Gravity Systems Gravity Rainwater systems rely on the properties of water and gravity for the motivating forces behind their operation. Water does not like heights. It tries to reach the lowest level possible and tries to spread out evenly over whatever surface is supporting it. This is exactly what happens when rainwater falls onto a roof and flows into a gutter. The depth of water accumulating in the gutter is the driving force, which causes the rainwater to flow towards the roof outlet. These outlets are merely holes in the sole of the gutter. As the rainwater enters the downpipes, air is also drawn in and a vortex action occurs reducing the efficiency of the system.
The most significant component within any rainwater system is the roof outlet. The capacity of the outlet determines the flow in the rainwater pipe and the depth of water in the gutter or on the roof Rainwater pipes are dimensioned to operate at atmospheric pressure and with only one third to one quarter of the cross sectional area occupied by water. Each outlet has its own downpipe, which conveys the rainwater to the underground drainage system, the design of such gravity systems is relatively straightforward.
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