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The principle of the siphon has been recognized for ages. A siphon is created by a tube or other type of conduit filled with the fluid to be siphoned, thereby creating a continuous and closed path. In any siphon, the discharge end of the conduit must be lower than the level of the fluid in the source reservoir. Atmospheric pressure at the reservoir surface becomes the driving force pushing the fluid through the tube to the lower point of discharge. Most examples of siphons include an inverted “U” shape, this configuration is necessary to lift the fluid out of the source reservoir that cannot be tipped, much like a car’s gas tank shown above. However, the actual path of the siphon tube is irrelevant to the fluid’s ability to flow. In the practical case of siphonic roof drainage, the drainage piping is installed in the simplest way possible: flat and level. This allows the piping to drain completely when it is not raining and then to prime full into a continuous and closed path on its own during a rain event.

People know that it is necessary to prime the tube in order to achieve the siphonic flow. Ask anyone who has received a mouthful of gasoline while trying to siphon gas from their car’s gas tank to fill a lawnmower. Therefore, the ability of a siphonic roof drainage system to prime itself might be counter intuitive. However, these systems prime up simply because the roof drain design and flat installation make full-bore flow occur. This tendency is exactly why plumbing codes have a set of rules for venting of sanitary waste systems. Without venting, water flow through even a pitched or vertical pipe will create zones of reduced or increased pressure that defeat fixture trap seals and under the right conditions can cause instances of full-bore flow. Still not convinced? The next time you flush a siphon-jet water closet consider why the water gets drawn out of the bowl.