Follow one drop from roof to use

A rainwater system is easiest to understand as a route with checkpoints. Identify the connected roof, follow each gutter to its downpipe, locate the inlet and then trace the outlet to the watering point. A large roof disconnected from the tank contributes nothing. Draw this route before comparing storage products; it often exposes the missing connection that changes a project.

Separate collection, storage and delivery

Collection describes incoming water. Storage describes water retained between events. Delivery describes whether the stored water can reach the intended task at a useful flow. These quantities must not be substituted for one another. A full barrel might have enough volume for a bed yet insufficient pressure for the selected emitters. Treat pressure and volume as separate design questions.

THE EXAMPLE AT A GLANCEOne rain event, three different volumes
Tank inflow576 L
Retained in tank400 L
Overflow176 L

Free space limits retained water, even when collection is higher.

60 m² × 12 mm × 0.80; 500 L tank, 100 L initially stored; no withdrawal.

A small, transparent example

Assume a connected horizontal roof footprint of 60 m², a 12 mm event and an illustrative overall capture factor of 0.80. Gross water is 60 × 12 = 720 litres; estimated tank inflow is 576 litres. If a 500 litre tank already contains 100 litres, only 400 litres of that inflow fit. The remaining 176 litres need a planned overflow route.

Decide what the water will replace

Write down an actual use before choosing hardware: watering ornamental beds, supplying a permitted outdoor wash point, or another locally allowed application. Separate non-potable outlets from drinking-water plumbing and obtain local advice for any building connection. Rainwater availability alone does not establish suitability for a task. A screen that removes leaves does not demonstrate safe drinking water.

Commission with observations

During the first suitable rain, observe inlet flow from a safe ground position. Compare the water-level rise with the event estimate and record any water spilling before the tank. Then time the outlet filling a known container. This simple commissioning log turns theoretical capacity into evidence about the installed system and provides a baseline for detecting blocked filters later.

What this model cannot promise

The example assumes one event, a fixed capture factor and no withdrawal while filling. It does not describe daily reliability, water quality, structural loading or storm-drain capacity. Use a time series for seasonal planning. Ask an installer to assess the foundation and overflow arrangement; a litre of water contributes approximately one kilogram of mass before the container itself is counted.

Before ordering

  • Confirm the contributing downpipes.
  • Measure the footprint rather than sloping roof surface.
  • List permitted uses.
  • Identify a safe overflow destination.
  • Allow access to screens and valves.
  • Check available delivery pressure.
  • Record who will maintain the system.