Abstract This paper represents the most thorough examination of siphonic system priming to date. Data is presented which has been obtained from a number of systems. The text details the priming of siphonic roof drainage systems, and outlines how knowledge of the priming process, if considered at the design stage, may be used to improve the operational system performance. The role of different system components in improving “ability to prime”, estimating priming time and reducing ongoing maintenance costs is considered and used as the basis of the “Priming Focused Design” philosophy proposed in this paper.