Siphonic Roof Drainage Installation

The major benefit of siphonic roof drainage (and the original inspiration for its invention) is the reduction in pipe diameter to achieve the same volumetric flow of water off of a roof. This is achieved by exploiting the kinds of flow patterns and behaviours of the flow of water through pipes that result in pressure fluctuations and priming to a full bore condition. These behaviours are systematically avoided and prevented in the design of traditional sanitary drainage and roof drainage systems.

The traditional design methods of sanitary waste and roof drainage piping systems limit the level of water in piping to leave a volume of air at atmospheric pressure to exist in every part of the system. For horizontal pipe runs, this level is about 50 percent water and 50 percent air. Roof drainage may go as far as 67 percent water to 33 percent air. Sanitary waste system design includes a set of rules for venting to ensure an atmospheric condition throughout with a variation of about 1 inch of water column in either positive or negative to avoid siphoning of trap seals or expulsion of sewer gasses through drains.

Siphonic Drainage Adaptability

If a conflict is encountered and the only resolution is to modify the siphonic roof drainage piping, the contractor must contact the engineer with their proposed solution. The engineer needs to then update the hydraulic calculations to determine the impact on system balancing. Any further changes or adjustments revealed by the calculations should then be noted and sent back to the contractor as an authorised modification. However, experience has shown that siphonic systems are very flexible and accommodating.

Pipe and Drain Protection during Construction

Drains may be used as temporary drainage, but the contractor needs to take care to ensure significant construction debris does not become lodged in the piping. Also, if a roof deck is constructed of poured concrete, roof drains must not be used for temporary drainage. Cement dust and other residual materials can enter the piping and cause a permanent blockage. Therefore, the roof drains should not be used for drainage until the roof waterproofing system has been put down over the concrete.


The 5 Myths and Misconceptions of Siphonic Drainage

Siphonic systems drain water off of the roof “faster” than traditional piping.

Reality: Although higher operating velocities are achieved, the drainage capacity is a function of pipe size. They can drain as quickly or slowly as desired. Siphonic roof drainage is very good for controlled flow requirements.

There is standing water on the roof at all times in order to maintain a siphonic operation, even while not raining (i.e., the pipe is always full).

Reality: When it is not raining, the roof and piping are dry. When it rains, a layer of water develops on the roof, but in the same way as with traditional atmospheric systems. Actually, this layer of water is typically less for siphonic systems.

Water builds up on the roof until a critical level is reached and then the drains “open up” to siphon the water off.

Reality: Water build-up on a flat roof or in gutters is not any different than traditional systems. Transition from partly full to full bore is a smooth transition. This property is a tested parameter for siphonic drains (15 second rule). Also, the drains are fixed and without moveable parts.

There must be valves, utility connections or mechanical controls to make the siphon work.

Reality: There are no valves, controls, regulators or moving parts of any kind. The system consists only of drains and piping. Siphonic systems prime due to natural hydraulic action.

Design Flexibility

The flexibility in siphonic roof drainage system design exists on several facets. Independence of pitch allows for the design of horizontal runs above ceilings and other spaces with overhead limitations. The placement of stacks within a building is much more flexible as a result. While the physical properties of pipe design enable the plumbing engineer to achieve more effective designs, there is also the fact that significant changes can be made to a siphonic system during construction without dire consequences.

The review of the hydraulic equations in this training program could lead one to believe that the calculations are “critical” and inordinately rigid. It is true that accuracy is a vital part of the engineering practice. An engineer must use the most reliable and most accepted methods and exercise the highest standard of care. This applies to any mechanical system and siphonic roof drainage is no different. On the other hand, the practice of siphonic roof drainage design does not press an engineer into a higher and unreasonable standard.

As mentioned previously, siphonic roof drainage technology was conceived in the mid-1960’s well before the introduction of the personal computer. Although the use of computer software is the current standard of practice in siphonic roof drainage design, there was a period when these systems were designed with hand calculations using charts and tables much like how automatic sprinkler systems were once calculated by hand before sizing software became available. Although automatic sprinkler systems and siphonic systems designed by hand could be shown today to have errors and inaccuracies when reevaluated by computer software, they still worked adequately. In other words, the best level of accuracy is any designer’s goal, but small errors are possible if in fact they occur.

In any construction project, many changes can occur. Such changes can result in conflicts that necessitate design alterations. Contractors sometimes regard siphonic roof drainage as too rigid a system because it is engineered. Traditional roof drainage can be altered by a contractor without the designer’s approval as long as the prescribed methods of the plumbing code are followed.

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