The main purpose of a rainwater system is:
Rainwater harvesting
This can save a householder large volumes of mains water and reduce the pressure on this resource.
These storage cisterns are installed below ground and collect rainwater from gullies. The average size may be 5,000 litres.
The fall of the gutter
Most manufacturers interpret this standard as a fall of:
1:600
(1mm drop every 600mm length)
25mm in 15m
This is now an accepted ratio in the trade, and allows the flow of rainwater to the running outlet in an efficient manner. It will also keep the gutter relatively silt free.
Modern systems will work even if there is little or no fall. This is used more often on large, commercial roof lines.
How to install guttering correctly