Conventional roof drainage systems generally comprise open outlets connected to vertical downpipes that are designed to operate at atmospheric pressure with a greater volume of air in the system than water. The size of the roof drains and the allowable depth of the water above them usually determines the discharge capacity of such a system.

A siphonic roof drainage system consists of a series of specially designed roof drains connected to a discharge point at or below ground level by pipe work that is assembled to specific dimensions such that all air is removed from the system resulting in more efficient solution.  When operating siphonically (known as 'full bore flow'), the driving head is equal to the vertical height between the roof level and the point of discharge. In many of today's commercial buildings this driving head may be in excess of 10m (30 feet) and can be used to advantage in many ways:

• Fewer siphonic drains are required than conventional drains thus the number of roof penetrations is reduced.
• The need for rows of vertical rainwater pipes inside a building can be virtually eliminated together with the extensive under floor drainage to serve them. 
• In a siphonic solution, the collecting pipe can be installed horizontally just below the roof structure freeing more space within the building and allowing greater freedom in architecturally design.
• The removal of air and dramatic increase if water velocity means that significantly smaller (and fewer) pipes are required.