Rainwater Harvesting.

As a rough calculation, roof area in square metres multiplied by annual rainfall in millimetres gives litres per year, then apply a loss factor of around 0.8 for run-off and filtration. A 100m² roof in a 700mm rainfall area yields roughly 56,000 litres a year.

That figure is annual, not available at any one moment — storage sizing depends on the gaps between rainfall, not the total.

Common practice is to size for around 5% of annual yield, or roughly 18 days of demand, whichever is lower. Oversizing wastes money because the tank never fills; undersizing means overflowing through winter and running dry in summer.

For garden-only use, a much smaller tank is usually sufficient since demand is seasonal and coincides with lower rainfall.

Toilet flushing, washing machines, garden irrigation, vehicle washing and general outdoor use. It must not be used for drinking, cooking or bathing without full treatment.

Any rainwater plumbing must be clearly marked and physically separated from potable supply, with backflow prevention where the two systems meet.

Above-ground tanks are cheaper, easier to install and simple to inspect, but take up space and are vulnerable to frost and algae from light exposure. Underground tanks are more expensive but keep water cool, dark and frost-free, which improves quality significantly.

For anything beyond garden watering, underground storage generally gives better results.

At minimum a leaf and debris filter before the tank, plus a calmed inlet to avoid stirring sediment. Systems supplying indoor use typically add finer filtration and sometimes UV treatment.

A first-flush diverter, which sends the initial dirty run-off to waste, makes a noticeable difference to stored water quality.

Any system supplying indoor fixtures needs a mains top-up so supply is maintained in dry periods. This must be via an air gap or approved backflow device — never a direct connection.

Garden-only systems can run without backup, since running dry is an inconvenience rather than a failure.