What happens when an architect’s starting point for a structure is the material they’re going to build it in?

The answer was there for all to see on the concourse behind the Royal Festival Hall in London through September. ‘Sclera’ was the all-timber pavilion designed by architect David Adjaye as the Size+Matter ‘installation’ at this year’s London Design Festival. 

The building, looking like some giant wooden puzzle, was constructed entirely in American tulipwood and originally sprang from a challenge thrown down to Adjaye by the Design Festival and the American Hardwood Export Council (AHEC).

According to Festival director Ben Evans, the aim of Size+Matter is to question     perceptions about structure and materials. Last year Zaha Hadid’s Urban Nebula pavilion did the trick for concrete.  This year, AHEC said it wanted to get involved as a sponsor and put forward tulipwood as the raw material.

It was a double first for Adjaye. He hadn’t previously worked with wood as a core structural material – “Like many architects, I’d seen it principally as a surface – a decoration”. And tulipwood was an unknown quantity too. “The first time I saw it was when AHEC dropped the samples on my desk,” he said. 

AHEC European director David Venables also acknowledged that the project was breaking new ground for tulipwood. It is not, he admits, the most prized of American hardwoods. It’s one of the most prolific, and so relatively cheap, but can vary widely in colour and grain and until now has largely been consigned to use as furniture   carcasing, or for mouldings.

“But this is precisely the sort of species we should be using more and in a greater range of applications,” said Venables. “It makes sense in terms of cost and for the environment, given increasing constraints on use of rarer tropical hardwoods.”

As part of its drive to get tulipwood more widely specified, AHEC submitted it to the BRE for structural strength testing. It passed. “The tests showed it had tensile strength characteristics comparable to oak and a remarkable strength to weight ratio,” said Venables.

Adjaye, it seems, was soon won over. “As designers, we’re obsessed with the surface effects of timber and having it even and monotone. But when I saw the samples of tulipwood I was shocked by the kaleidoscope of colour, from yellow to warm dark brown, sometimes the full spectrum in just one piece, and the thing designers have seen as a negative struck me as a potential positive. I also wanted to use the material as the bones of the building as well as the decoration and ornament… one seamless construction.”

This initial impression of the wood formed the kernel of the pavilion design. The concept was a building that created a range of perspectives and used light to play on the timber’s colour and grain – hence the name ‘sclera’, which is the outer layer of the eye.

The pavilion has two entrances to frame external vistas. But the walls and ceilings are not solid (comprising intermeshing frames of 1,500 60mm tulipwood single section and laminated boards) so the building “orientates to any view” and visitors snatch small glimpses of what’s going on outside through the timber. The light slicing through the structure also changes depending on where you stand, heightening the variation in the wood.

“I’m interested in the way light affects material and the way in which we receive and perceive it, how light hits the eye and affects us emotionally,” said Adjaye. “With the varying interplay of light and tulipwood we’ve created many kinds of structure in one and, I think, a very meditative space.”

After passing the BRE’s examination, AHEC was confident the tulipwood was up to the structural demands of Adjaye’s design. But the timber is not inherently an external-use species, so its performance with preservative treatments also had to be tested. So AHEC teamed up with treatment producer Osmose and showed that the species readily took up both water- and solvent-based treatments.

Because of time constraints, the pavilion timber was given two brushed and dipped surface treatments, one with Remmers Primer GN and the second, Osmose Primer Oil.  This will do the job, but will have to be renewed after two to three years.

“But we are also now working with Osmose on impregnation treatment schedules, which opens the door to large scale production and the use of tulipwood in a wider range of external applications, including cladding,” said Venables.

The task of bringing the treated wood and Adjaye’s design together was given to German timber building specialist Hess Wohnwerk. And international project manager Rensteph Thompson acknowledged it was a first for them too.

“Our work is principally in larch and spruce glulam, so using hardwood, and tulipwood in particular, gave us some new challenges,” he said. “The structure is also very intricate and all 1,500 pieces of timber are different lengths. It took some careful planning and we part assembled it in the factory and numbered every part.”

One hitch was that the melamine glue Hess uses for most of its softwood structures just didn’t take. “We face-glued some boards and they just came apart,” said Thompson. “So we tried a stronger Resorcin product, and that worked.”

The floor of the 12m-long, 4.25m-high, 8m-wide pavilion comprises edge-grain-screwed timber and into this are dowelled the wall’s 370 posts which, in turn, support 68 roof joists. From the latter hang 910 boards of varying lengths, like tulipwood stalactites. 

Because the pavilion was effectively free-floating for the month it spent on the Festival Hall concourse, steel bracing was added to give the open portal frame structure greater rigidity and meet wind-loading requirements. But when the building finds a more permanent home and is fixed to a slab or some other form of foundation, this will probably be removed.

Once the components were completed – a total of 50m3 of timber, weighing 25 tonnes - they were trucked to the UK and reassembled in 10 days by a team from Hess and British company Mtech. The work went smoothly, thanks in no small part, thinks Thompson, to the fact that all the components were cut and marked up on Hess’s CNC machining centre.

Once the month-long Design Festival was finished on October 16, the pavilion was due to be dismantled and auctioned – and prospective purchasers from abroad have already expressed interest.

Meanwhile, the whole experience seems to have made a lasting impact on Adjaye. He says he plans to do more with timber generally, and tulipwood in particular.