Siphonic Roof Drainage Systems Webinar
Question and Answers
Q1) Any concerns for pipe restraint or hanging or water hammer within the piping at full bore conditions?		  
The mass flow handled is the same as that of conventional piping (it only rains so much on a roof regardless of the drainage method). Reactive forces are really not much greater than "gravity" piping. However, the Cast Iron Soil Pipe Institute (CISPI) recommends lateral bracing of pipe hangers that are 16 inches or greater in length. There are also recommendations in ASPE 45. Believe it or not, there are engineering reasons for bracing any piping system, but unfortunately these rules are seldom enforced.

Q2) Are there any concerns that the higher design velocities could result in pipe erosion and premature failure?
No. If the velocity were to be maintained in a continuous duty, it might be a concern. However, roof drainage systems are in operation only a small percentage of the life of a building.

Q3) Are there any Pressure Considerations for PVC DWV Piping systems?
No. The range of pressures experienced (both positive and negative) are within the capability of PVC Schedule 40 pipe. However, cellular core PVC is not recommended because there is no data as of yet on its ability to withstand negative internal pressures.

Q4) Can you explain how the system is "primed"? Is it at the roof level or inside the piping system?
Inside the piping system.

Q5) Can you please explain how the system works without a pitch?
The same way that a siphon can draw water uphill to drain out an aquarium or gas tank. Although siphonic roof drainage systems don't go uphill, they still drain through flat piping under the same hydraulic principle. When it stops raining, the water level in the pipe drains down to zero and empties out.

Q6) Could you expand upon why siphonic roof drainage goes hand in hand with Rainwater Harvesting?
Siphonic design is an efficient means of conveying the rain water on the roof to the point of collection. The money saved with this method offsets the cost of the rainwater harvesting system as well.

Q7) Do the pipes have to be braced/restrained due the velocity of fluid in the pipes?
The mass flow handled is the same as that of conventional piping (it only rains so much on a roof regardless of the drainage method). Reactive forces are really not much greater than "gravity" piping. However, the Cast Iron Soil Pipe Institute (CISPI) recommends lateral bracing of pipe hangers that are 16 inches or greater in length. There are also recommendations in ASPE 45. Believe it or not, there are engineering reasons for bracing any piping system, but unfortunately these rules are seldom enforced.

Q8) For a 12 story application (175' tall) how is the terminal velocities avoided when the siphonic system is "full"?
Flow velocities are a function of pipe sizing and can be controlled to a point. My approach to high rise design is to bring the siphonic stack two to three stories down and then "break" it into a full size (conventional stack) where the water assumes terminal velocity the rest of the way down and avoids excessive pressure drop at the top of the stack.

Q9) How about leaves and other debris on the roof?
This is a roof maintenance issue that is addressed by the leaf guards on the roof drains and by the building owner. This is true for conventional roof drains and siphonic roof drains alike.

Q10) If there is a dynamic shift in the water stored on the roof, can the drain experience fluid hammer?
There is no quantity of water "stored" on the roof. Whatever falls on the roof is actively drained. The transition in and out of full-bore flow is gradual and there are no water hammer effects.

Q11) Is it feasible to mix drain system types on a multi-roof project, some conventional, some siphonic?
Yes. For example, a section of roof could be designed as siphonic and then tie into a conventionally sized stack. I've done systems where smaller roof surfaces drain by conventional downspouts down to a lower and larger roof surface with siphonic drains.

Q12) Is the pipe velocity a concern with wearing away the interior of the pipe?
No. If the velocity were to be maintained in a continuous duty, it might be a concern. However, roof drainage systems are in operation only a small percentage of the life of a building.

Q13) Is there an approximate percentage of savings in a siphonic system vs. a standard roof drain system. Does that take into consideration for pipe insulation where that would be necessary?
Many claim savings of 35% to 65% savings in cost. The actual amount does depend on specific factors like amount of trenching and back fill eliminated, exterior piping saved and other offsetting costs like insulation and hangers.

Q14) Just to clarify, it is okay to header the secondary siphonic system, just not connect it to the primary?
It is ok to manifold secondary drains together as long as those drains would be expected to go into overflow at the same time. Roof conditions can isolate drains from each other. If they are, they need to be piped separately so they can operate independently.

Q15) Please review how the siphonic action and self-priming of the system occurs.
Please see the instructional information at the www.jrsmith.com. Also, an article on the priming process will be appearing in PS&D Magazine later this summer.

Q16) What are the tolerances associated with design inputs and the actual installation?
This is addressed in ASPE 45. There is leeway on the order of +/- 4 to 6 inches from dimensions shown on the drawings.

Q17) What computer programs are available for sizing siphonic roof drain systems?
You can download the software (at no charge) at http://www.jrsmith.com/siphonitec.

Q18) What kind of support system is required as compared to a conventional piping system?
Normal pipe support methods and means for pipe full of water. Basically the same procedures for gravity pipe.

Q19) What method is the most effective way of reducing the velocity of storm water that is daylighting or when the piping is tied into the civil storm piping?
Increase the pipe size to the equivalent "gravity" or open channel flow size thereby "breaking" the siphonic system and resuming a slower open channel flow.

Q20) What software do you use and how much does it cost?
You can download the software (at no charge) at http://www.jrsmith.com/siphonitec

Q21) Where can the design software be obtained?
You can download the software (at no charge) at http://www.jrsmith.com/siphonitec

Q22) Where there are multiple roofs of different elevations, does each elevation of roof need to have a completely separate system to the point of discharge?
Not necessarily. Each roof elevation would need to tie in at a point of positive pressure in the piping. This is addresses in ASPE 45.

Q23) Will it be an alternative in hospitals also? I know the codes are different then other buildings.
It can be used in any type of building regardless of its use or occupancy.

Q24) With the Boston Convention center what percentage do you think you saved using a siphonic system versus a conventional system?
About 40%, but that was just inside the building. We also saved an estimated 10 to 15 million dollars on not having to build a pumping station to get the storm drains to the pipe left at the property line.

Q25) Would you consider siphonic drain system for area drains serving an industrial plant with water tanks that may be subject to failure?
I don't have the full picture of where these area drains would be. If they are at grade level, then there would be no vertical drop to drive the siphonic action. A conventional drainage system would have to be used in that case. If there is a vertical drop, siphonic could be used. In either case, you have to evaluate how much water is contained in the tanks and how much depth would develop while the (conventional or siphonic) drains are draining.
Q26) Any spreadsheet pipe sizing, calculation table available?
No, you basically start off sizing the system as a conventional system by referencing the sizing charts in the applicable code. Then you cut the pipe size in half and this would be your starting point for beginning the design for a siphonic system. And SiphoniTec® software is available to aid in validating the design.

Q27) Are clean-outs required in locations as conventional systems?
Yes, no different where ever the code would require them.

Q28) Are there any cold weather concerns with siphonic roof drainage systems that require different solutions than conventional roof drainage systems?
No, only the same concerns as associated with conventional drainage.

Q29) Are you going to make the PowerPoint presentation slides available to us?
The Webinar Presentation will be archived on ASPE website and the PowerPoint Slides and a Flash File of the Webinar are available on Jrsmith.com. Click here to view and download.

Q30) Can you define siphonic?
The act of siphoning.
Q31) Do you know if New York City makes an exception for the plastic material needed for siphonic systems?  Plastic is generally not accepted in NYC.
I assume you are referring to plastic piping? If so the siphonic drains can be used with any of the common piping materials used in conventional systems, cast iron, PVC, ABS, copper or galvanized steel pipe. So, no plastic piping is not the only piping that can be used with siphonic.

Q32) Does Siphonic roof drains eliminate overflow roof drains?
No, overflow drains would still be required.

Q36) Has IPC approved the Siphonic Roof Drainage System?
It has been submitted to be added for the 2009 code update. But the IPC does currently have a provision for "Alternative Engineered Systems" which the siphonic drainage has been submitted under for many projects and has been approved by local code authorities.

Q33) If you are using sizing software, why would a licensed PE be required to design?
Only licensed engineers can stamp and signed plumbing systems. Anyone under the PE's supervision can do the design using the software, but the PE still needs to review and stamp the drawings and specifications.

Q34) Is there a Program that will help design the system?
Yes, the SiphoniTec® software is available to aid in the design of the siphonic system. The SiphoniTec® will validate the system design by calculating the resistance through out the system and determine if the drains are balanced, draining at the same rate.

Q35) Are there any states or cities that discourage use of siphonic roof drains drains?
No, to our knowledge there are none that discourage the use of siphonic drainage.

Q36) Is this system applicable on a parking garage?
Yes. For the top deck receiving rain water, a siphonic system can be used. However, the drain needs to be placed in a sump to receive the water and not be subject to damage.

Q37) What does a siphonic roof drain look like?  How's it different from a normal roof drain?
It is very similar to a conventional roof drain. The only difference is the siphonic has an air baffle which eliminates the entrance of air into the piping when is becomes sealed over with water. You can find the drain on the Jay R. Smith website at www.jrsmith.com.

Q38)What happen to the rain water when we have a light rain and the pipe is not filled?
It is drained off conventionally just as with a standard conventional drainage system if the rainfall rate is not enough to fill the piping and go siphonic,

Q39) When does the system become "primed"? When all drains are covered with a layer of water?
Yes, when all drains on a horizontal lateral are covered over with water to prevent the entrance of air into the piping.

Q40) Why does ASPE #45 restrict the allowable flow to 70% of the drains rated capacity?
This comes from ongoing European practice. It has been shown to be advantageous to allow reserve capacity in each drain in order to handle extra water should one or more drains become partly or fully clogged. By all rights, this should be normal practice in conventional roof drainage as well.

Q41) Why must drains on siphonic secondary system be piped separately?  Isn't it possible for rain to be falling on one end of the roof and not on the other resulting in a similar condition in the primary system?
The secondary drains would be open (not draining) the majority of the time thus allowing air to be drawn into to the piping thus never allowing the primary drains to go siphonic.