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Since 2000 Keith William Architects has been working with artistic director Tony Graham on the Unicorn Children's Theatre's first permanent home.

Sutherland Lyall talks to project architect Sofia Kapsalis about selecting materials to convey the 'rough yet beautiful' design principle.

The main ambition of London's Unicorn Theatre is 'to connect children with high-quality theatre, ' says Keith Williams, whose practice has designed the building. Although it has extensive outreach programmes, the Unicorn is not a theatre school; its actors are adults whose core audiences range from four and five to around 12 years of age. The new theatre, due for completion in November, is being built on Tooley Street, halfway between London Bridge railway station and the southern approaches to Tower Bridge.

Keith Williams Architects was appointed at the end of 2000 from a shortlist of six, following an Official Journal of the European Communities pitch. The practice had shown the client an outline of its ideas, rather than a finite design solution, and the scheme evolved in collaboration with the Unicorn's artistic director, Tony Graham. Project architect, Sofia Kapsalis says: 'They knew what kind of spaces they wanted, but, apart from that, they were interested in taking forward the idea we had presented.' At the beginning of the briefing, Graham coined the evocative phrase 'rough yet beautiful', as a kind of guiding design principle. Kapsalis says: 'It squared with the way they work. Their stage performances are very true. They don't spend a lot of time hiding the mechanisms. They count on the children's imaginations.

And they wanted that to carry through in their building.' The design team was comfortable with that because it aimed to create a building that has a functional, structural and material logic. 'We worked with a set of interlocking volumes.

We felt the materials should reflect what went on inside. The building is in a tiny space, so we wanted to first create the main auditorium and, second, to create a transparent (street-auditorium) interface. We didn't have enough space to put them side-by-side.

So we moved the box into the air and put the street interface underneath it, ' says Kapsalis.

THEATRE PLAN The rest of the plan flowed from that decision. At ground level there is a square 120-seat black-box performance space, with associated dressing rooms and a scenery dock behind the full-width foyer. The main internal stairs lead up the Tooley Street side - from the foyer to the 350-seat auditorium, a rectangular sound-insulated box with fixed semicircular seating in one half and a three-row circle above. There is a semicircular technical gallery above that.

The other half of the floor is the stage space, with a scenery grid above. Planning restrictions precluded a conventional fly-tower, and the consultant, Theatre Projects, came up with the idea of a series of bridges with grey-painted panels. When folded down, they allow scenery to be flown up and down between them. The stage area can be used in a variety of configurations and a temporary proscenium can also be flown in. Behind the large stage space are more dressing rooms and a scenery dock accessed by a lift that also services the black box below and backstage rehearsal rooms above.

The problem for structural and services engineer Arup was how to keep the auditorium block in the air, or, more specifically, how to keep the ground floor clear of structure. The solution was to extend the inner half of the auditorium walls down to the supporting piles and cantilever the front 8m of the auditorium over the foyer. This could only be done in concrete, a material that Kapsalis says was a 'fundamental' part of the building's form.

THE CONTRACT This was a JCT98 stage-negotiated tender contract. 'The quantity surveyor Bucknall Austin put together the sum for the building works, and the six contractors put in their prices for prelims, overheads and profit independent of the contract price, ' explains Kapsalis. Then there was a selection process: 'With this kind of contract the winner comes on board early to give buildability and cost advice. You don't have to use the same contractor for the main contract, though you usually do. We didn't - with the mutual agreement of the first contractor. Mansell (who had tendered at the first stage) was the next lowest, so the final contract went to them.' Kapsalis says she has been 'very pleased' with the input of the main contractor: 'They give a lot of added value in terms of technical input, by suggesting more efficient ways of doing things - alternatives to over-complicated detailing. Even on aesthetic issues, where there is a bit of pipe that goes in a wrong direction, they ask us to take a look. They have a feeling for details and have become more proactive about buildability. When something doesn't work, they make a proposal, and have been very good that way too.'

EXTERNAL MATERIALS Surrounding buildings and the brick of existing old buildings affected the choice of external materials. But the architect also wanted to have a distinctive material to mark the theatre. Kapsalis says: 'We thought for a while about how to clad the auditorium box and looked at lots of materials. In the end we opted for a pre-oxidated copper, KME's Tecu Copper - not the usual green, but a deep burgundy colour. It's a live material and it will change over time and develop in a slightly unpredictable way - it could even be green in 15 years' time. The thing is that it evolves.' Kapsalis says that a cassette rainscreen had originally been specified for the roof, but budget constraints prompted a change to a standing-seam roofing system. 'So we played with the width of the copper strips, ' she says. 'There are four or five different standard widths, so we had them laid out randomly to create a composition that probably says a lot about what we have done in the building - which is to use conventional systems in an unconventional way.

'There is beautiful craftsmanship here (by Cambridgebased installer TR Freeman). Copper in rolls - bending, cutting and shaping it - it's such a craft. We really enjoyed it.'

The adjacent box high over the stair foyer is clad in insulated render from Alumasc and applied by SERS, as are areas of the back elevations. Here, the lower facade levels are finished in Staffordshire Blue engineering bricks from Baggeridge Brick, although the east ground-floor facade around the stage door is clad in bright-blue mosaic tiling from Domus Tiles. The original intention was to wrap the entire auditorium in copper, but like its other roofs, Sarnafil single-layer polymer membrane with copper copings (laid by Fenland Flat Roofing) has been used instead.

COMPLICATED GLAZING The most complicated element in the design was the ground-floor glazing. The dimensions of the glazed ground-floor elevations and the 2 x 3.8m glazing panels would normally have resulted in the use of heavy steel transoms and mullions. That would have negated the idea of the flying auditorium. 'We went to some trouble to get the transparency right, ' says Kapsalis. 'We talked to Arup Facades and came up with the idea of steel-flat transoms suspended on rods from the cantilevered auditorium and from the adjacent high-level stairway box.' Subcontractor Hynds Architectural Systems changed the Arup solution to structural tubes and used toggles to hold the glass in place. 'In a way, it's a traditional glazing system, fixed back to steel transoms and mullions, ' says Kapsalis. 'Usually, it would be done by gluing to small angles fixed to the main frame, but Hynds uses a toggle device that involves clips bedded into the double glazing that fix on to the toggle. As a result, the fixing disappears in the jointing.' Hynds has a design responsibility under the contract.

'The design service they provide is very good, ' adds Kapsalis, 'especially as we couldn't afford Arup Facades' solution. Hynds maintained our design intention on budget.' When the glass panels were fixed, they had to be slid down a narrow gap between the scaffolding and the facade. 'We were biting our nails, ' says Kapsalis.

Glazing elsewhere is by Cantifix, including the projecting upper foyer at the base of the auditorium box. Cantifix only works with aluminium systems and would have had to outsource the design and supply of the steel ground-floor foyer glazing.

FEATHERBEDDING Auditoria always involve acoustic issues. Here, Arup Acoustics specified a rubber connector-based separation between the concrete structure and the plasterboard lining. In addition, all the rigid structural connections to the auditorium box are separated by acoustic bearings and pads, so no sound is transmitted to, or from, the surrounding structures at any level. The black-box auditorium below is totally isolated, sitting on acoustic pads and bearings.

Tubular Erectors did the steel in the main auditorium.

'Their finished product is very good and accurate - thousands of small pieces of steelwork clad in carpentry work. It is extremely complicated and they got it right first time, ' says Kapsalis. 'The carpenter, GF Barrett, is also extremely proactive. They propose a solution if something doesn't work, rather than simply shoving it in.'

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