Such systems can be designed for any type of roof or building, although they are more commonly used on low-rise buildings with a large roof area such as warehouses, airports and schools.

The components of a siphonic drain. All photos courtesy of Zurn Industries.


In this day and age, efficiency plays a major role in our society. From finding a car with the best fuel mileage to using the brightest bulb that operates on the least power, people are able to save time and money by using a more efficient solution. So why should solving our plumbing problems be any different? While siphonic drainage is hardly a new technology to parts of the world, it is relatively new to the U.S. market and is one of the most efficient drainage solutions available.

Siphonic drainage operates with the piping system completely full of water, whereas a conventional piping system contains roughly 1/3 to 2/3 of water. The remainder of the piping is filled with air under atmospheric pressure. By increasing the pipe capacity, and in turn, efficiency, the pipe diameters can be reduced.

By combining multiple drains into one collector pipe, less material is used and less ground excavation is necessary.

How It Works

Siphonic drainage is made possible by the addition of an engineered baffle placed in the sump of a standard roof drain. The purpose of this baffle is to prevent any air from entering the pipe by forcing the water straight down instead of allowing it to vortex around the sump/outlet.

Once the water is forced downward into the tailpiece of the drain, the absence of air and atmospheric pressure creates a negative pressure in the pipe and the siphonic effect begins. The water then moves into a horizontal collector pipe where it flows towards the downspout, with the pressure constantly decreasing. Before the negative pressure reaches a level that could possibly damage the pipe, the pipe size is increased to alleviate the pressure. Eventually, the water hits a vertical downpipe to carry it to the sewer system.

As the flow moves down this pipe, the pressure starts to increase again and approach zero. Before the pressure reaches zero, the pipe size must be increased and the siphonic action is broken at this point. This allows the water to enter the sewer at the same rate as a conventional system would, and by increasing the pipe diameter, the flow velocities drop to an acceptable level that will not damage the sewer system.

Most manufacturers sell a drain with a tailpiece already attached to the drain; this tailpiece is customized to the drain and cannot be shortened or the drain may not prime correctly.

The driving force behind a siphonic system is referred to as the disposable head, which is the difference in height between the fluid level on the roof and the discharge point. This is the potential energy available to the system and helps create the siphonic action after the system is primed. In conventional drainage, the only potential energy available is the amount of water built up on the roof. The increased energy in the siphonic system assists in the higher flow velocities and increased discharge value of the system.

It is important to understand that a siphonic roof drain system will not always be operating with siphonic action during a rainfall. As rain starts to fall on a rooftop and enter the drains, the siphonic drains will initially act in a conventional manner. Air will continue to enter the piping until rainfall increases and allows water to build up in the sump of the drain.

As water builds up on the roof, the piping system is slowly purged of any air entering the system. The number of air pockets in the system will decrease until the piping is completely full of water. If the current rainfall intensity is less than the designed roof capacity, the water will quickly drain off the roof and the cycle will start over again.

Design Considerations

Siphonic drainage systems can be designed for any type of roof or building, but it may not always make much economical sense to utilize them. Because it is necessary for a fair amount of negative pressure to build up in the horizontal piping, siphonic designs for taller buildings become complicated. If the building is tall but not that wide, the required pressure build up makes it difficult to design a siphonic system. It is for this reason that siphonic drainage systems are more commonly designed for use on low-rise buildings with a large roof area such as warehouses, airports and schools.

When designing a siphonic piping system, it is helpful to have siphonic design software to assist in the calculations. The key to designing an efficient siphonic system is to create a system where the residual head pressure is as close to zero as possible. The residual head pressure is the difference in the disposable head pressure and the energy losses in the system.

Examples of energy losses include friction in the pipe, fittings, and the drain body itself. Values for these frictional losses are factored into the calculations of the software and can also be obtained from the fitting manufacturer if hand calculations are attempted by the engineer.

The hydraulic calculations can become quite cumbersome when done by hand so the software is really a time-saving tool. Necessary inputs for most software programs include: roof area, roof dimensions, rainfall rate and pipe lengths. The software program will then calculate the required pipe diameters and will detail exactly what is going on in each pipe section. Pressures, flow velocities (ft/s), and the flow rates (gpm) within the drainage pipes are monitored to check the effectiveness of the design.

Most of the software packages available also include built-in checks to ensure that the design will run correctly. Pressure in each pipe section is monitored to prevent damage to the piping system. Maximum velocity and drainage capacity are monitored at the discharge point to let the user know how much water will be entering the sewer and at what rate. Minimum flow velocities are also checked throughout the system to verify that the system will remain self-cleaning.

After the system is designed, the software should output a comprehensive data chart, as well as a graphical layout of the system, for use by the engineer or installer. This layout usually consists of a 2D line drawing that details the pipe lengths, diameters and orientations. When installing the system, the layout provided by the software must be rigidly followed. Any last minute changes to the piping layout, no matter how small they are, must be recalculated in the software to guarantee that the system stays in balance.

Most standard roof drain options are also available for siphonic roof drains.

Installation And Maintenance

A siphonic system is considered an engineered system by the plumbing codes. As such, certain precautions need to be taken when installing it. ASPE Design Standard 45 has a nice basic layout for installation procedures, as well as material and connection standards for the piping. Some important installation tips:

  • All reducers should be of the eccentric type, with the crown of the pipe installed level.

  • All branches should be created using a 45-degree lateral wye.

  • All 90-degree bends should be created using (2) standard 1/8 (2 x 45 degree) bends.

  • All horizontal pipes should run flat with zero slope.



    Like any roof drainage system, regular maintenance of the rooftop is required to ensure optimal performance of the system. Large debris caught on the outside of the dome or gravel guard of the drain will need to be removed periodically to allow water to enter the drain body directly. Small debris will be flushed through the system when siphonic action occurs.


    • Advantages Of Siphonic Drainage

    Besides allowing for the most efficient use of the piping material, a siphonic drainage system offers other advantages to the engineer and/or contractor:

  • The reduced diameter of horizontal and vertical pipes will lower building costs.

  • Horizontal piping is installed without pitch, saving ceiling space for increased storage or building capacity.

  • There is an ability to tie more drains together in one horizontal run and reduce the number of downspouts.

  • The downspout locations are flexible and can be placed conveniently around or within the building.

  • Because the downspout placement is flexible, they can be placed in a location that will require the installer to do less costly site excavation.

  • A siphonic system is considered a self-cleaning system due to the higher flow velocities in the piping. Any debris that enters the piping will be quickly flushed through the system or knocked loose by the fast-paced water. This eliminates the need for cleanouts in the siphonic portion of the system.

  • LEED points can be awarded for a siphonic drainage installation.


    • What The Future Holds

    Like any new product entering the market, no one wants to be among the first to try out the system when old, tried and true methods have served them well for years. But in today’s competitive market, every advantage should be utilized to save money throughout the process.

    Siphonic systems have been used effectively throughout Europe for the last 30+ years and many advances have been made in the way they are designed and installed. These European companies have now ventured with U.S. manufacturers to share their knowledge and designs on the subject.

    For additional information on siphonic drainage, consult ASPE Design Standard 45: Siphonic Roof Drainage.