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Over the past twenty years a Scandinavian system for transferring rainwater from the roof down to ground level has found its place in the UK roofing and cladding industry. Siphonic drainage can offer clear advantages over gravity systems:

- reduced number of down pipes
- connecting pipework can be laid level, with less headroom required on long horizontal runs
- higher water velocities such that the pipe becomes "self cleansing"
- less underground drainage.

Principle of Operation

During light rainfall, water flows under gravity through the pipework.
When the rain is heavier the relatively small bore pipes run full, see figure 1. At the top of the stack, the free falling water drags along the water further up the system with increasing speed. Hence a small diameter pipe can have a high rate of discharge, and thus drain large areas of roof.

A simple analogy is that of a metal chain draped over the side of a table, see figure 2. If the hanging length is steadily increased, there reaches a point when its self weight overcomes the surface resistance of the remaining length and the chain quickly runs off the table. A siphonic system acts in the same manner.

For the siphonic action to start, the pipework must be airtight, which requires:

- special rainwater outlets with baffle plates
- correctly dimensioned pipes
- fully sealed joints

Hydraulic Design

The Code of Practice for drainage of roofs, BS6367 and published in 1983, makes no reference to siphonic drainage systems. Accordingly the hydraulic design of the outlets and downpipes is normally carried out by the specialist manufacturer.
Firstly, the pipe friction resistances are determined from charts based on the Darcy formula. Subsequently the hydraulic pressure drops along the system are balanced using Bernoulli's equation, making allowances for bends and fittings. Not surprisingly, a computer is used to optimise the sizing of the pipes in the drainage network.

For outline design purposes, the shortest practical stack height required for siphonic action to start is 3m, and the maximum design flow rate per outlet is usually of the order 12l/s. For the design of gutters with siphonic outlets, the assumption is made that the rainwater discharges into a 'flooded outlet' and the BS6367 - Appendix B supplementary design procedures should be used.


The siphonic outlet is a specialist item with a carefully designed air baffle to prevent air entering the system at peak flows. The presence of a leaf guard is important to reduce the risk of solids entering the system of narrow bore pipes and creating a blockage. As with all rainwater goods systems, it is important that the outlet sumps are cleared out at least once a year.

The internal pipework is typically formed in high density polyethylene between 50mm and 100mm in diameter and needs to be supported at regular centres, often from a secondary framework with additional vertical bracing at changes in pipe direction. It is essential that all of the internal pipe joints are fully sealed, and one technique gaining in acceptance is the use of an electroweld sleeve coupling - a proprietary sleeve with a built-in electric heater coil operated on site from a battery pack.

Thermal movement in long lines of continuous pipework can present long term problems, and special provisions should be made for flexible expansion joints. If there is extensive silt or fine particles washed off the roof over a period of years, there can potentially be excessive wear on the inside faces of pipework at changes in direction.

Other Points to Note

It is a common experience on site to find 'follow on trades' sweeping debris into roof gutters and subsequently into the drainage system. This is of particular concern for siphonic drainage systems where the relatively small pipework can easily block up and where it is uncommon to find rodding access points within the pipework. The main contractors' attention should be brought to this potential risk.

When properly designed and installed, siphonic drainage systems can rapidly transfer large quantities of rainwater to a few points of discharge at ground level. On older sites where the underground drainage is somewhat of an unknown capacity, the client would be well advised to seek the advice of a civil engineer to check that the surface water system can accept the peak flows from a new siphonic system - otherwise a flooded roof problem could become a flooded floor problem!


1. BS6367: 1983: Code of Practice for Drainage of Roofs and Paved Areas
2. RCI Technical Note 22 - Internal Gutters: January 1992
3. RCI Technical Note 23 - Gutter Outlets: February 1992
4. RCI Technical Note 24 - Down Pipes: May 1992


© Keith Roberts, 1994

First published in Roofing Cladding Insulation, June 1994
Reprinted in Building Engineer, April 1995


RCI Technical Notes

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