¾«¶«Ó°ÊÓ

On 31 March, a new exhibition hall, topped by a spectacular glass dome, opens in Greenwich's Maritime Museum. The roof, spanning the 52 m wide courtyard between two wings of the E-shaped museum, is the most dramatic part of a £20m redevelopment scheme to increase exhibition space and improve circulation through the museum in time for the millennium.

The scheme's architects, Rick Mather Architects and ¾«¶«Ó°ÊÓ Design Partnership, set out to create a sheltered courtyard space using a minimal roof structure with no internal columns. The 52 × 45 m roof looks surprisingly delicate – a remarkable feat given the glass weighs more than 150 tonnes, and the lattice trusses and purlins another 100.

The roof's weight is carried by the building's three existing neoclassical facades and one rebuilt wall. To ensure lateral loads are not transferred to the facade, and to maintain the roof's gentle arch, steel ties span the roof and stop its base from spreading. A concrete ring-beam – part of the facade's rebuilt parapets – transfers the weight to the underpinned walls.

"Erecting the roof to achieve the specified tolerances was the biggest construction challenge," says Bovis Lehrer McGovern's project manager, Keith Page, "particularly as no lateral forces could be put on the building's listed facades during or after construction." The structure arrived on site in 10 m lengths of two lattice trusses with five purlins attached. These 10 m "ladders" were welded together to form a 20 m length to span from the parapet to the roof's centre, before being lifted into place and supported on props. The trusses were then welded to the roof's centre member and the purlins attached.