Design of roof and vaulting, as executed, c. 1792-93 (4)
The design of the hall's roof and vaulting was crucial to Soane's practical and aesthetic purposes. Structurally, Soane needed to stablise the hall's skeletal yet weighty construction that combined spacious top-lighting (for proper illumination) with heavy brick vaulting (for incombustibility). Thus the use of the iron tie-rods shown in drawings 28-29, embedded above and below the impost blocks to brace the hall's eight perimeter vaults and central pendentive dome. Soane also wanted to lighten the appearance of the vaults as much as possible, especially the pendentive dome with its wide-spreading oculus. Thus the system of concentric tie-rods at the base of the lantern shown in drawing 29, that allowed the inner edge of the oculus to float out over the crossing, giving the pendentive dome below the appearance of a taut, thin surface resting lightly on its corner supports. For the vaulting brickwork Soane employed hollow-cone pots. These bricks were hollow pots, square and closed at one end, but basically circular in section, with the other end domical and containing a small opening. The cones would have been made at a kiln on the construction site. It was the first time such construction had been employed in an English public building. Soane drew particular attention to the use of such fireproof material for vaulting in his Royal Academy lectures.