The design for Procession House had to answer both the developers' need for speed and the planners' conservation worries. It did so through the unusual approach of cladding the building twice.
Procession House in the City of London is an unusual building. For passers-by at Ludgate Circus, the office is clad in a variety of traditional building materials, including Portland stone, terracotta and hand-pressed bricks. For the tenants who moved in last week, however, the interior is modern and glass-walled.

The reason for these contradictory visions is the £22m building's cladding system. A full-height, glazed, curtain-walling system, supported from the building's steel frame, forms the rainscreen cladding to the offices. Outside this rainscreen the building has been wrapped in a skin of traditional materials. The cladding stands 300 mm proud of the curtain wall, its form mimicking the building structure. Horizontal masonry panels trace the line of the office floorplates over its six floors, while vertical panels repeat the layout of the structural columns. Secondary horizontal and vertical elements form openings for windows, to let in natural light.

When architect RHWL took on the job of designing Procession House, the palette of cladding materials had already been selected. Originally a bombsite belonging to the Corporation of London, the plot was sold to Heron Property Corporation complete with planning permission based on an office scheme designed by Edward Cullinan Architects. Its scheme detailed the office cladding, which was designed to break up its perceived size. Having bought the site, Heron decided to run with the scheme, but not with Edward Cullinan Architects. RHWL was appointed instead.

The cladding combination is a solution to the conflicting requirements of the developer and of the Corporation of London's planners. Heron wanted the office built quickly using modern construction methods and materials; the planners were concerned that, given the building's proximity to St Paul's Cathedral and its surrounding conservation area, a modern office would be inappropriate. They preferred the traditionally clad office Edward Cullinan Architects had originally proposed, which would blend into the townscape.

Grant Wood, RHWL's project architect, came up with the concept of a curtain-walled building with masonry overcladding. "It was perceived as the most sympathetic way of achieving approval," he says.

Separating the overcladding had other advantages for the developer. It allowed the inner face of the cladding to be completed earlier in the project, which in turn enabled fit-out to start earlier, speeding up the construction programme. It also removed the difficult-to-manufacture traditional masonry facade from the critical path.

Unusual outlook

For the building's occupants, this compromise has resulted in one of the strangest outlooks in London. Along with glimpses of St Paul's Cathedral and Ludgate Hill, they also have a clear view of the back of the building's masonry overcladding system.

Attention to detail was critical if the cladding contractor, Irvine Whitlock, was to succeed in producing panels that would look acceptable from both inside the building and outside. For the design, Irvine Whitlock teamed up with building materials procurement specialist James & Taylor and structural engineer Smart & Crosby International. James & Taylor was responsible for procuring all the masonry materials and overseeing the manufacture of the panels; Smart & Crosby worked out the loading on the panels, then designed the interface with the building's structure.

Fabricating an overcladding system from a mix of traditional, hand-crafted materials in a modern context and attaching it to a hi-tec, precision-manufactured, glazed facade was never going to be easy. Indeed, it was the main reason that the overcladding element was tendered so far in advance of the main construction package.

The cladding was designed as a bolt-on system. The masonry is attached to a stainless steel backplate using bolts that are resin-anchored into the stone. The panels are then bolted on to the main structural steelwork through holes pre-drilled by the steelwork fabricator. The team spent a considerable amount of time working out how the overcladding system would interface with the rainscreen and structural steelwork, and also how the mounting system would look from inside the offices.

Installing the cladding panels also needed careful planning. Because each panel had to be manufactured for a specific facade location, the delivery sequence was critical if the cladding was to be installed on time. The panels were delivered to site by lorry as and when they were needed, because of limited storage space on the site. This required careful co-ordination, as the panels were assembled in three different workshops, one for each material type.

Panel by panel

Irvine Whitlock and James & Taylor divided the building into floors by gridlines and on a floor-by-floor basis. The Portland stone cladding was installed first because it had a shorter manufacturing time than the terracotta. Panels were delivered to the site and craned straight into position, to be bolted to the facade.

Installing the panels was a test of nerves for Irvine Whitlock's six operatives, with sections of the cladding – the terracotta in particular – costing more than £2000 each and taking weeks to manufacture. The panels had to be lowered into position, by crane, through a small gap between the external scaffolding and the building's facade.

The last of the terracotta panels has now been fitted, much to everyone's relief. The building's first tenants moved in last week. They, more than anyone, will be able to appreciate the traditional materials and craftsmanship used to assemble the facade as they work in offices surrounded by the traditional cladding.

How the cladding was assembled

Cladding subcontractor Irvine Whitlock teamed up with building materials procurement specialist James & Taylor and structural engineer Smart & Crosby International for the design of the cladding. James & Taylor procured the masonry materials and oversaw the manufacture of the cladding, while Smart & Crosby worked out the loading on the cladding and then designed an interface with the building’s structure. It was clear from the restricted site and tight build programme that the cladding would have to be delivered to site as pre-assembled, ready-to-install panels. One of the first things the team did, in conjunction with the architect, was to produce a mock-up of a typical cladding panel off-site. As a result, glass-blast finished stainless steel was selected, both for its appearance and durability, as the backing plate on to which all the masonry materials would be mounted. Traditional surround Edward Cullinan Architects’ original scheme called for the facades to be clad in traditional materials. Portland stone, handmade brick and terracotta were specified for different areas. “All the materials are British,” says Bob James, a director of James & Taylor. “The stainless steel is from Sheffield, the terracotta is made in Leicestershire from British clays and the handmade bricks were produced close by, so they have an excellent colour harmony with the terracotta. Portland Bill, Dorset, provided the Portland stone.” Attention to detail and quality in assembling the cladding was a unique aspect of the project because the building’s occupants would be close enough to scrutinise it. “It’s almost unheard of,” says James. “Usually, nearly all attention is focused on what is seen from the outside. That attention to the outside is, of course, true here. But it is also true for the backs of the elements, indeed more so, as people are closer to them inside the building than on the street outside.” The Portland stone panels were the easiest to assemble because the stone can be cut to within a millimetre of the required dimension. Care had to be taken with the bricks, supplied in three shades, to ensure they were blended properly before being attached to the backing plate. The terracotta caused the biggest problem. Because of the traditional manufacturing process – each piece of terracotta is pressed by hand into a plaster mould, after which it is taken out and left to dry before being fired in a kiln – the variation in colour and size from one piece to the next can be huge. Before selecting terracotta for the spandril, mullion and transom sections of the cladding, each of the hundreds of pieces of terracotta had to be measured by hand and sorted into similar-sized bands. They also had to be sorted into colour groups to give an even distribution throughout the cladding grid. For James & Taylor, this meant that every piece was allocated a specific position on the cladding grid to give a random look to the facade and enhance its natural appearance. Fast-track delivery Irvine Whitlock used three specialist firms to assemble the manufactured material into panels, one for each type of masonry material. The masonry is attached to these panels using bolts resin-anchored into the stone. The panels were delivered to site as they were needed, craned straight into position, then bolted to the steelwork.

Suppliers

terracotta Hathernware handmade bricks Charnwood stone quarried in Portland stainless steelAcnon CCL

Project team

client Heron Property Corporation architect RHWL Architects main contractor Kier Build masonry subcontractor Irvine Whitlock specialist materials procurement James & Taylor structural engineer (cladding)Smart & Crosby