What is Siphonic Roof Drainage?

22 June 2022

There's nothing new about the idea of using syphons to drain water from a roof. A Finnish engineer, Ovlai Ebling, first devised it in the late 1960s, and a Swedish turbine manufacturer installed it in 1972. Because of the widespread use of siphonic roof drainage systems around the world, the UK market is home to a number of suppliers.

By installing systems based on 75 millimetre/hour rainfall intensity, the reputation of the UK siphonic industry was severely damaged in the 1990s. As a result, systems would inevitably overflow; however, the siphonic technique was held responsible rather than the low design rainfall levels.

Many businesses have gone out of business because they lacked the technical know-how to properly design siphonic roof drainage systems. The subpar systems installed often required roof repairs and tarnished the industry's good name.

SRDA was established in June 2004 with the goal of promoting best practices in the industry. The SRDA played a key role in the development of BS8490:2007, the British Standard for Siphonic Roof Drainage, in conjunction with the industry and BSI.

How Siphonic Drainage Works

The operation of all siphonic roof drains is the same, regardless of brand or model. A baffle plate over the outlet hole prevents air from entering the pipes, causing them to run full of water.

During heavy rain, the difference in elevation between the gutter and discharge point causes a negative pressure in the pipe system, which pulls water through the system. Increased potential energy and increased outlet flow capacity are directly proportional to drop size.

Because cavitation and pipe implosion can occur when negative pressure is too high, the designs must be carried out by a competent person using appropriate software. Horizontal siphonic piping at a high altitude Discharge manhole lid with vents. A collector main is used to connect multiple discharge points to a single discharge point via small-diameter tailpipes.

There may be a large number of these systems in larger buildings. Tailpipes must be able to fill the collector pipe in a reasonable amount of time to be effective. Roof drainage systems in the UK are designed using a two-minute storm, so if they take more than 60 seconds to fill, they are rendered useless. This has become increasingly important in the UK as the size of buildings and systems has increased. Drainage is often divided into primary and secondary systems in larger systems.

Primary and secondary systems divert rainfall on a daily basis, but when there is a large amount of rain or flooding, the secondary system kicks in. Because underground drainage systems cannot handle the full volume of drainage, these secondary systems often empty onto parking lots and other hard surfaces. Reduces stress on downstream drainage systems by using a long-term strategy.

Pros and Cons of Siphonic Drainage

Roof drainage

It's important to note that no matter which manufacturer you choose, all siphonic systems work the same way.

The following are the main benefits of a siphonic system:

  • The majority of the drainage pipework is horizontal at a high level, freeing up space inside the building.

  • In order to speed up groundwork, the system is put in place later in the construction schedule.

  • Is compatible with a green roof drainage system.

  • The amount of underground pipework in the building has been reduced to a virtual minimum.

The drawbacks include:

  • Poorly maintained systems are more prone to blockage.

  • It's possible for systems to be a bit erratic (only usually an issue if used in the wrong type of building)


For large buildings, siphonic roof drainage is the best option; however, regular maintenance is necessary to keep the systems operating at their peak efficiency.

Sizing the pipework, achieving system flow balance, and starting the system quickly all play a role in how quickly a siphonic system works. It is the responsibility of specifiers to ensure that all siphonic companies are able to produce calculations showing system balance, fill time, and gutter performance.