Siphonic drainage systems are the best way to protect your building against high-intensity storms and rising levels of rainfall. As a member of the Siphonic Roof Drainage Association, we have the expertise to design and install the right system for you.
What Is Siphonic Drainage?
Siphonic drainage prevents water ingress by draining rainwater more efficiently than traditional rainwater systems. It is specifically designed to cope with high intensity storms, while operating similarly to traditional systems for day-to-day drainage.
Those traditional rainwater systems rely on gravity alone for water to drain. Pipes are fitted on a gradient and a building typically needs more room within the roof space to operate efficiently. Access/rodding points are needed to enable cleaning and maintenance.
Siphonic rainwater systems use baffle plates (fixed to the rainwater outlet) to prevent air from entering the pipes during intense storms. This creates a vacuum effect that drains higher volumes of water from roofs using fewer, smaller diameter, horizontal downpipes.
What are the Benefits of a Siphonic Drainage System?
Siphonic drainage systems provide high-performance rainwater management by combining efficiency, flexibility, and cost savings. Compared to traditional gravity systems, they can handle extreme rainfall more effectively, require fewer downpipes, and free up valuable building space.
- Handles intense rainfall: High-velocity water flow enables the system to cope with heavy, high-intensity storms without performance loss.
- Greater efficiency with fewer downpipes: Each gutter usually needs only one or two downpipes, improving aesthetics and reducing the cost of large underground drainage networks.
- Flexible pipework design: Pipes can be routed horizontally and positioned to suit the building’s layout, maximising space within the building envelope.
- Reduced underground infrastructure: Minimising underground pipework makes above-ceiling pipework more visible and accessible for inspection, maintenance, or repair.
- Self-cleansing operation: High-speed water flow naturally cleans the system, with maintenance carried out from roof-level outlets rather than multiple access points.
- Space-saving and lightweight: Smaller diameter pipes run parallel to the ceiling, saving space and reducing structural loads compared to gravity systems.
- Multi-outlet integration: Multiple outlets can be connected to a single connector, simplifying installation and improving efficiency.
Why Choose NSS for your Siphonic Drainage System?
As a member of the Siphonic Roof Drainage Association, NSS Group has developed close working relationships with all leading industry suppliers, enabling us to select the ideal drainage system for your needs.
Does your Siphonic Drainage System need an Upgrade?
Many early siphonic systems were designed to the BS 6367:1983 standard. This was replaced by a new standard (BS EN 12056-3:2000) in 2000, leaving existing buildings under capacity.
In 2025, BS 8490:2025 was introduced to reflect increasing levels of rainfall, adding secondary drainage systems to all internal gutters.
If your system was installed before 2000, it is likely to fall short of BS EN 12056-3:2000 and BS 8490:2025, and is therefore likely to need attention in order to provide adequate rainwater protection.
Over the years, debris builds up on the roof a building, while a lack of adequate maintenance systems installed since 2000 can lead to operational failure. These older siphonic drainage systems typically require additional maintenance to bring them back to full capacity, so they offer the necessary protection.
Professional maintenance is also essential to protect guarantees and ensure high performance. Gutters and baffles should be inspected and cleaned approximately 3 times each year, depending on temperatures and how close the building is to trees or industrial sites.
Our engineers are fully employed in-house, with the expertise to design, install, repair and maintain any system. Which means there is no need to rely on (or wait for) external contractors.
We operate nationwide and accommodate portfolios of any size with minimal lead times. Support teams are available around the clock, 365 days a year.
What can you Expect?
Our detailed siphonic drainage consultations include:
- Thorough inspection of current systems and gutters, including checks of fixings, welds and rainwater outlets.
- System calculations to determine the appropriate intensity, accounting for rainfall criteria in line with both current British standards.
- Details of any remedial work required to bring the system back to capacity.
- Full calculations of additional capacity necessary to bring protection in line with current and future standards.
- A full report with recommendations for the most viable system/additional systems and the steps to achieve suitable rainfall protection.
- NSS now offer third party impartial checking and assessment of new build siphonic systems and installations.
Siphonic Drainage FAQs
If your system was installed before 2000, it may not meet BS EN 12056-3:2000 or the latest BS 8490:2025 standards, which account for increased rainfall and require secondary drainage for internal gutters. Older systems may have reduced capacity and require maintenance or retrofitting to meet current performance standards.
Professional maintenance is essential to protect performance and warranties. Gutters and baffles should typically be inspected and cleaned around three times per year, with frequency depending on climate and proximity to trees or industrial sites.
Yes. Existing gravity systems can be upgraded to siphonic drainage, and older siphonic systems can be modernised to meet the latest British Standards. A full inspection, capacity calculation, and remedial work plan are recommended before installation or upgrade.
NSS engineers conduct a detailed inspection of gutters, outlets, fixings, and welds, followed by rainfall intensity calculations in line with British Standards. You’ll receive a full report outlining any remedial work, additional capacity requirements, and system recommendations.
Traditional gravity drainage relies solely on gravity, requiring larger pipes and sloped gradients, which take up more roof space and need multiple downpipes per gutter. Siphonic drainage uses a vacuum effect to achieve faster flow, requiring fewer, smaller pipes, offering greater design flexibility and reduced underground drainage infrastructure.
Benefits include faster rainwater removal, fewer downpipes, reduced structural loads from smaller pipe sizes, greater flexibility in pipe routing, improved aesthetics, space savings within the building envelope, and lower underground drainage costs. Siphonic systems are also self-cleansing and easier to maintain since pipework is more accessible.
Siphonic drainage looks great and is simple to maintain, but also offers the best possible protection against storms.
Billy Gordon, NSS Technical Manager and Siphonic Drainage Consultant
