Siphonic roof drainage systems: no space to breath Marc Buitenhuis Hydraulic research engineer Akatherm International BV, Panningen, The Netherlands 23-11-2009AbstractThis article describes the main difference between conventional and siphonic roof drainagesystems, ie: the prevention of air ingress in siphonic roof drainage systems. The airbaffle ofthe roof outlet is the most important part to eliminate the air from the system. The airbaffleconsists of a disc to prevent direct air ingress, but also fins to prevent the occurrence ofvortices which can drag in air.The above mentioned phenomena and their prevention will be discussed in detail in thisarticle. 1. Introduction Special precautions that have to be taken to avoid air ingression are discussed belowSiphonic roof drainage systems aredesigned to create full bore flow. Expelling 2. Avoiding air ingress into siphonicair from the system leads to a roof drainage systemsdepressurized system that sucks the waterfrom the roof at high speeds. Full bore As stated above there are two ways for theflow can only occur when air is eliminated air to enter the system. The direct way isfrom the system. This means that air not the inflow of air that is dragged in by theonly has to be removed from the system, friction at the water surface. This is easilybut air ingress also has to be prevented. To solved by separating the water surfaceprevent air from entering the system one from the water flowing into the roof outlet.has to concentrate mainly on the inlet of A disc, called air baffle, directly over thethe system. When water flows in an open roof outlet opening will do the job from thesystem the friction at the water surface will moment the disc is fully covered by water.drag the air along with it. Thus when the Beware that this is not when the overallwater is flowing into the inlet of a roof water level is at the same level as the discoutlet air is being dragged in as well. height, since the suction pressure in theEven when the water fully covers the roof vicinity of the roof outlet will graduallyoutlet opening, air can still enter the decrease the water level as the flow issystem through a vortex. The suction accelerating towards the outlet.pressure in the siphonic system can be sostrong that a vortex is being developed thatis able to drag in air (like the well knownbath tub vortex with its slurping sound).