The new gallery is 26.5m long and 7.2m wide with a pitched roof, matching the proportions of the original building. The walls and roof are clad with prepatinated zinc. Viewed obliquely, the building envelope appears entirely sheeted with zinc but close inspection reveals that the north-east roof pitch and the wall below are glazed; the combination of narrow bands of glazing and projecting (215mm) zinc-clad mullions gives the appearance of solidity.
The gallery has a cast-in situ concrete frame with 200 x 50mm timber joists on the south-west roof pitch, clad with standing-seam zinc sheet. It has a protective coating to the underside so that it could be cleated directly to the foamedglass insulation, avoiding the traditional ventilation gap and thus preventing wind uplift.
On the north-east side the zinc-clad aluminium mullions run at 450mm centres from ridge to ground-floor level and are tied to the cantilevered edge of the first-floor slab. They are fixed to spigots set in a continuous ridge plate bolted back to the 356 x 171 UB ridge beam. The zinc cladding to the ridge extends to overlap this connection and is then welted over the return lip of a zinc rainscreen which runs between the mullions. The inner pane of the glazing is laminated with PVB (polyvinyl butyral) which is clear at ground-floor level and translucent above.