Gravity Drainage System (Roof Drainage)

As defined in BS EN 1253-1-2003, Gravity drainage system is a system where flow is caused by gravity and where the pipe normally operates partially full.

Gravity roof drainage system is a drainage system that employs gravity for water to flow from the roof to the pipe system and finally to a discharge location. The fundamental design principle is open-channel without the presence of pressure.

The typical criteria to be considered in gravity roof drainage systems are;

1. Water depth around the roof outlet – The capacity of gravity roof drainage system is typically limited by the water level around the roof outlets. This is because water needs to rise to a certain depth to fall through the roof outlets.
2. Leaf strainer – Leaf strainers are required in most countries as it is a means to prevent foreign objects from entering the pipe system. However leaf strainers in roof outlets have a significant reduction factor to the flow rate. In accordance to BS EN 12056-3, a reduction factor of 50% has to be applied to the effective capacity of the roof outlets.
3. Vertical downpipes – The important criteria for vertical downpipes is the fill rate. Most standards limits the fill rate of vertical downpipes to a maximum of 0.2 to 0.33. This is to prevent the fluctuation of pressure inside the pipes.
4. Horizontal pipes (with slope) – BS EN 12056-3 allows the filling degree of maximum 70% for horizontal pipes (defined as drains which are calculated based on the principle of open channel flow). In this, the slope/gradient of the pipe is the key factor that contributes to the drainage capacity. Typical accepted gradient ranges from 1:40 to 1:100. If the gradient is not steep enough, i.e. less than 1 in 110, then the pipe could block if solids slow down and become stranded.

The design of the gravity roof drainage system has to take into consideration of the above 3 criteria and the largest diameter required by anyone of the above is adopted as the necessary pipe size for the design flow rate. In accordance to BS EN 12056-3; it is not allowed for pipe size to reduce in the direction of flow because this will induce pressure within the pipe system.

References

1. BS EN 1253-1:2003
2. BS EN 12056-3:2000