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working details

Weald & Downland Open Air Museum Edward Cullinan Architects

A timber gridshell with a propped timber roof The museum enclosure,58m long and 16m wide, is a series of undulating domes and valleys. It is formed of a gridshell of green oak laths, connected by galvanised steel nodes.

The lower levels of the shell are clad with vertical cedar boarding; the upper levels are clad with triple-wall polycarbonate sheet. A roof runs above the apex of the shell, propped on paired Douglas fir pillars, which are flanked by polycarbonate clerestories. It is flat in crosssection but follows the undulations of the shell.

At their bases, the paired 75 x 100 x 25mm pillars are bolted to transverse oak rib laths which run over the gridshell at 700mm centres to brace it after assembly.To maintain the roof soffit and clerestory head at a consistent height yet accommodate the undulating shape of the gridshell, the pillar bases fan out and back as they run along the roof. They run from maximum inclination where the cross-section through the roof is narrowest, to vertical where the cross section through the roof is widest.

The paired pillars are bolted at their tops to a series of 150 x 25mm Douglas fir joists which oversail the clerestories, giving shelter to the structure below. The tops of the joists were notched to receive 100 x 25mm purlins laid flat with 100mm spaces in between.The purlins are covered with a succession of layers - moisture barrier, insulation, breather membrane and plywood.The final roof covering, Roofkrete, comprises four layers of steel mesh stapled to the ply deck and covered with a trowelapplied cement layer.The roof edges were formed by dressing the mesh up to stainless steel stop beads and rendering down to them.

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