Only five wooden gridshells have ever been built, and none without broken timbers. Here's how the team behind this one, the roof of a Sussex museum, is cracking the conundrum.
"We are all on a journey where we haven't quite understood how we are going to get to the destination." Andrew Holloway, director of Green Oak Carpentry Company, is talking about his firm's latest and most challenging project: the construction of the UK's first major timber gridshell.

Gridshells are extremely strong structures that need no internal support. Built from timber laths woven into a diagonal lattice mat, they have no strength when flat. When formed into curves, however, the grid locks into a rigid shell. In this instance, a gridshell will form the 50 m long, 15 m wide roof of the £1.6m conservation centre being built at Weald & Downland Open Air Museum, near Chichester in West Sussex.

As the specialist carpentry contractor on the project, Holloway's team is not only responsible for building the gridshell roof. It has also had to pioneer a new method of erecting it. Only five timber gridshells have ever been built before, and in each case, timbers were damaged during erection. These gridshells were erected either by pulling the delicate timber lattice upwards from above or by prodding it from below. On the last one that was built – in Mannheim, Germany – 60% of the timbers had to be replaced.

At the museum, a new method of erection is being used. The gridshell was built on top of a 10 m high, temporary scaffold table. Then, the scaffold table was carefully dismantled section by section, allowing gravity to pull down the lattice until it came to rest on a series of props. These props are now being adjusted to sculpt the wooden shell.

Almost a year into the construction programme, the roof is slowly starting to take shape. Supported on a jungle of of gleaming metal tubes, yellow props and scaffold towers, the part-formed gridshell hangs like a stiff wooden blanket, its domes and valleys just starting to emerge.

Green Oak's Holloway describes the props as "Acros with brass knobs on". The length of each prop can easily be adjusted by turning a knurled wheel in its middle; a numbered scale indicates the prop's length.

On site, a team of eight carpenters clambers into position, ready to begin the next manoeuvre in a sequence that will allow the lattice to droop a few millimetres closer to its final, curvaceous form. They stand on the scaffolding, each stationed next to a prop, awaiting instructions. On a grass bank overlooking the site, Green Oak's foreman Steve Corbett stands like a conductor before an orchestra.

On Corbett's command, the lowering begins. The carpenter in the middle of the group shouts out a prop's reference number and its revised length. The carpenter next to that prop springs into action and spins the adjusting wheel, shortening its length and gently lowering one section of the lattice. Then, the carpenter standing by the next prop is given his instructions. The sequence is repeated until all the props on one section have been adjusted. All the while, Corbett, puffing on a roll-up, scrutinises the operation, his experienced eye scouring the laths for signs of stress.

While the carpenters sculpt the shell, Olli Kelly, Buro Happold's project engineer, monitors the position of the timber nodes supported by the props. The sequence of the prop's adjustment has been carefully planned, using sophisticated software developed by the engineers. At the end of each day, the revised node positions are fed into Kelly's spreadsheet to check that the structure is behaving as predicted. "From gridmat to gridshell, we know the precise position each node is meant to be in at any particular time," says Buro Happold's Richard Harris, the structural engineer responsible for the gridshell's design.

As work progresses, realisation of the architect's concept is now firmly in the hands of the specialist trade contractor. "The carpenters have taken ownership of the gridshell," says Harris. Architect Ted Cullinan nods in agreement. "Skilled craftsmen just love showing off," he adds in admiration. Harris acknowledges the contribution made by the craftsmen's empathy with wood and understanding of the way it behaves. "The carpenters don't adjust the props blindly – they move them to what they think the structure can comfortably bear at the time," he says. This explains why only five of the laths have been damaged during erection.

One area where Green Oak's experience had a big impact was in the design of the node clamps holding the laths in place where one crosses another. The original proposal called for a bolt to be passed through a slot cut into each lath, but this, says Holloway, would have weakened the structure. Instead, a clamp was designed that straddles the laths and allows the timbers to move at the joints. "This was a significant development," Holloway adds – so significant, in fact, that the design team has patented it.

For the main contractor, too, this is a challenging project. "We always say in construction, 'we are building a prototype'," says Mike Wigmore, managing director of West Sussex-based EA Chiverton. "In this case, we most certainly are." The contractor started work on the scheme early last summer with construction of the building's basement archive store. "There was a burst of activity on the substructure until September. Since then, we've had a watching role while the roof is constructed." Once the roof is complete, Chiverton will move back onto site to begin the internal fit-out. "It will take more than 70 weeks to build a modest building," says a bemused Wigmore. Chiverton was appointed to the scheme on a JCT98 contract, following competitive tendering.

Green Oak Carpentry's appointment was more unusual: it was appointed to the design team before the main contractor came on board. Then, the carpenter became a named subcontractor. "We couldn't tender competitively because of the unknown risk in erecting the roof," says Holloway. The arrangement "caused some fun", adds Wigmore, "because Green Oak can design a detail and instruct themselves to carry it out".

On such a pioneering construction, this form of appointment could have shouldered the main contractor with a large amount of risk. "As a named subcontractor, rather than a nominated subcontractor, any problems the subcontractor encounters become Chiverton's responsibility," says Wigmore. However, he explains: "Chiverton was protected from most of the risk because erection of the gridshell was defined as part of the design process."

Once the shell has been manipulated into its final form, it will be fixed into position by attaching the timber to the edge of the ground floor. Longitudinal laths will then be attached along the sides of the shell to lock the grid structure solidly into position before the cladding is added. Wigmore's concern at that point will be how to clad the curving gridshell in Western Red Cedar boarding without putting the operatives at risk. "The Construction (Design and Management) Regulations mean we can do a lot less climbing around the structure than when the last gridshell was constructed in Germany," he explains. The internal scaffolding will remain in place to allow safe access for the carpenters to fit the cladding.

"There is a lot of talk among the design team that the new method of assembly will give gridshells a huge future in construction," says Wigmore. "But we have to remember that we have a client and a delivery date: we have to keep everybody's feet on the ground."

Craftsmen’s master: Andrew Holloway

Director, Green Oak Carpentry Company
Andrew Holloway started his professional career as a potter before discovering that his skills were better employed as a carpenter. He founded Green Oak 10 years ago and has since worked on restoration projects, the construction of traditional and modern timber homes and even a Buddhist monastery. His association with the Weald and Downland Museum goes back to his days as a self-employed carpenter, when he worked on the restoration of one of the museum’s buildings. He finds his return to work on the gridshell exciting. “Once I saw what the project was about, I had to do it,” he says. “The team spirit working on such a pioneering project is exceptional,” he adds.

Shells angel: Steve Corbett

Site foreman, Green Oak Carpentry Company
Steve Corbett has worked with wood for the past 25 years. In that time he has been a cabinetmaker and an interior designer, and he has even built timber barns. Corbett says the “completely untried” building was an irresistible challenge. To meet it, he had to recruit a team of sufficiently skilled woodworkers. “Half of them were boat-builders, but the whole team was selected on the contribution they could make,” he adds. He sees his role on the project as that of an interpreter between the engineers and their spreadsheets and the craftsmanship of the carpenters.