... is not gold. In fact the striking cladding on the foyer of Bristol’s Colston Hall is an alloy of aluminium and copper. And while the result has the gleam of a high-tech machine, traditional techniques and craftsmanship were critical to achieving it

Buildings frequently impress or inspire, but few can make your eyes water like the newly-built foyer to Bristol’s Colston Hall concert venue. Approaching this curved, golden-clad structure on a sunny day, caution – and a pair of sunglasses – is recommended, as the building emerges from the streetscape like a brilliant sun.

Despite its appearance, though, Colston Hall is not made from gold. The cladding is a copper/aluminium alloy product called TECU Gold, supplied by German manufacturer KME. It wraps around three facades, which are either concave or convex and punctured by a large glazed public entrance at each end, plus numerous long, narrow windows, bays, and gill-like louvred vents.

As ‘organic’ cladding options, copper alloy, aluminium, bronze and other metal finishes are gaining in popularity with architects. However, fabricating and installing these bespoke metal rainscreen systems adds layers of complexity for contractors. At Colston Hall, Willmott Dixon and manufacturer and installer Richardson Roofing had to revert to hand-crafting hundreds of panels on site to clad the building’s complex curves, and even fitted its feature louvres by eye.

‘In some respects, working with hand-made panels is easier, because you’re not relying on exacting drawings, but making things on site takes a lot of time and space,’ says Willmott Dixon operations manager Richard Jones, a former Construction Manager of the Year gold medalist. ‘It often came down to the operator’s knowledge and how he felt when bending the sheet, so we have this scenario where some panels are different here and there, but that’s part and parcel of the “organic” look.’

The Colston Hall foyer is designed as an extension to the existing 1868 grade-II listed concert venue and is part of a cultural quarter near Bristol’s harbour. Built around a five storey atrium, the foyer incorporates performance spaces, public rooms and access to all levels of the adjoining auditorium.

The client, Bristol City Council, wanted a signature building and the gold-coloured cladding was seen as fundamental to achieving this. ‘We went to see a building in Graz in Austria called the Golden Nugget, which I think was the defining moment when choosing to use TECU Gold,’ explains Thomas Lloyd, project architect at Levitt Bernstein. ‘The curved facades will create an interesting effect in terms of the way they catch the light and the panels are also thinner than most standard copper panels. But mainly we specified it because it retains its colour and shine, the metal is 60% recycled and can be completely removed and recycled in future.’

Music is also expressed in the cladding. The copper alloy material is similar to that used to make brass instruments, and the way it wraps around the building recalls the skin of a drum. The narrow strip windows, twisted louvres and projecting bay windows also mimic the punched holes on an old musical roll.

Each shingle measures 600mm by 3,000mm and is set at a stagger of 300mm to its neighbours, creating a musical rhythm. In addition, each strip and bay window has a relationship to the size of the shingles, so the windows became multiples of the the shingles’ 300mm module (600, 900, 1,200mm) to create a sense of variation with an underlying theme.

Fundamental to the design was the decision to attach the cladding to a steel frame fixed to the building’s concrete frame. Unlike the originally proposed masonry inner leaf, this new solution accelerated work on the exterior and helped achieve water-tightness roughly six weeks earlier for internal works to progress.

Due to the very long windows – some are 15-20m long – Willmott Dixon devised a secondary frame of hot-rolled steel posts and horizontal beams to form the apertures for the windows and other openings. The spaces between were then in-filled with lighter cold-rolled sections supplied by Albion Sections.

Drilling and bolting the structural steel posts to the concrete frame was ruled out because this might damage the in situ post-tensioned concrete and compromise structural integrity. Instead, a series of channels were cast into the concrete to allow the steel frame to be attached and adjusted easily without any drilling.

The 150mm-wide posts and beams of the secondary framework were either connected directly to the main frame or bolted to the slab using minimal fastenings. Bands of decking were then installed between the posts to create a vertical surface, and bonded to increase air-tightness (see Fig. 1). EPDM rubber was then placed between the decking and the windows to achieve near air-tightness.

A 110mm layer of insulation was placed on top of the decking, followed by 50mm x 50mm vertical timber battens fixed to the metal structure through the insulation to create an air gap. Next, 18mm plywood was screwed to the battens, and finally the shingles were nailed to the ply over a vapour barrier.

‘Because the shingles follow a specific raking pattern around the building we had to start installing these in the south-east corner of the south elevation,’ says Jones. ‘The cladding went up in the opposite direction to the frame and substrate structure, which was started at the south-west corner.’

Preparation was key, and the lengthy second stage of the two-stage contract allowed Willmott Dixon to involve the client and consultants early on to resolve any problems. ‘The team spent a long time preparing life-sized samples of tricky window details, which were checked and changed several times,’ says John Boughton, operations director for Willmott Dixon.

But even that level of detailing could not account for all the uncertainties. ‘The shingles came from KME in set sizes and 60% went straight on the building,’ says Ted Tibbs, contracts manager at Richardson Roofing. ‘About 40% were altered on site using a guillotine and press to fit with the sides of windows, corners and ends of elevations. It should have been 25% but we had to adapt a lot and some sheets were dented where they’d been hit by scaffolding. I had to get a 3m bender forming machine on site because we hadn’t anticipated anything that long to be made.’

The shingles also interlock, which meant that finding a problem with a sheet high up in a stack meant removing a whole section. ‘It’s not as precise as some rainscreen cladding since the specification was varied,’ admits Jones.

The biggest challenge came during the installation of louvred vents covering plant rooms. These eye lid-like apertures are the first of their kind on a curved building, according to Lloyd. The curved profile of the louvres, combined with the curved nature of the facade itself, meant each set of louvre blades had to be pitched at a slightly different angle. Richardson Roofing spent the best part of two weeks preparing dozens of drawings to set it out.

‘The way the panels bend, no matter what the drawings showed they couldn’t describe how it looked on site,’ says Jones. ‘So we had to get the design team and the clerk of works to come to site to see the louvres on the building. The architect spent the whole day viewing them from different aspects around the building and advising the specialists on how to alter the profiles. It’s very tricky to get a flat piece of copper to bend to the right shape.’

Colston Hall foyer will be handed over in April, but the public already has full view of its gleaming exterior.

‘We saw the cladding as a key risk and a key feature of the building, it’s what makes it in many respects,’ adds Jones. ‘It’s great that people are now saying: “Colston Hall, what you mean the place next to the golden building?”’

Ore-inspiring metal cladding

The Cube

The Cube, Birmingham
Designed by Make Architects for Birmingham Development Company, the Cube is the final phase of Birmingham’s Mailbox development. It will feature 42,000m² of space for retail, offices, apartments and a boutique hotel, housed in a 53.1m x 53.1m x 53.1m cubic structure.

The building will be clad with unitised anodised golden aluminium and a metallic fretwork screen comprising 16,000 panels supplied and installed by specialist German contractor Haga.

The confines of the site mean that when cladding work starts early this year the panels and the fretwork will be hung from inside the building using a special manipulator.

Essentially a mini-crane, this will be driven along the floor, picking up panels, pushing them off the floor slabs, then hanging them on the outside of the building.

Apart from improving safety, this installation method also ensures that the characteristically wet English weather won’t stop any work.

Cornerstone, Didcot
Credit: Daniel Hopkinson

Cornerstone, Didcot
This recently completed £5m arts centre by Ellis Williams Architects includes a 260-seat auditorium, art gallery, cafe and multi-purpose spaces. Aluminium panels form a pattern of interlocking boxes, with projecting folds to catch oblique sunlight.

Sited directly on the town’s main square and with three public facades, self-finished aluminium was chosen for robustness and longevity, whereas coloured, powder-coated finishes would be more easily damaged. Compared with stainless steel, the anodised aluminium could also be folded and modelled with relative ease.

Contractor Leadbitter tendered the entire envelope – cladding, curtain walling, windows and doors – as a single package, and selected Sorba Projects of Essex. This decision meant that the sub-contractor resolved all tolerance issues.

Fabrication was based on the architect’s 3D CAD model. But at the corners, the geometry was so irregular and the tolerances so tight that Sorba had to measure dimensions on site and send them back to the factory. The panels are fixed to Sorba’s standard rainscreen carrier system, over a steel frame with blockwork infill.

Q-Park car park, Sheffield

Q-Park car park, Sheffield
Design and build contractor JF Finnegan of Sheffield had overall responsibility for the complex facade of the Q-Park car park, concept-designed by architect Allies & Morrison.

The £2.5m rainscreen cladding package was sub-contracted to facade specialist McMullen, which brought in Ascalit as fabricator and installer.

Each anodised aluminium panel is 1200mm by 1200mm, but is hung in four different orientations. The 3mm-thick panels are fixed to the substrate frame with key hole slots, while the gaps between the panels allow for ventilation and the passage of rainwater. Individual panels can also be replaced for maintenance.

The internal face of each panel is painted a lime green colour, using a technique from the marine industry to apply the paint over the anodised surface. When lit from inside at night, the light picks up the colour and the facade appears to glow green. A mesh barrier on the inside protects the public from falls or the danger of projectiles.