Cladding: The walls of St Mary’s

The walls of St Mary’s

At last the long-running saga of the cladding at St Mary’s Hospital, Isle of Wight, has come to an end – at a cost of £10m. The building was opened in 1992 as a trailblazing example of what the next generation of NHS hospitals should be. Unfortunately the building’s cladding didn’t quite live up to the billing. Within 18 months of the hospital opening, a catalogue of serious cladding defects had become obvious.

The salt-laden sea air did not agree with the shiny stainless steel that clad the hospital. Water trapped at junctions was causing the thin sheets of stainless steel to corrode. Electrolytic reactions between the aluminium support rails and the stainless steel were also causing corrosion of the stainless steel fixings.

To make matters worse, the rooflights and the curtain walls leaked, the double-glazed units in the windows were failing and the red paint on the aluminium cladding sheets, intended to provide a colour contrast to the stainless steel, was too thin.

Something had to be done. “The main danger was the stainless steel sheets being pulled off by the wind in the next five to 10 years,” says Richard Mathieson, partner at architect Frederick Gibberd Partnership. In 1996, the firm won the job to sort out the mess, as it had prior experience of stainless steel cladding – it was the architect for the cladding on the original Canary Wharf tower. It had also gained expertise with cladding failures.

The architect extensively evaluated a range of options to solve the problem. The solution had to be cost-effective over a 60-year period – the required life expectancy of a public sector building – and had to be no heavier than the original cladding because of structural implications. Its installation had to be straightforward to avoid disruption to the hospital, which had to remain open while the work was being carried out, and of course it had to be corrosion resistant.

The solution was a product called Rheinzink, a zinc titanium alloy sheet on the roof. This was laid over a deck of softwood planks that were in turn nailed to timber rafters fixed to the building’s steel frame. Mathieson says the detailing was crucial. “The key thing is to ensure they don’t come into contact with anything that causes them to corrode,” he explains. “We looked carefully at a range of timber species and the preservative treatments used on these. If we had chosen the wrong timber this would have caused it to corrode from below.” For the walls, rainscreen cladding sheets were fixed to aluminium cladding rails (see detail, below).

The aluminium cladding was replaced with a blue version, the rooflights were replaced and a series of overheating, cantilevered conservatories had smaller windows fitted to reduce solar gain.

Contractor Banner Holdings had the job of replacing the wall and roof cladding – an area equivalent to five football pitches. This took three-and-a-half years because the work had to be carefully phased to prevent disruption to the working hospital. The job was made easier because Rheinzink is a soft, lead-like material that can be fabricated by skilled craftsmen on site to suit the complex junctions that were part of the hospital’s original design. A pre-patinated version of Rheinzink was specified because of the length of the programme. If the bright version of the material had been used, some areas of the hospital would have looked shiny while areas that had been reclad earlier would have looked dull.

The project is now finished and the cladding will only need light maintenance over its 60-year life.