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Silver top with a green twist

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technical & practice

It is becoming a cliche to talk of a roof as a fifth elevation, but in the case of the new Sainsbury's at Greenwich, designed by Chetwood Associates, the cliche is justified. Indeed one could argue that the roof is the first, the most important of the elevations.

This is in terms both of its function and of its visibility. Designed with the aim of reducing the energy consumption of the standard supermarket by half, and of being generally 'environmentally friendly', the store is set low between landscaped bunds, making the roof extremely visible. One of the key factors of the design is to bring natural daylight into the building - a concept which is still radical for supermarkets. With the large floor depth this means bringing light in through the roof - but stopping too much coming in at inconvenient times. The roof also plays a key part in the ventilation strategy, so has been designed to be as aerodynamic as possible.

The conceptual design long predates the Greenwich store. Chetwood was originally appointed by Sainsbury's to do a research project. The brief, explained the practice's regional director Paul Hinkin, was 'to create a fresh look at the way they designed stores... from first principles.' Serendipitously, the practice issued its document to Sainsbury's in the same week that the retailer received its first enquiry about developing the site with a supermarket that reflected the environmental aspirations of the adjacent Millennium Village.

Sainsbury's was shortlisted against asda and was appointed in 1998, according to Hinkin, because 'the approach was the most holistic'. Chetwood then worked up the design.

The natural lighting in the roof is a development of the kind of sawtooth roof commonly used on factories, but with the 90degrees angle smoothed off to make it more aerodynamic. The sharp angles of the traditional 60-30 roof generate turbulence and the resulting negative pressure, says Hinkin, 'can suck heat out from inside the building'.

Choosing the material for the roof was of fundamental importance. Chetwood considered both the mill-finished aluminium standing seam roof that it eventually selected, and a membrane. Most membranes were still pvc-based which Hinkin rejected for 'environmental and political reasons' (pace the controversy over the Millennium Dome roof).

Hinkin was also put off having a membrane roof after visiting several, including at Foster's Duxford, and deciding that at best they 'looked mediocre'. In contrast, the aluminium roof makes a distinctive design statement. Hinkin was keen to use a material that didn't require any surface treatment, since this made it possible to minimise the emission of volatile organic compounds in construction. He particularly likes the mill-finish aluminium because it helps to create a building that would 'patinate rather than age'. And, he said, 'with a standing-seam roof you are not relying on a tape or a seal. It is formed out of material rather than relying on sticky-backed plastic.'

Insulation of the roof is mineral-wool based, using Rockwool, a continuation of the determination to avoid oil-based products wherever possible, and it is 10 per cent thicker than is required by the building regulations.

Having decided to go with the mill-finish aluminium roof, Chetwood looked at five potential suppliers before deciding to go with Euroclad. 'The criteria we judged on included the ability and personnel to provide detailed design advice. They became an integral part of the design team.' The roof was actually manufactured on site, in standard module widths of 400mm.

Euroclad delivered a rolling plant to site in a container, and brought in the raw material in rolls. From this it rolled out strips up to 80m long, using the motive force of the rolling plant to get the strip up onto the roof.

Most of the roof is in effect a shallow barrel vault, with inclined surfaces above the saw teeth. The way the manufacturer dealt with the edge detail where the inclined surfaces meet the barrel vault in an S-shape, - which Hinkin described as a 'rolldown' - was key. Originally it was thought that it would be necessary to create special tapering aluminium sheets, turning this into a hand-crafted operation. But the architect and the manufacturer built a full-scale 10m by 15m model of the roof at Euroclad's works, and were able to show that, because it is possible to vary the joint on the standing seam every 300mm, there was actually enough flexibility to make the curve possible without the need for the tapered sheets. This simpler method resulted in considerable savings in both time and money. A similar technique was used around the four large vents on the roof.

The north sides of the saw teeth are glazed, creating in total about half a kilometre of glazed elevation, albeit only 1800mm high. The glazing is standard double glazing with external automated louvres to control both the direction and total quantity of light that comes in. Although natural light is desirable, too much could be deleterious to the produce. The sensing equipment is similar to that used in art galleries except, said Hinkin, 'In their case they are protecting paintings, in our case fruit and veg.' There are also automated dampers on these north-light facades, as part of the control system for passive ventilation of the store.

Colt was originally appointed to provide the automated solar-control elements of the system, but then offered to co-ordinate the whole of the construction of these facades. 'As we started working through the design process we became more concerned about the programming and co-ordinating of the roof,' Hinkin explained. By taking on this entire role, Colt was able to control this process.

Construction of the supermarket started in January and it opened on 14 September, showing that innovative buildings do not have to be slow to construct. The design and construction of the roof had a significant part to play in this achievement.

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