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Architectural clean sweep

technical & practice The Commonwealth Institute's 40-year-old leaking roof has been refurbished to something like its former glory

In contrast to T E Collcott's Imperial Institute in South Kensington, founded as a result of the Colonial Exhibition of 1886, the Commonwealth Institute, designed by Robert Matthews, Johnson-Marshall & Partners, was meant to represent a new spirit of the commonwealth of nations. By the time the building was completed, there were 16 member states. Today there are 55 member countries.

Planned since 1958, commenced in 1960 and completed in 1962, the building is deemed by English Heritage to be the most important public building in London after the Festival Hall, which was also designed, in part, by Matthews. The architect, later rebranded as RMJM, was founded in 1956 and today ranks 17th largest in the world.

Material benefits According to Cherry and Pesvner's Buildings of England, the Commonwealth Institute was 'full of post-war optimism' and its purpose was to reflect the new mood in Britain after the collapse of empire, the independence of many former colonies and the experiences of war.

That sense of new community was emphasised through the materials and form of the building. Constructed with timber and copper - 'gifted' from Nigeria and Zambia respectively - the complexity of the roof form was intended to symbolise the simplicity of tent-like structures.

Cherry and Pesvner say: 'The form reflects the search at the time for alternatives to the rectangularity of the modern tradition, influenced especially by Candela's experiments with parabolic roofs in Mexico and by Hugh Stubbin's Kongresshalle in Berlin, but externally it lacks their confidence, and is not helped by the indifferent opaque glazing used for the walls.'

Indeed, the principal feature of the building is the dramatic sweep of the roof; a vast hyperbolic-parabaloid spanning 55m (the largest concrete shell span in the UK), which projects the elevations high into the air. A geometric feat achieved without the aid ofa computer and constructed without the aid of a net.

Grounds for divorce Situated in the south-east corner of Holland Park, the grounds of the Commonwealth Institute were designed by Dame Sylvia Crowe (1960-62), developing a strong unity between the architecture and the designed landscape.

The park, which came into public ownership after the Second World War, was handed over from Foreign Office control on 1 January 2000. At this point, the government formally announced that the Commonwealth Institute had become an independent agency and the site was transferred over to its ownership.With the 'severance' payment, the Commonwealth Institute had the funds to start the much-needed reparations. It set up an open architectural competition to upgrade a building that had been leaking for 30 years.

Avery Associates began by undertaking an investigation of the building; from original records, maintenance reports and intrusive surveys.

Because of the events scheduled by the institute, all works had to be programmed with the aim of minimising the visual impact of scaffolding, noise and ensuring safe public access.

The building comprises a prestressed concrete roof supported in the corners. Blockwork walls around the perimeter support a fully glazed cladding system of blue glass with an opaque white enamel backing. The gap between the blockwork and glazing acts as a vast ventilation duct, taking air from the central air handling plant and distributing it to the interior through high-level vents.

The consequent thermal shock on the rigid glazing, together with vandalism, has meant that many of the original glass panels have been replaced over the years. However, these repairs have been poorly carried out and have included inappropriate painted glass panels or, in the worst instances, unpainted plywood panels, resulting in a patchwork of colours and reflections. The solution was to replace the entire cladding layer with new, retaining the original fixing brackets and framing.

New aluminium secondary framing, cappings and heat-soaked toughened glass panels were installed to recreate a level reflective surface.

Given that this is a Grade II*-listed building, negotiations on all matters pertaining to the appearance were critical; and apparently amicably resolved. Although the proposed glazing colour did not match the original building (given that the building was listed when it had a patchwork finish), English Heritage was more than happy to agree to a better specification.

Treating the leaks The work to the roof was more complicated. The leaks were the result of two factors. First, because the original roof had been created as a threedimensional form, the upstand seams in the short-strip copper, laid parallel, were bound, at some point, to lie such as to trap water. Even if the seams followed the falls at the top, for example, by the time they reached the bottom the planes had twisted so that the seams ran perpendicular to the fall.

Second, by forming the roof covering in 1,500 x 525mm copper short strips, movement became significant.

The number of original joints increased the opportunity for water ingress.

At the high points, the wind turbulence had forced open the clips holding the copper sheets and panels had become dislodged at the joints. The remedy that had been used by the building owners was as ingenious as it was comical; a Heath Robinson repair that involved drilling holes through the roof on either side of the dislodged copper sheets, lying a scaffold pole across the face of the sheet and feeding tie wires through the holes into the building. These were tensioned and tied off on weights inside the building, to keep the scaffold poles tight up against the roof slope. An ingenious solution, but one guaranteed to leak.

Once again, the argument that to replace the existing materials to match the original design would only replicate the inherently poor detailing convinced English Heritage to accept a modification to the specification;

substituting long strip copper and allowing the joints to follow the roof line. The new copper has been laid in three distinct areas, separated by a batten roll; each area fixed either at the head, middle or bottom to accommodate differential expansion.

The original roof structure has been retained throughout and comprises a central area of pre-stressed concrete which tapers to a mere 50mm in the centre. Even during the inspections, it was deemed advisable that only a few people should congregate in one place at any one time, as the concrete would flex underfoot. A 2m-wide gantry scaffold was constructed around the perimeter of the building during the works to ensure that no materials were stored on the roof to overload the delicate structure.

The perimeter areas of the roof comprise pre-stressed concrete beams, fanning out to the outer edge with steel channels supporting woodwool slabs and topped with 32mm vermiculite as an insulation layer. On both sections of roof, timber battens were inlaid within the roof build up to take the copper roof fixings. The concrete beams can be clearly seen on the inside of the building, creating wave-formation ceiling supports for the untreated woodwool slab soffit.

On inspection, the woodwool was found not to be saturated. Water seeping through the copper covering over the years had been running down the trowelled finish of the vermiculite/concrete, penetrating only through weak joints and shrinkage cracks. The architect resolved to retain as much of the structure as possible, but because of the severe loading constraints, this limited the options for replacement build up.

Snow worries Fortunately, the snow loads criteria that had been used in the 1960s were greater than those required today, giving slightly more leeway for adding more weight. Similarly, the copper, because of the reduced number of joints, was lighter than the original roof. In consultation with KME UK and Foamglas (the copper and insulation suppliers respectively), the roof build-up increased the insulation, weather tightness and run-off capabilities, while reducing the condensation risk and load; but with slightly greater thickness. The new roof has been thoroughly tested for leaks and because there is nowhere for an overflow to discharge safely, an electronic warning device sounds if the gullies block and cause rainwater to back up.

The roof has now been completed and is finally weathertight. It will take about 10 years to develop the green patina of the original. Currently, work is beginning on Phase II; the refurbishment of the west wing, including lift installation, new accommodation, offices and general upgrade.

Even though we tend not to live in an era of post-war optimism, at the end of the building's 40th year, the Commonwealth Institute should at least be restored to something of its original architectural glory.

Broad brush strokes We were selected to work with the Institute through a competition process more than two years ago.

At that stage we knew the scope of the work would be very broad, ranging from masterplanning studies to the restoration of the Grade II*-listed building. Obviously, the range of skills needed are quite different, but there is at least one common denominator to all the work - understand the problem, then look for appropriate or achievable solutions.

Undoubtedly, the most difficult part of the building restoration was the new roof. The challenge was to detail a new copper roof that would:

lproperly deal with the large-scale expansion and contraction (the existing roof did not);

lprovide a decent level of insulation (the existing roof had none);

lnot overstress the existing structure (there was very little spare load capacity);

lprovide a degree of weather protection during installation without the use of temporary structures (a temporary structure was ruled out because the roof is so big).

Finding the right people and right products to achieve a lasting solution and demonstrating the same to the Kensington and Chelsea conservation officer and English Heritage was indeed a challenge, but one that the design team relished and enjoyed.

COSTS Preliminaries £231,766 £57/m 2Copper roof £887,703 £217/m 2Glazed facades £405,614 £99/m 2Scaffolding £256,870 £63/m 2Electrical £63,067 £15/m 2Latchway systems £53,363 £13/m 2Total £1,898,383 £463/m 2 PROJECT TEAM CLIENT Commonwealth Institute, High Street Kensington www. comw. org ARCHITECT Avery Associates, tel 020 7233 6262: John Dawson, Joanna Podmore, Julien Odile, Andrew Brown QUANTITY SURVEYOR/ PLANNING SUPERVISOR Munro White Hilton, tel 020 7383 8181:

Richard Hilton, Roger Duncombe, Roy Barber STRUCTURAL ENGINEER Buro Happold, tel 020 7927 9700: Mike Cook, George Keliris, Tom Hay, Caroline Collier SERVICES ENGINEER John Brady Associates, tel 020 7939 7800:

Peter Hammond, Matt Phillips, Roy Russell ADVISORY BODY Copper Development Association, tel 01727 731 205 (Kian Power) CONTRACTORS Management contractor Wallis; roof works NDM; glazed facades Glazing Vision;

scaffolding Coventry Scaffolding; concrete repairs and cleaning PAYE Stonework; drainage Dyno-Rod (J&L Services); asbestos removal Crips; lightning protection RC Cutting; trace heating Trace Heating Installation Services;

latchway system HCL Safety; electrical installation Nightglade; general carpentry GS Construction; hoists GB Access; rainwater goods PW Goddard Plumbing; decorations D&L Decorating Contractors; roof access equipment Online Safety; access works Scanmoor; glazing repairs Codu Glass SUPPLIERS Long strip copper KME UK; roofing insulation Foamglas; glass to cladding Hansen Glass

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