Hail Siza

The Serpentine Gallery pavilion is a wide-span timber vault made of short, flat timber panels joined at their midpoints

Like many contemporary works of art, this year’s temporary pavilion at the Serpentine Gallery in Hyde Park, London, arouses a bizarre combination of conflicting responses. Externally, the building lives up to its creator’s vision of “an animal whose legs are firmly attached to the ground but whose body is tense from hunger, with an arched back and taut skin”. Inside, however, it presents itself as an impressive wide-span vault brightened by sunlight shining between timber arches.

At the same time, it has a ticky-tacky DIY character, evident in the gauche way it skirts around a large tree and in its capping pieces of unfinished spruce plywood that do not meet properly. If anything, the building evokes those Whole Earth Catalogue-inspired domes built by hippies in the 1970s.

The pavilion, which opens to the public from 29 June, is the creation of Portugal’s leading architect, Álvaro Siza, his long-time collaborator, Eduardo Souto de Moura, and Cecil Balmond, Arup’s inspired deputy chairman.

Turning to the detailed design and construction another curious hybrid of the rough-and-ready and the highly sophisticated emerges. The 17 m clear-span vault is made up of thick, flat plywood panels, all more or less 2000 × 550 mm in size, and fitted together on site with chunky mortise and tenon joints. What could be more rough and ready than that? Except that it relies on an obscure structural system known as “reciprocal offset beams”, pioneered in Germany in the 1920s. And without the latest computer-aided design and manufacturing technique, it wouldn’t have been possible at all.

You see, all 427 flat plywood panels vary slightly – but critically – in length and angle of end cut. They fit together to form a self-supporting vault of intersecting arches. But the flat panels do not clip together at every intersection of arches – that would be too fiddly and structurally too dicey. Instead each panel is two bays long, and pairs of panels meet head on with slight overlaps at the mid-point of the cross panel. Hence the more stable reciprocal offset, or mutually supporting, structural system.

“At first, we all thought why not just make the pavilion out of ready-made arches,” comments Steven Evans, divisional director of Bovis Lend Lease. The answer from Arup’s structural engineer, Martin Self, is that the flat panel system is “transportable, manhandlable and demountable”. The 427 panels were simply transported to site in just two lorry-loads from Finnforest Merk’s fabrication plant. Each was handled on site by two men, who fitted and bolted them together while supported on a cherry-picker. And after three months at the Serpentine, the arches can be dismantled and re-erected by whoever buys it.

The building is sheathed in clear polycarbonate panels that cover each 1 m square bay. Each panel comes with a top-hat ventilation flue containing a photovoltaic solar cell that lights the pavilion at dusk.

This year’s Serpentine Pavilion is an inspiring example of architecture as art. Even more so, it is also an example of creative structural engineering.