Roof drainage and guttering

Broadly, there are two types of roof drainage system in common use: gravity systems and siphonic systems.

Gravity systems rely upon larger and more frequent downpipes than siphonic systems but are the traditional 'low technology' approach. Siphonic systems can  discharge water at a faster rate and rely upon smaller diameter outlets connected via short tail pipes to horizontal connector pipes, which lead to a downpipe. Water flows into the pipework via a special outlet designed to exclude air, the pipe fills and the force of gravity pulls water downwards thus creating a vacuum and forming a siphon.

Both systems rely upon proper gutter design and a sensible assessment of the probability (expressed as a return period) of a 2 minute storm intensity, the value of which varies according to geographical location. Flat roof and eaves gutters can usually overflow without causing harm (albeit temporary inconvenience) but parapet and valley gutters are likely to cause leakage into a building if they overflow, so must be given a higher risk rating. This means selecting a longer return period. See Gutter and outlet sizing.

Older codes of practice and Building Regulations gave a design value of 75mm/hour but this is now proven to be inadequate for buildings where the consequence of overflow cannot be tolerated.

Early siphonic systems ran into difficulties owing to the underestimation of rainfall intensity and the consequent inundation of the system. In the 40 or so years that the systems have been in production, design and performance standards have become better understood and documented, although specialist computer aided design is generally required. While there are several specialist contractors and suppliers, all systems work in fundamentally the same way. See Types of guttering.

Gutter maintenance (and access for maintenance) is also essential for both gravity and siphonic systems. With their smaller diameter pipes and outlets, siphonic outlets are prone to silting and blockage with organic materials and so can lose efficiency quickly. See Siphonic drainage systems.