Beginner • Intermediate • Expert • Design
Beginner Level
The natural height of the building creates a negative pressure that pulls the water from the roof. Based on demand and pipe routing, a siphonic system is hydraulically balanced using roughly ½ the size diameter of traditional gravity drainage, with the pipe installed with no pitch.
Gravity drains require 2/3rds air mixing with 1/3rdwater to transport efficiently. Siphonic drains have a baffle plate eliminating air from entering the pipework; without the need for air the pipe diameter is roughly ½ the size.
Because of increased velocities in the pipework, there are 2 new bracing requirements (4 total)
A. Sway bracing every 30 ft
B. Pipe restraint 1ft away from every change of direction (i.e. 3 points for a WYE)
C. Pipe Support (Follow PVC or CISPI standard)
D. Bracket pipe to wall/column in the vertical every 10ft
Water always seeks to find its own level, its own low point. To visualize, if you pour water into a container, the water levels out the entirely of that container. If we poke a hole in the bottom of the container, the water will level to that hole.
No, there is nothing within the ASPE 45 Standard that requires a MIN vertical length. We aim to have between 2’ and 3’6” of initial vertical drop when possible.
Yes, siphonic drainage works on high rises. No there is not a MAX height limit. Siphonic drainage on high rise projects offers the benefit of eliminating vertical stacks and providing the most optimal routing to the civil connection.
A siphonic drain needs to have a MIN of 23gpm flow rate to even be considered for a siphonic option. Reason being, 1-1/2” pipe is the smallest allowed diameter by the ASPE 45 standard, and 1-1/2” takes 23gpm to reach full pipe capacity. MAX for a siphonic drain is 1820gpm on our MH-600” – that is the highest in the market. We recommend a MIN 50gpm when designing, MAX is up to designer.
No. If there are separate roof levels, they need to be on separate siphonic risers. After breaking back to gravity flow the separate systems can be combined.
Technically, a siphonic drain could be used on a dead flat roof; however, building a manmade flat roof is not really possibly, there will always be depressions. For insurance purposes as well, pitch towards the drain is required. Siphonic drains do allow for a less of a roof pitch where code permits.
Siphonic drains should be treated the same in cold weather conditions as traditional gravity would. If heat tracing cable is used normally, the same would be true for the siphonic design. No issue with snow.
When the rain event ends, the system will evacuate any remaining water within a minute.
There are no pumps in a siphonic system. The natural height of the building creates a negative pressure that pulls the water from the roof.
Please reach out to Brennan Doherty with any questions.
Intermediate Level
Siphonic drains will ALWAYS have less ponding than traditional gravity drains.
For gravity drains, once the ponding reaches a point where the weir collapses (and there is no longer air getting to a gravity drain), it actually starts losing efficiency, ponding at a quicker rate.
The increased ponding is also created when the leaves and debris collect on the side of a gravity drain.
IPC 2015
1107.1 General Siphonic: roof drains and drainage systems shall be designed in accordance with ASME A112.6.9 and ASPE 45.
UPC 2018
1106.2 Siphonic Roof Drainage Systems: The design of a siphonic roof drainage system shall comply with ASPE 45.
1106.3 Siphonic Roof Drains: Siphonic roof drains shall comply with ASME A112.6.9
ASPE 45 Standard: Design standard for rules and regulations governing siphonic roof drainage
ASME A112.6.9: Performance standard for siphonic roof drains
Siphonic drainage is an engineered system that must be stamped off by a licensed plumbing engineer. They provide calculation reports from the manufacturer’s design program to show compliance with the metrics laid out in the ASPE 45 Standard.
Yes. Pipe in a siphonic system runs dead level flat not only in the ceiling, but also underground. If coordinating with the civil scope, the siphonic action can travel more than 5’ outside of the building. The cost savings would be smaller diameter pipe and linear trench with a smaller width.
ASPE 45 7.9.3: Minimum velocity in horizontal piping sections shall be 0.9144 m/s (3 ft/s).
ASPE 45 7.7.5.4: In fully primed flow (i.e., full-bore flow), the velocity in the stack shall be greater than 2.2 m/s (7.2 ft/s) for stacks 150 mm (6 in.) and smaller. For pipes larger than 150 mm (6 in.), refer to appropriate testing data from drain manufacturers. Minimum velocity is a function of pipe diameter.
MAX velocity is determined by the balancing program which prevents the chance of cavitation, or pipe implosion.
No. Standard DWV or service weight cast iron are approved.
Distance is based on vertical height, and demand. The rough rule of thumb is for every 1 unit vertical, you can achieve 12 to 14 units horizontally, but there is anyways ways to manipulate a design to achieve more.
No. A siphonic drain requires the pipe diameters and lengths to be sized appropriately.
There has to be a vent to atmosphere, but it does not necessarily have to vent to the roof. The vent could be connected to the vent stack, which would be sized accordingly, or connected to an oversized area drain on a balcony, or out a sidewall hidden by a grille.
Any light rainfall, a siphonic system will act as a traditional gravity system. As the rain increases a siphonic system will have less ponding whereas the gravity system will start becoming inefficient.
Noise in a storm system is directly related to air in the pipework.
A. Stage 1 – light rain – equal noise to gravity
B. Stage 2 – drizzle – 5% louder than gravity
C. Stage 3 – storm – 5% quieter than gravity
D. Stage 4 – heavy downpour – 7% quieter than gravity – no air in pipe
Cast iron should be used with noise as a concern, never PVC. Insulation can also be used to help, but 5% swing either way is negligible.
No. Flow control drains limit the amount of water discharging from a roof. A siphonic drain evacuates the roof as quick as possible.
One the line is broken back to gravity flow, it can be combined with any other system. Traditionally if code allows for the overflow to combine with the primary, it is in the vertical stack.
The system would still function. If there is pitch there is a potential for air to be at the top of the pipe.
No. The leaves and debris are still entering the storm system, weather it is through the roof, or a curb inlet on the ground. This is not an issue.
Please reach out to Brennan Doherty with any questions.
Expert Level
A ponding curve is created during the ASME A112.6.9 performance test. Each drain has it’s own unique ponding curve. Based on the GPM design for the drain being used, we can understand what the ponding depth would be as the system enters full siphonic mode.
MH-300 Ponding Curve: https://www.dropbox.com/t/1CrAtXpbfQnbhFpe
Example: If an MH-300 is designed for 175 GPM, we would expect 1.55 inches of ponding when the system enters full siphonic mode, the entire pipe system is 100% full of water.
If designing drain areas under 300 GPM a standard 2” overflow is used. If the project uses over 300 GPM, the ponding curves dictate we should increase the water dam. The water dam height should also increase for gravity drains but is taboo topic.
Ideally the vent should be as close as possible; however, there is nothing within the ASPE 45 Standard that calls out a MAX length.
Anything changing from the calculated model more than 8” needs to be resubmitted for calculation.
Please reach out to Brennan Doherty with any questions.
Design Level
Yes, 23 GPM. GPM takes into account the area feeding the drain and the design rainfall for the project (location based). 2″ = 1,107 SF; 3″ = 738 SF; 4″ = 554 SF.
If a siphonic drain is within a parking sump, the area above will be draining gravity unless the water fills up the entirety of the catch basin at which point the surface of the parking lot would then be pulling the water siphonically towards the drain. This method will promote ponding to achieve the siphonic action quicker.
No. A siphonic drain is anti-clog and does not require a debris guard. One can be purchased as an accessory but serves no real purpose for a siphonic drain.
Yes. Because the drain can handle more demand, less siphonic drains can be used on the same gutter length compared to gravity – less vertical downspouts.
Same roof materials traditional gravity drainage is able to accommodate.
No proprietary pipe material. PVC or Cast Iron is what we see on 90% of our projects. Galvanized steel, Copper, and HDPE are also options in our program. Multiple materials can be used on the same design.
Standard fittings, they do not need any special pressure rating. Cleanouts at every change of direction are not required. Crosses are not used. Both eccentric and concentric reducers are approved. Standard duty no-hub bands are all that are required.
A siphonic system always breaks back to gravity flow at point of termination by increasing pipe diameter back to gravity sized flow, and reintroducing atmosphere.
Cleanouts are not required anywhere by the ASPE 45 Standard. Cleanouts at every change of direction should not be installed as they would create an air pocket. We recommend installing a spool type cleanout anytime they system is about to go underground to keep inspection easy.
No. Whatever the standard method to install a gravity drain, the same accessories are offered for siphonic drains.
The siphonic system will be ½ the diameter of the traditional gravity setup. Are there any verticals that can be eliminated? Any civil drainage that could be eliminated?
For a siphonic overflow the downspout nozzle acts as the atmospheric break, it is recommended to revert to gravity sized pipe before leaving the building. Adding any fittings or horizontal pipe will also create friction loss to slow down the water before exiting the building should the overflow be needed.
No. If working with the civil engineer, general contractor, or larger team it would be 100% be beneficial to the project, but is not required.
A siphonic design has to calculate how long it takes to fill the entirety of the pipework to achieve full siphonic mode. The standard is to have the system fill up under 60 seconds. If it was over 60 seconds, the system will still be functional, the case might be that the system has long runs of pipe.
Please reach out to Brennan Doherty with any questions.