Heatherwick Studio has created a flamboyant kissing roof to unite two diverging buildings in this conversion of a Victorian coal depot into an ambitious shopping destination
Public displays of affection were frowned upon by the Victorians, but perhaps they’d have set their inhibitions aside had they seen the dramatic ‘kissing roof’ that crowns the Coal Drops Yard redevelopment at London’s King’s Cross.
Two former coal depots are connected by the twisting steel and slate structure some 25m above ground, which adds a new upper storey to the 10,000m² retail scheme being constructed in the depots for developer Argent.
Architect Heatherwick Studio worked closely with structural engineer Arup on the bridge-like structure, which in the spirit of a 19th century feat of engineering, appears to hover between the existing buildings and merges seamlessly with their pitched timber and slate roofs.
Tamsin Green is project leader at Heatherwick Studio. ‘When thinking about the rationale and engineering of this grand gesture, including the small hand-crafted details people will touch, we would often ask ourselves “what would the Victorians have done here?”,’ she says. ‘We drew on their industrial building typology and the rich existing material palette of cast iron, timber, brick and slate.’
The coal drops were built to receive and sort coal as it arrived from the north of England by train, but as coal use declined they were converted for light industry and storage. In the 1970s a major fire burnt out part of the eastern building and by the ’90s the buildings had fallen into disrepair, being used sporadically as workshops, studios and nightclubs.
Both are very long; the 150m eastern block was the longest continuous brick building in Europe when it was built. A 26m gap at the northern end of the site splays to 39m at the south by Regent’s Canal.
This unusual arrangement did not lend itself to a destination retail space with a central focus, says Green: ‘A light refurb would not have created the draw we needed to compete with Covent Garden or Regent Street. Rather than disrupt the long elevations and repeating brick arch motif, we focused on the roofscape.’
The kissing roof is designed to peel up and away from the existing pitches to meet at the centre point between the buildings, and creates a 2,000m² suspended floor for the scheme’s anchor tenant. A double height space in the courtyard below can be used for concerts and events.
Step inside this new hovering retail floor and the vibe is more light and ethereal than grimy industrial, with a sinuous white ceiling bathed in natural light and column-free spans.
Where the roofs come together, on either side of the central ‘V’, a new serrated structural glass facade of 64 separate panels zigzags along. ‘The serrations allowed the glass to be structurally self-supporting without the need for additional framing elements, while conveying a human-scale regular bay window,’ explains Green. Undulating strips of oak boarding run along the ceiling edges of the glass, in an oblique reference to timber sarking boards (traditionally attached to the rafters of a pitched roof to boost its strength) on the coal drops roofs.
The roof’s chunky bridge structure comprises a pair of inclined ribbon trusses that follow the line of the existing buildings and a primary A frame that spans at right angles to them. These three main structural lines meet at a crucial ‘kissing point’ node at the roof’s apex.
Suspended on vertical hangers from the bottom cord of the ribbon trusses is the floorplate for the anchor store. Primary loads from the roof are spread across new steel frames and some 52 new columns with piled foundations threading through the existing structure.
‘It was vital that loads from the new roof pass through the new columns and do not find their way to the existing walls, which could damage the brickwork,’ says Edward Clark, associate director at Arup. ‘The new steel trusses want to move as the roof deflects and absorbs the load of the new floor and the columns want to spread sideways, so we had to carefully detail the junctions between the old and new structure to prevent distress to the heritage brickwork.’
Steel fabricator Severfield manufactured the ribbon trusses to very tight tolerances to ensure a seamless join where they transition from retained sections of timber trussed roof at either end of the building.
‘The fact that we have chosen to do something so ambitious and daring, both architecturally and structurally, is very much in keeping with the spirit of the Victorian heritage round King’s Cross and St Pancras, most notably William Henry Barlow’s single-span iron shed at St Pancras, which was for many years the longest span roof in the world,’ says Clark.
The build-up above the steel trusses comprises a secondary layer of metal purlins covered by insulation then a strata of timber battens and counter battens that support an exterior layer of Welsh slates selected to match those used on the existing buildings.
Some 80,000 slates were installed across the roof in total, of which around 25,000 over the curved sections had to be hand cut. None of the original slates was reused due to their deteriorated condition.
As construction continues, the roof structure is already freestanding, but close monitoring is required to detect any unexpected deflections as finishes, cladding and glazing are added. If the contractors can pull that off everyone will breathe a sigh of relief – and, who knows, perhaps sneak in a cheeky kiss or two to celebrate?