What is siphonic roof drainage?

In a siphonic system the rainwater pipes run completely full of water instead of water trickling along the invert with air above it. That creates negative pressure, and the system actively pulls the water off the roof.

Some brand names have become so common that they are used generically — in the Netherlands Pluvia, a Geberit system, is used almost as a household word. The neutral terms are siphonic roof drainage or full-bore drainage.

How it works

In a conventional system, water flows down a falling pipe with a layer of air above it. In a siphonic system a special roof outlet — with a baffle plate — prevents air being drawn in. The pipe fills completely. The column of water in the downpipe then pulls on the horizontal collector, negative pressure develops, and the water travels far faster.

Advantages

• Smaller diameters. The same volume of water through a considerably smaller pipe, so it is easier to conceal in the ceiling zone.
• No fall required in the horizontal collector. It can run level under the roof without losing headroom.
• Fewer downpipes. One stack can serve many roof outlets, which limits the number of penetrations.
• Self-cleansing. The high velocity flushes silt and leaves through rather than letting them settle.

Points to watch

• This is calculation, not installation. A siphonic system only works at the design point it was calculated for. Diameters, lengths and heights are fixed; you cannot simply add an outlet or extend a run afterwards. Have the supplier calculate it, and design the rainwater system to BS EN 12056-3 with BS 8490 for siphonic design.
• Negative pressure. The pipework operates under suction. Only pipes, joints and brackets approved for that duty may be used — an ordinary above-ground drainage pipe can collapse.
• Roof debris. The system is sensitive to anything that disturbs priming. Leaf guards, ballast and vegetation around the outlets need maintenance.
• Overflow provision is still required. A flat roof still needs a second, independent route for the water if the main outlets block or the storm exceeds the design event.
• Noise. The high velocity generates flow noise. Where pipes pass habitable rooms, use an acoustic pipe or a boxed enclosure.

Related terms

• Rainwater drainage
• Flat roof
• Overflow
• Roof outlet
• Above-ground drainage

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