Neptune Court has been enclosed with a glazed roof, creating a large exhibition space. Covering 52 x 44m without the use of columns, the roof is one of the largest of its kind in Europe. The apex of the roof is relatively low (about 4-5m above the parapet) to reduce intrusion and to preserve views from the Old Royal Observatory.
Structurally, the roof acts as a shell, in which the members, a grid of 168mm-diameter chs tubes, are held in compression by ties consisting of pairs of 60mm-diameter Macalloy bars set about 8m apart. The tubes are braced against buckling by fins made of 76mm-diameter CHS tubes; the fins are 750mm deep to minimise roof-level intrusion.
The roof had to be supported on the nineteenth-century historic structure of the courtyard - three-storey walls, mostly 12m high, of mass brickwork rendered with stucco. Non-destructive tests established the characteristics of the walls. The rusticated arches at the western wing had to be strengthened with steel inserts. The foundations on three sides of the courtyard were underpinned to spread the load.
Stone parapets topped the courtyard walls. They were strengthened with a concrete spreader beam which was dowelled to the original brick piers which run along the facade. A series of vertical 139mm-diameter chs posts, bolted to the spreader beam, supports the roof grid of 168mm-diameter chs tubes. Sealed double-glazed units, about 2m square in size, rest on the grid at their edges, and are faceted to accommodate the curve of the roof. An inverted channel is welded to each tube to support a drained and vented gasket system which lies under the glass joint.