Stretching the truth

But an conversion of the grade II-listed Boiler Shop in Chatham's old dockyards to a shopping centre is changing all that – and in doing so, is marks a turning point in the development of tensile membrane roof structures. This key project is the first such structure to be thermally insulated, and its construction should lead to a wide range of applications for the fabric building material.

The specification of a tensile membrane structure may seem like a brave move for such a sensitive site, but in fact it was determined by the historically significant building it adjoins. The Boiler Shop is one of the earliest examples of a cast and wrought-iron building in the UK, so local planners and English Heritage were adamant that any extension be able to stand comparison to the innovative engineering of its 19th-century neighbour. Also, a white, sail-like structure would serve as a reminder of the area's nautical history.

The Boiler Shop was first built as a cover for a slipway at Woolwich dockyard in 1847 and then re-erected in its present location at Chatham, Kent, in 1876. It comprises a clear central nave with lower isles running along the north and south elevations. The building was the forerunner to some of the greatest clear-span buildings of the 19th century. After finishing the Boiler Shop, the ironwork contractor Fox Henderson went on to build Sir Joseph Paxton's Crystal Palace as well as Waterloo and Paddington stations.

The last thing English Heritage wanted was a pastiche: the brief was for a building with a contrasting modern style that would serve to highlight the brilliance of Victorian engineering.

After weighing up the alternatives, John Muir, partner at Kemp Muir Wealleans, decided that a structure with a large tensile membrane roof would be the most appropriate construction. Tensile membranes roofs are ultramodern in appearance – think the Millennium Dome and the tented roof at Lord's Cricket Ground – and offer a double curvature that is markedly different to the angular lines of the Boiler Shop's iron structure.

English Heritage was satisfied with the architects' initial designs – being particularly impressed with the incorporation of a 60 m steel arch, from which the fabric is draped, that echoes the spans of the neighbouring Victorian structure. Medway council's planners also approved. They felt the design addressed the need for a landmark building that reflected the history of the docks, and the sail-like form of the membrane indicated to drivers emerging from the nearby Medway Tunnel that they were visiting a place with a seafaring past. The semicircular site opposite a roundabout also lent itself to a flexible tensile structure, according to Muir. "Rigid structures can look awkward in curved locations," he says.

Kemp Muir Wealleans' tensile membrane roof is 20 m high and supported by a steel arch, which transfers the compressive loads to the ground by way of two concrete buttresses. The membrane is fixed to a steel perimeter structure supported by seven steel columns: these carry the tensile loads to the foundations. The problem of an insulated connection to the Boiler Shop was solved by incorporating a built-up flat roof system from Sarnafil.

The specification of the structure would have been relatively straightforward if it hadn't been for the thermal performance requirement: the structure had to comply with Part L of the Building Regulations. But Muir had taken a calculated risk in designing this roof; he knew that tensile membrane maker Architen Landrell had been researching insulated fabric roofs for a number of years. And he had quoted for a job that required the retrospective insulation of a tensile roof. The quote was too expensive for the client, but Muir was confident that membrane roofs could be insulated. "I knew it could be done because of our experience, but we needed the expertise of Architen Landrell to actually design the structure," says Muir.

Muir couldn't have approached Architen Landrell at a better time, as the Chepstow-based company was keen to put its theories on insulated membranes into practice. Architen's method of incorporating insulation requires three membrane layers: an outer membrane similar to those used on normal tensile roofs, made from PVCu coated with polyester fabric; an insulated membrane consisting of a sandwich of Isowool insulation, air pockets and aluminium foil; and a lighter inner lining made from a higher grade PVCu-coated polyester fabric that gave the underside of the system a smooth appearance.

Architen's insulated membrane roof has one drawback: it is opaque. To let in natural light, Architen incorporated rooflights into the steel arch that forms the spine of the building. "Access to roofs can be a problem," says Martin Hall, architectural principal at Architen Landrell. "We have riggers who are trained to work on roofs in a way that glazing subbies aren't. As we are responsible for watertightness of the building, we want to make sure it's done properly."

The steelwork and rooflights are designed and erected by Architen. This made the detailed specification and design of the roof structure itself relatively simple for Muir and the main contractor, Galliford Try. "The demanding details are our responsibility," says Hall. "This is important because clients can get very nervous if they use something new. We offer a single point of responsibility, so there is none of the usual dilemmas about which contractor you should approach if thing go wrong."

Hall says that if an architect has not worked on tensile roofs before, they should involve a company such as Architen Landrell at as early a stage as possible. "It is common to underestimate the structural loads in a tensile structure, and we can give the architect's engineer an early load analysis," he says.

Hall is at pains to point out that the early involvement of his company does not mean the designer's style should be cramped. As well as defining the shape of the tensile structure, Kemp Muir Wealleans also specified the glazing in the rooflights. Originally Architen suggested using clear polycarbonate panels, but Muir chose obscure polycarbonate panels because he was worried that the build-up of bird droppings on the horizontal glazing panels would be visible from below. Muir believed that the shopping centre's cleaning regime would not be able to cope with the local seagull population's copious output.

Hall says the Boiler Room extension is a fantastic starting point for the development of thermally insulated membrane roofs. For now, Hall says, Kemp Muir Wealleans has created a landmark structure that meets the tough brief of the client and English Heritage. Not only that, says Hall, "it's a remarkably cheap roof: the tensile membrane, steelwork, insulation and glazing all comes in for less than £300 a square metre". And with thermal insulation, the structure now has what Hall describes as "the performance spec of a grown-up building".

Ultimately, though, Architen Landrell's goal is to provide a tensile structure that is both thermally insulated and transparent. Of course, there will always be those that say it can't be done …