Developed by two engineers in Finland, siphonic roof drainage made its first appearance in Europe in 1968. Unlike traditional roof drainage systems, a siphonic system is designed to operate with the piping completely charged with water during a rainstorm. A siphonic roof drain has a special insert that acts as an air baffle and anti-vortex vane such that only water is drawn off of the roof, not air. Several of these drains tie into a horizontal collector that is routed to a convenient point where it transitions into a vertical "downpipe." This downpipe, once it reaches the ground, is piped to a vented manhole or inspection chamber where the water is discharged at atmospheric pressure into the storm sewer.
Despite the popularity of this system in Europe and its increased application around the world, siphonic drainage is virtually unknown in the United States. Roof drainage is probably one of the most crucial of the fundamental building mechanical systems. For the most part, roofs in the United States have been drained of rainwater the same way as they have been for centuries. Building and plumbing codes clearly outline the minimum requirements for rainwater pipe systems drawing off of tried and true techniques.