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A roof does not simply have to provide shelter: with a little creative thought and a willing client, the design of a roofing component can be highly innovative, interacting with the site's local environment and bioclimatic conditions.

Suddenly your roof is a rainwater collector, daylight and natural ventilation provider, a platform for solar panels and wind turbines and even an ecological habitat.

RAINWATER RETENTION / DISPERSAL Harvesting rainwater for irrigation and WC flushing use is attractive as it can help stabilise the local water table and reduce operating costs. In this case, the roof design has to be co-ordinated with the design of underground cisterns and a detailed overflow strategy.

Metal - in the form of galvanised steel and aluminium- or single-ply membrane roof finishes - has been favoured for rainwater collection. While aluminium is both recycled and recyclable, the energy demands are three times those of galvanised steel and some suggest almost the equivalent amount of energy is required for recycling as for forming the original material.

Membrane finishes consist of petrochemical substances so their use must be checked carefully, ensuring that PVC is not a constituent. Some interesting alternative materials to specify include timber (shingles of homegrown species like oak, Douglas fir or larch), natural rubber membranes or tiles like the Ardesia Max range from EBC, unbaked clay tiles and recycled slate.

The effect of Britain's fl ood and drought-prone climate on buildings can be mediated by the roof. Dependence on mains storm sewers or local ponding can be reduced by adopting a sustainable urban drainage system (SUDS), with the roof being a key component of the water attenuation strategy.

Green or brown roof systems assist in retaining water temporarily during big storms, mediating demands on SUDS or local storm sewers.

ROOFSCAPE AS ECOLOGICAL HABITAT While promoting the use of brownfield sites for construction can be good for land use, it also results in diminished wildlife habitats. Green or brown roof construction offers the opportunity to replace, or even improve, the ecological diversity of a site. Having constructed many grass and sedum roofscapes over the years, we at Architype are now constructing a brown roof, along with a green roof, for the new Sustainable Construction Centre at Somerset College of Art and Technology. This brown roof uses recycled building waste - rubble, local stone, timber - with the sharps content eliminated to protect the Firestone EDPM waterproofing membrane.

The roof forms part of a carefully planned SUDS system, which attenuates the site's rainwater through a series of landscape elements.

Structural loading of the materials used in the build-up of a brown roof needs to be reviewed early in the design process, due to the weight of materials such as crushed concrete. Asymmetric loading on the roof also needs to be considered, due to the variable depths created when laying the build up in natural drifts.

A brown roof can be designed as a specific habitat with the inclusion of certain plants and live feed, attracting particular types of birds. The thicker construction build-up found in a brown, grass or combined roof creates conditions for more diverse planting and provides a superior habitat to a sedum blanket finish, which is thin and remains more of a monoculture.

An advantage shared by both a brown and green roof is the reduction of 'heat island' effect in urban situations.

GENERATING ENERGY Solar water heating can generate up to 60 per cent of domestic use, and the payback is in a matter of years, so even in the UK this is cost-efficient.

In one housing scheme, we met 40 per cent of the electrical demand with photovoltaic tiles with a slate-like appearance from Solar Century.

WIND TURBINES We are also using the roof to mount a new type of helical wind turbine, the Quiet Revolution, developed by XCO2, on a renovated building for media training use on Southwark Bridge Road in London. As wind speed is higher in an unobstructed field, an urban location like this has to be assessed carefully.

Positioning a turbine on an existing building provides additional structural challenges, and the potential vibration effects have to be considered.

However, mounting turbines on existing buildings can contribute 20 per cent of the annual electrical demand. The study on www. sustainingtowers.

org gives further details.

DAYLIGHTING PROVISION One means of reducing electrical demand is through carefully placed rooflights.

A rooflight provides better use of natural daylight than a window, as the sky is brightest overhead. Carefully positioned skylights angled toward the north should ensure that the building does not overheat through too much solar gain.

Atria, light wells, skylights and sunpipes can be used for natural ventilation and air extraction, and by raising the height of any of these roof apertures a stack effect can be created - especially useful in a deep-plan building. An ideal construction for solar harnessing is a continuous angled band of skylights of north orientation with photovoltaic panels erected on the south side.

Roofscapes present a great creative opportunity to both implement and visually express buildings' environmental agenda: the city of the future may have a completely different skyline.

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