01Measure
the catchment in plan
Use the horizontal projected roof area draining to the chosen gutter,
not the sloping surface area. One millimetre of rain on one square metre represents one litre before losses.
Some rain wets surfaces, spills, or is diverted before reaching the tank, so a collection efficiency factor
keeps the estimate realistic. Roof material, debris, and maintenance affect both quantity and quality. Do not
collect from a roof with unsuitable or uncertain materials without appropriate advice. Divide a complicated
roof into simple rectangles and include only the sections connected to your inlet. Seasonal rainfall patterns
matter more than an annual total when the garden needs water during a long dry spell.
02Keep debris out and overflow planned
A gutter screen
and a covered, screened inlet help keep leaves
and insects out of storage. A first-flush diverter sends an initial portion of runoff away from the tank,
reducing some accumulated roof debris; it does not make the remaining water drinking quality. Keep the tank
covered, stable, and inaccessible to children. Water is heavy, so place the container on a base designed for
its full load and follow the tank maker’s installation guidance. Route overflow away from foundations and
neighbouring property using an appropriate drainage arrangement. Inspect screens and gutters regularly, and
check local requirements before altering drainage. A full tank during a storm still needs a safe path for every
additional litre.
03Match storage to
garden use
Stored roof runoff can be useful for ornamental garden watering where
local guidance permits. Avoid treating untreated collected water as drinking water, and follow local advice
for edible crops or any use involving contact with people. Keep a separate, clearly identified watering setup
and never create an unprotected connection to drinking-water plumbing. Water near the soil when appropriate
rather than spraying it into the air. The collection estimator reports potential inflow before the storage
limit; it does not know how much space is already available in your tank. Compare expected collection with
actual use over several weeks. A larger tank cannot create rainfall, and a small tank may still provide useful
water between ordinary showers.