Siphonic Drainage Systems

UK rainfall is getting worse if you can believe it. 10 of the wettest years on record have happened since 1998, according to the Met Office, and it’s only getting wetter.

For building owners and facilities managers, it’s creating a drainage problem because gravity drainage systems weren’t designed to handle so much water at once. Siphonic drainage systems are different.

What is siphonic roof drainage?

A siphonic roof drainage system uses specially designed outlets and pipework to move water off a roof at high velocity.

Unlike conventional gravity systems, which rely on the natural fall of water and always carry a mix of air and water, siphonic systems are engineered to run completely full-bore under negative pressure.

In practice, that means smaller pipes, fewer downpipes, and horizontal runs that don’t need any gradient.

How siphonic drainage systems work

Siphonic drainage has an anti-vortex baffle plate that’s fitted inside each roof outlet, and removes air from the pipework as water levels rise.

Under light rainfall, the system runs part-full, behaving much like a standard gravity drain.

As rain intensity increases, the baffle makes it harder for air to get in, the pipe cross-section fills with water, and negative pressure builds until the system primes.

Once primed, the vertical difference between the water level at the roof outlet and the discharge point below drives flow at high velocity through the collector pipes, which run full-bore.

Debris has nowhere to settle, so the system is largely self-cleansing, and drainage happens faster than any gravity drainage systems could handle.

A siphonic system has four core components:

•   Siphonic outlet: the roof drain incorporating the anti-vortex baffle, rated to a defined flow at a specified head
•   Tailpipe: a short vertical section dropping from each outlet to the horizontal collector below
•   Horizontal collector: the main level pipe run, sized to run full-bore under design conditions, with no gradient required
•   Downpipe: the vertical riser that carries water to ground level or into a buried collection pipe

Siphonic vs traditional gravity drainage systems

Main benefits of siphonic roof drainage

The main benefits of siphonic drainage are:

•   Fewer, and smaller downpipes, which means fewer penetrations through the building fabric.
•   Horizontal collector pipes run level near gutter height, which gives you more usable internal ceiling space that a gradient-dependent system would consume.
•   Faster roof drainage during heavy rain or storms, cutting the risk of ponding and water ingress.
•   Reduced underground drainage requirements, which can cut civil engineering costs on larger sites.
•   Self-cleansing pipes that rarely block because there is nowhere for debris to gather.

You will still need to maintain your drainage systems. We recommend that a qualified roof maintenance technician inspect siphonic systems at least three times a year.

Design standards and compliance

There are two governing standards that your system will need to comply with.

BS EN 12056-3 covers roof drainage design and rainfall intensity calculations.

BS 8490 is the code of practice written specifically for siphonic systems. It was updated in 2025 to make secondary and emergency siphonic drainage systems mandatory for internal gutters where any blockage could result in water ingress.

Suitability and applications

Siphonic drainage performs best on large flat or low-pitch commercial roofs like warehouses, distribution centres, retail parks, airports and industrial facilities that have a large roof area with a substantial drop.

Smaller roofs with limited height differential can benefit, but any benefits would often be barely noticeable. You can also technically retrofit existing buildings, but it requires a full structural and drainage survey to assess outlets, pipe penetrations and support fixings before any design work starts.

Systems designed to BS 6367:1983 or early BS EN 12056 implementations may now be under-capacity. If your drainage system was built before the current standards came in, you should consider having it reassessed before a problem comes up.

Installation, commissioning and maintenance

Siphonic systems are sensitive to deviation. Installation must follow the manufacturer’s drawings precisely, and any material change to pipe routing, outlet position or support arrangement must go back to the designer for recalculation before the work continues.

You should install horizontal pipes level or to the minimal gradients specified in the design. 

Adding a fall (thinking it will improve water flow) is one of the most common installation errors because it disrupts the siphonic action and can cause the system to struggle or fail.

Once everything is installed, you’ll need to do hydrostatic pressure and leak testing along with a controlled flood test to make sure the outlets prime and pipes run full as designed.

Not doing this means you’ll only find out there’s a problem after something has already gone wrong.

Ongoing maintenance covers outlet cleaning, baffle plate inspection, checking of pipe supports and bracing, and confirming that secondary drainage routes are clear.

Choosing a siphonic drainage contractor

Siphonic systems that fail do not fail quietly.

Ponding, water ingress and, in serious cases, structural damage from excess roof loading are the consequences of poor design or poor installation. Specialist certification exists for a reason.

When selecting a contractor, ask for full hydraulic design calculations produced using validated specialist software, case studies from comparable building types, and evidence of installer certification.

NWIR adheres to all guidelines set out by the Siphonic Roof Drainage Association.

Every system we install is designed to that standard, and commissioned to confirm it.

To arrange a free site survey, call NWIR on 0800 046 1500 or contact us online.