Green Roofs


What Are Green Roofs?

Green roofs are vegetated layers that sit on top of the conventional waterproofed roof surfaces of a building. Whilst green roofs come in many different forms and types, usually a distinction is made between extensive, intensive and biodiverse or wildlife roofs.

These terms refer to the degree of maintenance the roofs require.

Intensive green roofs are composed of relatively deep substrates (20cm+) and can therefore support a wide range of plant types: trees and shrubs as well as perennials, grasses and annuals. As a result they are generally heavy and require specific support from the building. Intensive green roofs (what most people think of as roof gardens) have in the past been rather traditional in their design, simply reproducing landscapes found on the ground, such as lawns, flower beds and water features. However, more contemporary intensive green roofs can be visually and environmentally exciting, integrating water management systems that process waste water from the building as well as storing surplus rainwater in constructed wetlands. Because of their larger plant material and horticultural diversity, intensive green roofs can require substantial input of resources – the usual pruning, clipping, watering and weeding as well as irrigation and fertilization.

Conversely, the green roofs that have received the greatest interest recently are extensive green roofs. They are composed of lightweight layers of free-draining material that support low-growing, hardy, drought-tolerant vegetation. Generally the depth of growing medium is from a few centimetres up to a maximum of around 10-15cm. These roof types have great potential for wide application because, being lightweight, they require little or no additional structural support from the building. Furthermore, because the vegetation is adapted to the extreme roof top environment (high winds, hot sun, drought, and winter cold), extensive green roofs require little in the way of maintenance and resource inputs. Extensive green roofs can be designed into new buildings, or ‘retro-fitted’ onto existing buildings.

Biodiverse or wildlife roof are becoming more popular, as people become more aware of biodiversity issues, and options for conservation. These are designed either to replicate specific habitat needs of a single or small number of species, or to create a range of habitats which can maximise the array of species which inhabit and use the roof.

Because of their very wide range of environmental and economic benefits (in particular their insulation and cooling properties, ability to significantly reduce rainwater runoff from roofs, and their value in promoting biodiversity and habitat in built-up areas), green roofs have become important elements of sustainable and green construction in many countries. Moreover, because they can be highly visible, green roofs clearly signal the intent for sustainable building and can provide a positive and distinctive image to a building or development.

Wildlife roofs

As more land is built upon, ensuring that an area's biodiversity is retained is a key requirement for local authorities and bodies such as Natural England and the Environment Agency concerned with conservation. Whilst green roofs do not directly replace ground-based habitats and are not part of a ‘green corridor’, they can be thought of as green islands and, if well planned, can cater for a variety of flora and fauna currently not found on traditional roofs.

Wildlife roofs are becoming more common and are often referred to as: biodiverse; brown; rubble; living roofs; brownfield; eco; habitat or even by the name of the species they are aimed at i.e. black redstart roof.

Wildlife roofs are designed for one of the following two reasons:

                • to replicate the habitat for a single or limited number of species

                • to create a range of habitats to maximise the array of species which may inhabit the roof

Creating a wildlife roof is not an opportunity to recycle unscreened site-won waste on the roof of a new development. British Building Regulations outlaw the direct transfer of demolition waste to roof level without adequate screening for potentially polluting elements. Demolition waste can be very fertile, or have had seed banks mixed in to it at some time, which may lead to any resulting green roof requiring an unexpectedly high level of maintenance.

Installing a wildlife roof is often considered where the budget for installing a green roof is being squeezed. Wildlife and biodiverse roofs are sometimes assumed to be less costly to install, due to the recycled materials they may include. However, in reality this is seldom the case, a wildlife roof that will both deliver genuine habitat replacement and compliance with good green roof practice will cost approximately the same as an ‘off the shelf’ green roof.

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