Rainwater harvesting & water storage: a plain-English guide
How harvesting works, how to size a tank, when harvested water is safe to drink, and the rules on boreholes and greywater. Then run your own numbers and get quotes from vetted suppliers.
Reviewed on 2026-06-24. General guidance only — water quality and local rules vary. Confirm potability with proper testing and check your municipal by-laws and the Department of Water & Sanitation before acting.
How rainwater harvesting works
Rain that lands on your roof runs down the gutters into a downpipe, through a screen and (ideally) a first-flush diverter, and into a storage tank. From there a pump or gravity feeds it to taps, the garden or the toilets. The harvest you can expect follows a simple formula: roof area in m², times annual rainfall in mm, times a runoff factor of about 0.8 — one millimetre of rain on one square metre is roughly one litre.
Why you need a first-flush diverter
The first rain after a dry spell washes dust, bird droppings and debris off the roof. A first-flush diverter throws away that initial dirty flow before water reaches the tank, dramatically improving quality. It is one of the cheapest, highest-impact parts of a harvesting system — never skip it.
Sizing your tank
Tank size is a trade-off between how much you can capture, how much you use, and your budget and space. For backup storage, work from the essentials: people × days × litres-per-person-per-day. For harvesting, a bigger tank captures more of a heavy downpour but costs more and takes more space. Tanks come in common sizes from a few hundred litres up to 10,000 litres and beyond; many homes link several together.
Is harvested rainwater safe to drink?
Not without treatment. Rainwater collected off a roof can carry bacteria, dust and contaminants. It is excellent for gardens, toilets, washing and irrigation as-is, but to drink it you need proper filtration and disinfection (such as fine filtration plus UV or an approved treatment step). When in doubt, keep harvested water for non-drinking use.
Boreholes and greywater — know the rules
A borehole can be a powerful backup, but drilling and water use are regulated: depending on the volume and purpose, you may need to register the borehole and obtain a water-use authorisation from the Department of Water & Sanitation, and many municipalities require registration and metering. Greywater reuse (from baths and basins) is encouraged for irrigation but has its own health rules. Always check your local by-laws before you start.
Frequently asked questions
How much rainwater can I collect from my roof?
Use: litres ≈ roof area (m²) × annual rainfall (mm) × 0.8. One millimetre of rain on one square metre of roof is about one litre. For example, a 120 m² roof where it rains 700 mm a year could capture roughly 67,000 litres over the year, before allowing for overflow when the tank is full.
What size water tank do I need for backup?
Work from essential use: people × days of backup × litres per person per day. Around 100 litres per person per day covers drinking, cooking and basic hygiene, so a family of four wanting a week of backup needs roughly 2,800 litres — about a 5,000-litre tank once you allow headroom. Normal full household use needs far more.
Can I drink rainwater harvested from my roof?
Not safely without treatment. Roof-harvested water can carry contaminants and needs proper filtration and disinfection before it is potable. It is ideal as-is for gardens, toilets and washing.
Do I need permission for a borehole in South Africa?
Often, yes. Depending on the volume and use, a borehole may require registration and a water-use authorisation from the Department of Water & Sanitation, and many municipalities require registration and metering. Check your local rules before drilling.
Plan your system, then get real quotes
Start with honest numbers — then let NanoLeap match you with vetted suppliers for tanks, pumps, filtration and boreholes.
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