Siphonic Drainage

In a siphonic roof drainage system, the drains benefit from the addition of an air baffle to prevent the entry of air into the system and by ensuring that the pipework is assembled to specific dimensions, air can be totally removed from the system thus filling the system entirely with water.

The calculation of the exact pipe dimensions requires a full understanding of the forces acting within the system, the type of material used for the construction, the maximum storm conditions, the roof area to be drained and the height of the building. The calculations are complex and can only be solved by iterative sequences – which is where our specialist software becomes essential!

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.

Siphonic roof drainage solutions are ideally suited to modern high volume structures. In particular, large footprint buildings such as warehouses, factories, airports and distribution centres are prime candidates for siphonic roof drainage. The ability to transfer the water at high levels to the building boundaries, avoiding roof penetration and internal downpipes, allows for maximum use of both floor and roof space. In addition to these building types, the flexibility and innovative design of siphonic solutions lends itself to 'non-standard' designs such as sports stadia and designs of a radical nature! 

• Reduced costs due to fewer, narrower pipes and limited discharge points
• Less complex underground drainage requirements
• Horizontal pipes laid internally make positioning of other services easier
• Greater architectural freedom
• Smaller and fewer pipes lead to easier installation and maintenance
• Self-cleaning of pipes through higher water velocity