Siphonic Roof Drains :: Introduction, Overview, & Anatomy of Siphonic Roof Drainage Systems built by Jay R. Smith and Engineered through Rainwater Management Systems

A siphonic roof drain looks much like a traditional roof drain. The distinguishing feature of a siphonic roof drain is the air baffle. This air baffle is engineered and tested to prevent air from entering the piping system at peak flows.

Other than the baffle, a siphonic roof drain has the same features as a traditional roof drain including a drain body, flashing ring, dome strainer, and fastening hardware.

In contrast to traditional roof drains, siphonic roof drains are not designed with a large diameter or deep sump bowl because their operation is by means of sub-atmospheric pressure generated at the under side of the baffle and outlet. The depth of water maintained on the roof is dependent only on the resistance value of the drain assembly while operating under siphonic conditions. Any viscous weir effect of the drain body becomes minor and the flow is determined by simple inertial hydraulic effect of flow from a high pressure (atmospheric pressure at the roof surface) to low pressure (within the piping system).

Unlike a traditional roof drain system, a siphonic system is designed to operate with the piping completely filled with water during a rainstorm. Several drains tie into a horizontal collector that is routed to a convenient point where it transitions into a vertical stack. This stack, once it reaches the ground, is piped to a vented manhole or inspection-chamber where the water is discharged at atmospheric pressure and low velocity into the storm system.

Siphonic Roof Drains from Jay R. Smith Mfg. Co.
Introduction After several years of prototype development and testing, Jay R. Smith Mfg. Co. is pleased to provide owners and the plumbing engineering community patent-pending, specified roof drain products for siphonic roof drainage systems. • Jay R. Smith Mfg. Co.’s siphonic roof drains promote full-bore flow within engineered siphonic roof drainage piping systems. • Fully tested and certified in accordance with ANSI/ ASTM A112.6.9 “Siphonic Roof Drains” and representing the first-line of specified roof drains complying with this American National Standard. • Cast of solid ASTM 48 grey iron and utilizing the same set of accessories already familiar to the plumbing engineer and installer, thus making specification and installation as easy as traditional roof drains. • The castings contain mainly recycled metal content making the products a part of a sustainable consumer cycle. • The low-profile nature of the baffle component of competitive siphonic roof drains can make them prone to quicker blockage by debris. Smith’s siphonic roof drains have a polyethylene dome strainer to help protect the baffle from debris accumulation and allow for the passage of water even if debris collects around the strainer base. This design makes our siphonic roof drains behave in the same manner as traditional roof drains in all types of rainfall and roof conditions.	Overview In a siphonic roof drainage system, Smith’s specially engineered and tested roof drain baffle allows and sustains negative atmospheric pressure in the connected piping and inhibits the admission of air into the piping system hence sustaining full-bore flow and higher flow volumes and velocities. The hydraulic balance in a siphonic roof drainage system is achieved by an engineer employing hydraulic calculations to ensure that the piping system fills up automatically in cases of moderate to heavy precipitation. The resulting full-bore or siphonic action allows for the installation of horizontal, i.e., level, drainage manifold piping serving multiple roof drains. Siphonic roof drainage systems are powered by and discharge to grade by means of a vertical stack into the point of discharge through the influence of gravity making them true gravity systems.

A siphonic roof drain looks much like a traditional roof drain. The distinguishing feature of a siphonic roof drain is the air baffle. This air baffle is engineered and tested to prevent air from entering the piping system at peak flows. Other than the baffle, a siphonic roof drain has the same features as a traditional roof drain including a drain body, flashing ring, dome strainer, and fastening hardware. In contrast to traditional roof drains, siphonic roof drains are not designed with a large diameter or deep sump bowl because their operation is by means of sub-atmospheric pressure generated at the under side of the baffle and outlet. The depth of water maintained on the roof is dependent only on the resistance value of the drain assembly while operating under siphonic conditions. Any viscous weir effect of the drain body becomes minor and the flow is determined by simple inertial hydraulic effect of flow from a high pressure (atmospheric pressure at the roof surface) to low pressure (within the piping system). Unlike a traditional roof drain system, a siphonic system is designed to operate with the piping completely filled with water during a rainstorm. Several drains tie into a horizontal collector that is routed to a convenient point where it transitions into a vertical stack. This stack, once it reaches the ground, is piped to a vented manhole or inspection-chamber where the water is discharged at atmospheric pressure and low velocity into the storm system.