Siphonic roof drainage systems are finding increasing acceptance amongst architects and builders of large commercial buildings in Australia. The benefits of siphonic roof drainage over conventional roof drainage systems include underground excavation savings, higher flow volumes, fewer outlets and stormwater harvesting and reuse potential. Siphonic roof drainage systems are generally designed using the steady state Bernoulli and the Colebrook-White equations. Both of these equations assume a full bore flow regime which only occurs for short periods in siphonic systems. This means that the majority of storm events do not cause the system to run at its full capacity or efficiency. In Australia, systems are designed to cope with a maximum design rainfall intensity, usually the 1 in 100 year storm event. While the performance of siphonic roof drainage systems at this rainfall intensity is well understood, the system performance at lower rainfall intensities and unsteady flow regimes is largely unknown. This research aims to investigate and identify the improvements which can be effected to the overall performance of siphonic roof drainage by the removal of air from the system. The research will also investigate the effect the air entrainment has on siphonic flows. / PhD Doctorate