Overview of Traditional Drainage

Overview of Siphonic Drainage

Open outlets. The most common, but least efficient roof drainage solution

“Closed” Outlets. Drain has an air baffle that promotes “full-bore” flow.

Pitched horizontal piping. Gradient of the pipe induces “downhill” flow to the point of discharge.

Horizontal piping is not pitched. Flow is induced by natural hydraulic action of siphoning.

Atmospheric pressure throughout the system. Potential energy is not exploited.

When system primes, the piping depressurizes. Atmospheric pressure pushes the water into the drains with force of 14.7 pounds per square foot.

Capacity is limited by the size of the drain and the depth of water around it during a rain event.

Capacity is determined by the piping system and the height of the roof above the point of discharge. Makes full use of gravity (i.e. potential energy).

Piping is about 1/2 to 2/3 full. Only 1/3rd full in vertical pipes.

Piping primes and operates 100% full (i.e. full-bore flow).

Water flow is a function only of drain rim diameter and slope.

Water is drawn through the outlets and piping faster than gravity “channel flow” alone due to negative pressure.

Inefficient material use due to pipe diameters sized to be only part full even during maximum storm intensity.

Lower material expenditures due to smaller pipe diameters.

If below grade, the longer the horizontal run, the deeper the pipe trench must go to accommodate pitch thus requiring additional costs for excavation, bedding, and backfill.

Pipe inverts leaving the building are at a minimum, making deep trenching on the site beyond the building unnecessary.