ºÃÉ«ÏÈÉúTV

It could be a giant timber caterpillar munching its way across the South Downs. In fact, it is Britain's first permanent timber gridshell structure, due for completion by Easter at the Weald & Downland Open Air Museum in West Sussex.

The revolutionary design is by Edward Cullinan Architects and structural and services engineer Buro Happold. Its central feature is a rippling clear-span vault of criss-crossing oak laths resembling a giant wickerwork basket measuring 50 m x 15 m. From the outside, the timber gridshell is invisible, as it has been clad in three tiers of western red cedar boarding, a strip of polycarbonate patent-glazing, and rooftop waterproof screeding.

The £1.6m, 1800 m2 building will serve as the conservation centre of the museum, which consists of 45 reconstructed historic buildings scattered around a 20 ha site. Although it is devoted to the study and practice of building conservation, particularly English timber-frame construction, the new building is intended by the museum "to serve as an exemplar structure for modern rural buildings".

The building contains workshops, a classroom and a lower-floor archive, sunk into the hillside to help provide constant environmental conditions.

But the building's most revolutionary feature is the vaulted roof structure, designed with the help of advanced computer modelling. The structure consists of a diagonal grid of hundreds of pairs of slender green-oak laths, each measuring just 35 x 50 mm and connected at each joint by a steel clamp. The grid was assembled as a flat and floppy mat on a temporary platform erected high above the building site. It was then painstakingly lowered over adjustable scaffolding to assume its final rippling form. Finally, it was bolted to the perimeter of the first floor and to heavy timber arches at either end, which locked the whole undulating vault together as a rigid self-supporting shell.