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Complex project within sight of St Paul's

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When RHWL designed an office building at 25 Cannon Street in the City of London, it could scarcely ignore that it was opposite St Paul's Cathedral. Not surprisingly, this led it to adopt a number of traditional features in the design.What this meant for Hanson Bath & Portland Stone was that it was carrying out work of a level of complexity that seemed to have disappeared quite some time ago.

'This building is right at the top in terms of complexity, ' said technical sales representative Clinton Haigh. 'It was like going back 10 or 15 years. Cannon Street has very bold features. It is a different type of building.'

The facade of the five-storey building has several impressive features including decorative modillion cornices and a massive pediment. It is built in two Portland limestones - Coombefield Whitbed for the main ashlar areas, and Fancy Beach Whitbed for the sills and band courses.

Portland stone is a traditional City material, and responds magnificently to the climate and pollution - providing it is used properly. Haigh issued a reminder: 'You have to put Portland stone in its natural bed.' This means that the ashlar stone is in the same orientation as it was in the quarry. But the weathering courses, i. e.

the stone that projects from the building, is rotated through 90 degrees.

This change in orientation is a reflection of the fact that, whereas the ashlar stone receives most attack from the elements on its exposed vertical face, on the weathering courses the stone is most susceptible on its upper, and near-horizontal, face.

This may be a more complex building than many, but it shares with its simpler cousins the fact that what appears to be a massive stone facade is in reality simply a well-crafted illusion. Just as the ashlar stones are only 75mm thick, so the weighty cornice at the top is built up around a projecting concrete slab. 'It looks very big but was made up of several sections, ' said Haigh. 'It was clad in stone to look solid. This was very detailed and involved complex engineering.'

The largest elements were the columns that go from ground to first-floor level. These were massive rectangular blocks which were then turned to a circular form. Each finished stone was 680mm in diameter and 1200mm high.

In total there are 631m 3of finished masonry on the exterior of the building, made up of approximately 12,700 stones. All of these stones were produced on Portland, individually numbered and sent to site for erection by Grants of Shoreditch.

Uniformity is one of the joys of Portland stone but, nevertheless, the architect carried out an intense sampling process before awarding the contract to Hanson Bath & Portland.

One of the reasons that the company was able to handle the job with such a degree of confidence is that at any one time it has £1 million worth of stone on stock at the quarry, so the architect is able to make a selection from this range of material on stock that it can actually see. The Cannon Street project may in some ways be a throwback to the days of more intricate workmanship, but if it turns out to set a trend it is one that Hanson Bath & Portland is poised to take advantage of.

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