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Re-roofing Leicester's engineering faculty

20 Oct 17 One of the UK’s most celebrated 20th century buildings has been restored to better-than-new condition. David Taylor reports

The entire roof was enclosed under special sheeting
The entire roof was enclosed under special sheeting

The word ‘iconic’ is generally over-used  these days, but for fans of 20th century British architecture, Leicester University’s Engineering Building deserves the description.

Built in 1959, it was one of the first projects designed by up-and-coming young architects Stirling & Gowan (the RIBA’s coveted annual Stirling Prize is named after practice partner Sir James Stirling) and is now regarded as a modern masterpiece.

The most remarkable feature of the Grade II*-listed building is its complex geometric roof. The university’s requirement to provide plenty of natural light into the building’s interior resulted in an audacious design comprising 2,500 glazed panels assembled in a series of 45o diamond-shaped units that presaged modern patent-glazing systems.

However, 50 years after its construction, the roof started to show its age, not only letting water leak into the building but also allowing heat to leak out. So in 2011, after three years of consultation, the University announced a major project to preserve the building, replacing the famous roof and the glass façade of the laboratory block to extend the building’s life for a further 50 years.

Architect Berman Guedes Stretton was brought in to oversee design of the replacement roof with help from Arup’s façade engineering department. Contractor Lendlease was awarded the £19.5m main contract and the contract to manufacture the new roof units was awarded to Austrian roofing and cladding specialist Fill Metallbau. 

Preserving the original roof glazing was not an option; designed before the advent of modern factory-built glazing systems, it was put together in-situ by hand, using cheap materials and rudimentary techniques. 

According to the university the roof, although “an awe-inspiring display of geometric dexterity”, was not actually built to last. 

So the only option was to completely replace the roof, a solution that initially seemed so heretical that it could only be discussed in “hushed tones” according to the university’s project manager Pete Bale.

A refit would require rebuilding every individual section, each one unique in form and construction, from scratch. 

Torturous negotiation was required with the local planning authority, English Heritage, the 20th Century Society, Historic England and numerous other interested parties, until finally a plan could be agreed. 

The university drew up a special project charter, signed by Lendlease, the trade contractors and all other stakeholders to commit them to working in partnership to maintain the historic status of the building.

After that, there were still enormous technical challenges to address, says Bale: 

“The plan was to honour the intentions of the original architect, but that also meant working around the large deviations and misalignments of an older structure and today’s much stricter performance and safety standards,” he explains.

“The idea of removing the old roof and installing a new one with the same shape and using similar materials, might sound simple but in reality it is immensely difficult,” he adds. Each of the 2,500 glass panels was unique and the steel frame supporting the roof structure had moved and warped over the years so that every new panel had to be shaped to fit.

Furthermore, to meet current standards, the glass thickness had to be increased from 9mm to almost 30mm, doubling the weight of the roof and requiring the glazing bars to be strengthened.

To satisfy the building’s listed status, the design team had to come up with some creative solutions including the production of exact heritage replicas of the original air handling units in the façade for ventilation. 

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Another complication was the considerable inconvenience of having to keep the building operational at all times. This was made possible by the innovative use of a tensile netting under the roof line. A fabric envelope encapsulating the building was mounted on a bespoke scaffold designed by Lyndon Scaffolding to allow work to continue in all weathers.

Besides ensuring that the building remained weatherproof, the scaffolding structure had to be carefully designed to withstand high wind up-lift forces. Heavy ballast was used to pin down the scaffold towers supporting the sheeted structure.

“The reproduction of the original detailing has brought new meaning and life to the old structure. The way the light changes and reflects on the facets throughout the day is truly beautiful, and is testament to the original design.” -Peter Bale
“The reproduction of the original detailing has brought new meaning and life to the old structure. The way the light changes and reflects on the facets throughout the day is truly beautiful, and is testament to the original design.” -Peter Bale

The original roof was dismantled, leaving only the original triangular steel trusses, and the new glass and aluminium skin was installed over the top. To cope with the complex geometry and accommodate the myriad distortions in the design resulting from decades of settlement in the structure, Fill Metallbau developed a bespoke computerised model which the client says exceeded the capabilities of current BIM software

The new roof is designed to resemble the original as closely as possible but it is a very different type of structure. The original glazing was a composite consisting of a fibre-glass mat sandwiched between two thin layers of glass to create a translucent unit just 9mm thick. The new glazing, manufactured by German firm Okalux, comprises 30mm double-glazed units tinted grey to reproduce the appearance of the original panes. 

Historic England insisted that the original geometry of the roof was preserved as well as its overall appearance. Stronger glazing bars were required to support weight of the replacement glazed units, but they could not be significantly wider than the originals. In fact they are just 38mm wide, much narrower than the standard 50mm sections required for a conventional façade and only 6mm wider than the original bars.

Work began on the project in 2015 and was completed in August 2017. A special ceremony was held earlier this month to officially open the refurbished building.

Project manager Peter Bale says the new roof and façade “has exceeded the expectations of all those that can now see it, including many within the project team."

He says: “The reproduction of the original detailing has brought new meaning and life to the old structure. The way the light changes and reflects on the facets throughout the day is truly beautiful, and is testament to the original design. I am immensely proud to have been involved in the restoration of this iconic building for the University and future generations."

The £19.5 million project to replace Leicester University’s Engineering Building roof was funded partly by a loan from the European Investment Bank and partly from the University’s own capital. 

Arup Group provided professional consultancy, facade, structural, M&E and principal designer services for the project and the main contractor was Lendlease. 

Steve O’Connor, director of development at the University of Leicester, said: “The replacement of the roof and ongoing preservation works will be essential to the future of this iconic building. The university has launched a fundraising appeal to help complete the current project and provide essential philanthropic funds for the future preservation of the building."

Annie Provan, Leicester City Council building conservation team leader, said: “From a building conservation and planning perspective, this project was a huge challenge and we are immensely proud of what has been achieved. To replace the entire roof of such a complex and iconic building was a major undertaking and we all shared a tremendous sense of responsibility to this major piece of twentieth century architecture.”

This article was first published in the October 2017 issue of The Construction Index magazine, which you can read for free at http://epublishing.theconstructionindex.co.uk/magazine/october2017/

UK readers can have their own copy of the magazine, in real paper, posted through their letterbox each month by taking out an annual subscription for just £50 a year. See www.theconstructionindex.co.uk/magazine for details.

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