Is this the world’s most complex roofing project?

When the glass roof of one of Britain’s most iconic postwar buildings had to be replaced, the solution was to wrap it in a high-performance skin designed to meet the needs of regulations and heritage bodies. Stephen Cousins reports.

Among the many experiments and devices that fill workshops at the University of Leicester Engineering Building, one in particular draws the eye. Suspended in mid-air are a series of open-topped plastic containers connected to thin copper pipes that snake off towards the perimeter.

These strange contraptions were developed, by the Department of Engineering, as an ingenious solution to a longstanding problem with the 1960s-constructed building – a badly leaking roof. Water drips down from broken and damaged panes in the vast glazed roof above into the containers, then trickles along the pipes into permanent internal drains, keeping students and their work nice and dry.

This is no ordinary leaking roof, it is the iconic sculptural saw-tooth glass roof that covers the Grade II*-listed building, designed by renowned British architects James Stirling and James Gowan.

Plagued by problems since completion in 1963, the structure is now the subject of an ambitious £19.5m refurbishment project that will see all 2,500 glass panels on the roof and the glass facade of the laboratory block replaced with the intention of extending the building’s use for another 50 years.

The project must be a contender for the most technically challenging roofing job ever attempted and has pushed the construction team, led by construction manager Lendlease, lead consultant and facade engineer Arup and Austrian roofing/facade specialist Fill Metallbau to the limit of their abilities.

Tensioned netting enabled operatives to remove the old glazing

It involves stripping back the original roof to expose its triangular trussed steel frame, then wrapping it in a weathertight glass and aluminium skin – a visual simulacrum of Stirling and Gowan’s original, but compliant to contemporary thermal performance, longevity, safety and access standards.

Every double-glazed panel in the prefabricated system had to be unique, to enable the skin to mould around subtle curves and twists in the steel frame, which had moved and warped over time.

The tight tolerances required during installation led to the development of bespoke parametric modelling software that “exceeds what is currently possible in BIM”. To complicate things further, all the refurbishment work, including disassembly of the original structure, is being completed while the faculty is fully operational, with students working directly below.

Peter Bale MCIOB, project manager at the University of Leicester’s Estates and Facilities Management Division, told CM: “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.

“Essentially we are following the same path as Stirling and Gowan, but also taking into account movement in an older structure, much stricter performance and safety standards, all under the scrutiny of the Local Authority Conservation Office, Historic England and the Twentieth Century Society.”

Landmark building

Architects travel across the world to see the Engineering Building, which stands on the edge of the University campus flanking 8.9ha of open green space in Victoria Park. The first example of postmodern architecture in the UK, it has appeared on postage stamps and artworks and effectively launched Sir James Stirling’s stellar career.

The factory-like construction was a declaration of war against the predominant trend for dour functionalism. Critics were wowed by the dramatic 12-storey tower with two auditoriums cantilevered from its side – the tallest in the north at the time – and the bulging rooftop with its rows of diamond-shaped skylights, set at a 45-degree angle to allow north light into workshops and research laboratories.

An anodised aluminium subframe was manufactured offsite with mullions, transoms and panels installed individually

But the geometrical ingenuity of the design was far ahead of its technical performance and from day one the building was plagued by problems stemming from the basic palette of low-cost materials and technologies employed. The roof was fabricated using a flimsy and lightweight stick system of aluminium sections, transoms and mullions – all hand crafted, and cut and installed by hand.

The glazing comprised just two thin sheets of float glass, separated by a fibreglass matt. Much like a greenhouse, it caused intolerable extremes of hot and cold in summer and winter. The thin glass and lack of safe access made it too risky to send anyone up to carry out repairs, hence the ad hoc system of containers and pipes.

Stuart Savage, senior construction manager at Lendlease, comments: “It is questionable how the original system lasted as long as it did – the aluminium was screwed into rotting blocks of wood cast into the concrete frame. The building leaked in 1963 and has leaked ever since.”

The refurbishment is being part-funded by the University of Leicester, with a loan from the European Investment Bank, and delivered under a construction management contract. The form of contract is the first ever implemented by the client, which Peter Bale admits “could have been an absolute nightmare” were it not for the “passion and dedication” of everyone involved.

The project involved the construction of 84 geometrically complex “diamond ends” that required glass panes to interact at complex angles

A special “project charter” was drawn up by the University and signed by Lendlease, the trade contractors and other stakeholders, to commit them to work in partnership to maintain the historic status of the building.

The project team explored options for refurbishment, but a drive to remain faithful to the original informed the decision to retain the truss structure and cover it with a contemporary, precision-engineered system. “A preferable heritage option might have been to simply replace the original glazing, like for like, but there was no way of safely installing or maintaining it and no contractor would have built it due to the risk of litigation,” says Peter Bale.

The new system is similar to the stick system used in 1963, and comprises an off-site manufactured subframe of anodised aluminium, plus individually installed mullions, transoms and insulated double-glazed panels. The intricate structure will include 84 new “diamond ends” – geometrically complex frames at the end of each truss with glass intersecting at various angles.

High-tech heritage

Designing a system to meet the needs of a Grade II* listing while achieving 21st century levels of performance required a balancing act of historic proportions.

Thomas Pearson, senior designer and conservationist at Arup, told CM: “The changes we are making had to be legible but seem entirely natural. The new glazing has to look ‘right’, but establishing what that means has taken a long time. There have been many important factors to consider, such as the appearance of the translucent glass, but the finesse of the aluminium framing has always been our top priority.”

The new A-rated double-glazed units, manufactured by Okalux, replicate the grey tint and interlayer of fibreglass matting used in the originals, but increased in thickness, from around 9mm to just under 30mm – doubling the weight of glass on the building.

Top: The new glazing has to meet heritage and performance standards. Above: Work is carried out under a white tent of fabric-wrapped scaffolding

The glass and its support structure are robust enough to resist a person falling onto it from above without collapsing or shattering into shards, and to counter the negative pressure of wind trying to suck the glass out, something the previous
roof, with its leaks and cracks, had avoided. As a result, the aluminium glazing bars are 38mm wide, 6mm wider than the originals, but well below the equivalent standard facade solution of around 50mm wide, says Peter Bale.

Efforts to refine the design bordered on the obsessive. Discussions over what was visible behind the line of the pressure plates on the glazing took almost two months to resolve, says Mark Brennan, senior design manager at Lendlease:
“We managed to get the width of the gasket down to 10mm and could not go any lower because it is holding the whole thing together and we had to be able to deliver a warranty to the University.”

Apart from heritage requirements, the building imposed physical limits on how much the new roof could expand in size. For example, the base of the diamond ends had to align with permanent cast-in concrete gutters along the top of the brick walls to allow rainwater to run off.

Every component of the structure had to be assessed and passed by the client and the heritage stakeholders before installation. The star rating on the Grade II* listing required that certain fixtures and fittings were renewed or refurbished to maintain their original appearance. Bespoke heritage replicas of several air-handling units in the facade were produced at a cost of £27,000, as part of the new mechanical ventilation strategy.

Operatives worked on the site while students continued to use the library

The new roof is designed to trap warm air in winter, but heritage experts rejected a plan to integrate automated windows to naturally ventilate the workshops in summer because the openings would be visible from the tower. Instead, the air-handling units and new chillers installed under the roof will cool the spaces.

Two full-scale mock-ups of the diamond ends were produced, an initial visual mock-up and a test rig, built in Austria by Fill Metallbau to industry standards, to demonstrate to the heritage stakeholders that the lines of geometry and the structural system would function effectively.

A spokesperson for Historic England told CM: “The challenge has been to preserve the architectural significance of the original design while sustaining the building in its original use by improving technical performance and longevity. The achievement we hope will be a faithful recreation of the different geometric forms and aluminium profile with a new bespoke patent glazing system.”

Ready for lift-off

The refurbishment works are being carried out under a giant white tent of fabric-wrapped scaffolding, designed
by Lyndon Scaffolding, to weatherproof the building and allow the workshops and laboratories to remain occupied. A layer of tensioned walk-on netting under the roof line, with protective matting below, has enabled operatives to remove the old glazing and install new glass without dust or screws dropping on students or staff beneath.

Every component of the roof, including glass and screws, had to be removed without damage then stored, ready for reinstallation should the new structure fail.

“The challenge of having a kite the size of a football field hovering over the building is not only holding it up, but holding it down,” says Peter Bale. “The scaffolding is designed to resist large uplift forces created if wind gets underneath.”

The roof of the scaffold over the workshop is supported on towers that plunge through the trusses below onto the workshop floor. Large amounts of kentledge ballast weights were inserted around the base of the scaffold to hold it in place. The complex scaffolding structure took a lengthy eight months to assemble.

Installing a precision-engineered roof with many complex intersections on top of a 1960s-built structure with multiple large deviations and misalignments has been a huge challenge for Fill Metallbau.

Historic England insisted that the original lines of geometry flow into one another across the entire skin. Taking into account any expansion in the aluminium and settlement in the trusses during deconstruction and installation, installers are working to tolerances of +/-2mm.

Numerous point cloud surveys were carried out to map the trusses and develop a detailed three-dimensional model. The complexity of the challenge prompted Fill Metallbau to develop a new form of 3D parametric modelling software to monitor the installation procedure and identify any issues in real time.

Individual panels are temporarily fixed in position, then sample co-ordinates are recorded by on-site operatives and relayed to Austria. The figures are then run through the software to assess the impact on the overall system and ensure that cumulative errors are not adding up and sending those all-important lines of geometry out of kilter.

As work progresses, more than 50% of the new roof is complete and the first diamond ends are currently being installed. The level of complexity has resulted in a few teething problems, and the expected completion date has been extended to March 2017 from the end of this year.

Given the hefty £19.5m price tag, some have questioned whether re-roofing a technologically challenging building is worth the effort, when the same budget could buy a brand new building with the same floor area, plus state-of-the art equipment.

But that argument misses the point, says Peter Bale: “The University is committed to the long-term preservation and conservation of this building, and rightly so. At end of the day, it is not just our building it is everybody’s.”

And what about Stirling and Gowan, what would they make of the herculean efforts to recreate their design?

“I imagine them smiling wryly about the whole thing,” says Arup’s Thomas Pearson. “Some of the conservation discussions might have appealed, particularly to Gowan’s surrealist side: this is a building which resists conventional heritage thinking. But ultimately I see our project as quite a light-touch change to the architecture, and I don’t think they would have minded that at all.”