Smirke's Great Court, originally an open courtyard with the copper- domed Reading Room at its centre, has been restored. A new 6000m2 glass roof will cover the courtyard, creating a space for visitor facilities.
The outer edge of the roof rests on a new concrete ring beam at the cornice level of the original Great Court buildings; the inner edge rests on a circular 350 x 350 fabricated steel ring beam supported by 20 new composite steel and concrete columns set on a 18 degree radial grid at 7m centres around the Reading Room walls. Horizontal loads from the roof structure are taken by a 2.7mm-wide concrete slab which encircles the dome. The slab forms the floor of the Snow Gallery, a platform for roof main-tenance and contains access hatches and extract fans to vent smoke from the basement. The fans are maintained by an aluminium platform which runs on rails around the gallery.
Below the Snow Gallery the dome contains a series of Venetian windows. Pre-viously infilled, they have been restored with new capitals and columns on the outside, and newly glazed with reverse-frit glass panels. Fire compartmentation for one hour is provided by a rolled fire-curtain kept within a slot at the window head.
The columns are enclosed within a wall of 60mm thick Cabra limestone panels fixed to an I-section steel sub-frame. The enclosed spaces between them are used for services - smoke and air extract from the basement, electrical conduit for lights fixed to the roof ring beam, vent pipes and rainwater downpipes from syphonic drainage outlets which drain rainwater from the dome and the glass roof.