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What you will see, looked at geometrically, is a series of swooping curves, somewhat resembling a saddle.

Look at it historically, and you'll see what English Heritage describes as "the most important public building of architectural ambition raised in London between the time of the Royal Festival Hall and the Hayward Gallery". If you look at it from the point of view of a roofer, however, you'll see a big problem. This spectacular roof, which earned the institute a grade II-listing, has leaked since the day it was built. In 1962 there simply wasn't the skill or knowledge to waterproof such a complex design.

Now, 40 damp years later, architect Avery Associates has had the task of fixing the roof once and for all. The practice was awarded the commission after winning the competition to masterplan the restoration and expansion of the building. But Avery's job wasn't just to find a way to plug the leaks. It had to make the roof comply with current ºÃÉ«ÏÈÉúTV Regulations, which meant installing insulation on a roof that was not designed to take any extra loading. Just to add a little spice to the challenge, Avery was further constrained by the building's listed status, which meant that any design change could be vetoed by EH.

Among metal roofing contractors, the Commonwealth Institute roof has reached mythical status. "I knew of the job because, over the years, I'd spoken to men who had been asked to repair the roof," remembers Ted Tibbs, contracts manager at metal roofing company NDM, which has won the contract to replace it. "These men came back with horror stories about leaks that were impossible to fix."

For 40 years, the institute had learned to live with its weird and wonderful roof. Where possible, rainwater has been directed through temporary guttering to the basement, where it often forms a two-inch deep pool.

To design out the flaws in the new roof, Avery needed to understand why the original leaked. The reasons were numerous. It was partly down to workmanship: in some cases, copper seams weren't bent properly and flashing did not overlap. The design of the seam patterns also caused problems: the cross welts – where the head and tail of two sheets of copper join – were often positioned on flat areas of the roof, which meant that standing water eventually penetrated the joins.

Cross welts were also threatened by water where inadequate sumps and gutters caused water to pond during heavy rainfall. The maintenance team at the institute had to install electric pumps on the roof to keep gutters and sumps free of water. Wind suction had also lifted and loosened copper strips – for years, the stop-gap solution was to hang weights from them.

To come up with satisfactory design and product solutions, Avery had to work in close collaboration with the material suppliers and industry associations. Avery was able to check the robustness of some of its specifications during pre-tendering interviews with roofing contractors.

The use of copper for the new roof was never in question, and Avery's brief was to install new copper strips and make it work second time round. "The copper is fine. It was the way it was used that was the problem," says Sandy Harrison, business development manager for architectural products at KME, the company chosen to supply the 4500 m2 of metal required. Avery made extensive use of KME's technical advisory service, and this advice was checked against information from the Copper Development Association. This gave Avery a stronger case when it approached EH about potential changes. "By discussing technical issues with EH, it meant that it was warned of any dangers that were looming," says John Dawson, a partner at Avery.

One of the biggest changes to the roof's design was the change in the seam pattern. KME suggested that to reduce the number of vulnerable cross welts, 10 m long copper strips be used instead of the original 1.8 m lengths. Avery then mapped the constantly changing gradients of the roof and positioned the copper strips to ensure that there were no cross welts on flat sections. One consequence of using longer strips was that each would expand and contract by a greater amount, relative to the other strips. To control this expansion, the strips were secured onto the insulation with fixing clips at the top of gradients and movable clips at the bottom.

EH also agreed that KME could supply the more affordable mill-finished brown copper rather than pre-patinated green copper that would have resembled the copper it was replacing. Harrison says that after 10 years, the mill copper will oxidise, turn green and grow a protective white salt layer.

NDM won the roofing contract because it was the only contractor that was confident that it could lay the copper on a double curvature roof. This was largely because of Tibbs' experience tapering metal strips onto mosque minarets. The same principle, which Tibbs calls "orange peeling", applies on the institute roof. When laying copper strips in areas of double curvature, NDM widened every fifth or sixth strip halfway along its length by bending one of the seams by hand, rather than using a metal profiling machine. NDM also suggested that at the peaks of the roof, the copper trays be turned through 90°, to allow the rainwater to flow down along the strips rather than over the standing seams.

To bring the roof up to current thermal standards, insulation had to be added. Avery used Foamglas by Pittsburgh Corning as its insulation material because of its low weight and the fact it could be temporarily weatherproofed while the copper was being replaced. Originally, a different architect had considered using a built-up timber roof for the insulation, but the engineer, Buro Happold, said the load would have been too great. The original dead load was 36 kg/m2 and Buro Happold calculated that the maximum weight the roof could withstand was 48 kg/m2 – an increase of 12 kg/m2. Buro Happold was able to justify this additional load because E E of a change in the code that determines how snow loads are calculated.

The copper was attached to the Foamglas using steel plates. The copper is clipped directly to these plates, which are pushed straight into the Foamglas. Even with this lightweight system, Buro Happold calculated that the additional weight of the build-up was still 2 kg/m2 too much. To reduce the weight, the continuous steel plates were replaced by 600 mm long sections spaced 600 mm apart. The low load tolerance also restricted the number of people that could work on the roof simultaneously. Only one person could work in a 4 m2 area, and NDM had to check with Buro Happold before it placed any equipment on the roof.

The Foamglas solution also had to be adapted to cope with the gradients of the institute roof. Health and safety regulations prevented the use of hot bitumen on the steep sections, so the Foamglas had to be cold-bonded. This method hadn't been used on such a curvy roof, and extensive testing by the project team had to be carried out to ensure that it would work. In areas of the roof affected by wind suction, NDM used mechanical fittings as well as cold-bonding to hold the Foamglas in place.

To protect the roof after the copper had been pulled off and before the new layers had been installed, a 2 mm thick SBS torch-on membrane was used to provide instant weatherproofing to the insulation. Without this, an expensive temporary scaffolding shelter would have to have been erected over the roof.

The job took 24 weeks, the project finished on time and – at £900,000 – within budget. The new roof has been tested in one violent storm and no leaks were reported. One consequence of the watertight roof is that wooden flooring within the Commonwealth Institute is beginning to creak. Like a ship raised from the deep, the timber is not used to the dry conditions.