Understanding roof drainage systems

One of the most important things needed to carry rainwater away from your building is a properly installed roof drainage system. There are two types of roof drainage systems available in the market.

1. Conventional Roof Drainage Systems (Gravity Systems)

Gravity roof drainage systems channel rainwater from the roof down through gutters, outlets and downpipes. One disadvantage of this system is the vortex action, which draws air into the pipe system and significantly reduces the drainage capacity by two thirds due to the large volume of air.

2. Siphonic Roof Drainage Systems (JAS Siphonic)

A siphonic roof drainage system differs from a gravity system in two main ways. First, the outlets are specially designed to restrict the entry of air. Second, the rainwater pipes are designed to run full-bored from gutter level to downpipe. The “priming” process, which is needed to make the system operate siphonically, allows water to discharge at a faster rate, thus clearing the gutter more quickly when compared to a gravity system. (See comparison)

Gravity System VS JAS Siphonic


Gravity System

JAS Siphonic


In gravity system, when water flowing in the downpipe it spirals the inner wall of the pipe (Vortex), and there is an air-filled core down the center of the water flow, resulting in an inefficient drainage

JAS Siphonic can restrict the entry of air into the system to create a full bore flow. Water flows with negative pressure, extremely efficient drainage.

Flow Condition

Approximately 1/3 of the cross section of the pipe is water and the remaining 2/3 is air

100% water (full-bore flow)

Drainage capacity
(Diameter 75 mm, 5 meter head)

Gravity flow

Flow Capacity = 1.65 liter/second
(Gravity With Leafguard Outlet)

10 times faster than gravity flow

Flow Capacity = 16.5 liter/second
(Single/Stack System)

Pipe sizes



Flexible in Design



Underground pipe



A Number of Manhole



Gutter size



The ability to run the pipe to storm tank




High (in case of large building)

Cost Saving up to 60%