Siphonic drainage: Wrekin position and guidance

Siphonic drainage is a specialist method of removing rainwater, most commonly used on large flat roofs such as warehouses and distribution centres. These systems are carefully engineered so that pipework runs full of water, known as full bore (rather than partially full), creating a high-velocity flow.

This is achieved by controlling air entry at the inlet and designing the system so that negative pressure forms within the pipework. The result is efficient movement of large volumes of water using fewer outlets and smaller pipe diameters than traditional gravity systems.

It is important to recognise that siphonic drainage is fundamentally different from conventional surface water drainage:

• Siphonic systems rely on specialist outlets and precisely designed pipework
• The inlet must control air entry to maintain full-bore flow
• Discharge is typically directed into below-ground drainage networks, not onto open surface elements like gullies or gratings
• System performance depends on hydraulic calculations and exact configuration, rather than general drainage principles

Because of this, siphonic systems are considered engineered hydraulic systems, not standard civils drainage solutions.

Wrekin does not manufacture siphonic drainage systems, and our ductile iron gullies and gratings are not designed to function as siphonic inlets.

• Siphonic inlets must carefully regulate air to maintain system performance
• Standard gully gratings allow air entry and therefore cannot support siphonic operation
• As a result, Wrekin ironwork should not be presented as part of a siphonic system itself

While Wrekin is not part of the siphonic system itself, our products may still have a role in the wider drainage environment, depending on the project. For example:

• Surface water collection in surrounding areas (yards, roads, hardstanding)
• Conventional gravity drainage downstream of the siphonic discharge point
• General civils infrastructure such as gullies, catch pits and associated ironwork

However, these are separate functions from the siphonic system. There is no direct correlation between siphonic drainage and products such as V channels, catch pits, or standard gully gratings.