roof are just as applicable to a suburban home as to a cityscape. Green roofs, usually the simpler ‘extensive’ type, can be retrofitted to an existing structure. However, if you are considering a green roofing project, please always consult an expert. Without proper planning and installation, a green roof failure may mean more than a few dead plants – it might mean leaks and building damage.

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