Every rainy season, millions of litres of clean water fall on the roof of an ordinary rural house and run off the eaves into the mud. A few months later the same household is walking kilometres to fetch water, and the vegetable garden is dust. The gap between those two moments is bridged by three humble pieces of technology — a gutter, a tank and a first-flush pipe — and the arithmetic of what they capture surprises almost everyone who does it for the first time.
The arithmetic first
The rule is simple: one millimetre of rain on one square metre of roof yields one litre of water. A modest iron-roofed house of 60 square metres in a region receiving 900 mm of rain a year is rained on with about 54,000 litres annually — call it 45,000 after losses. That is 120 litres a day, every day of the year, falling on a roof most families let drain into a puddle by the door.
You will not capture it all, because storage costs money and the rain comes in bursts. But even a 2,000-litre tank, cycled — filling and part-emptying with each rain — typically supplies tens of thousands of litres over a season: drinking and cooking water at the door in the wet months, and a decisive buffer for a kitchen garden at the start of the dry.
The catchment: your roof, slightly improved
Corrugated iron, aluminium and tile roofs are excellent catchments. Thatch is poor — it stains and seeds the water — so families with thatch often roof a single small building (a store, a latrine block) with iron specifically as a rain catchment. Whatever the roof, the improvements are the same: keep it clean, cut back overhanging branches (leaves, bird droppings and rats travel by branch), and fit gutters along the eaves with a steady fall — about one centimetre of drop per metre of run — toward a single downpipe. Gutters can be bought PVC, folded from flat galvanized sheet by any tinsmith, or improvised from split bamboo replaced every second season. Support them every metre; a water-filled gutter is heavy.
The detail everyone skips: first-flush
The first rain after a dry spell washes the roof — and everything on it — toward your tank: dust, soot, insect husks, bird droppings. A first-flush diverter discards that first wash automatically. The classic version is a vertical length of pipe teed off the downpipe ahead of the tank: the first flow fills this chamber (sized to hold roughly the first half-millimetre of rain — say 20–40 litres for a family roof), a ball or flap seals it when full, and the now-clean flow passes on to the tank. After the rain, a small weep hole or tap drains the chamber ready for next time.
Screen every opening — tank inlet, overflow, vent — with mosquito mesh, keep the tank dark and lidded (light grows algae; open water grows mosquitoes), and draw water from a tap, never by dipping containers. Rainwater stored this way stays remarkably clean; for drinking, boiling or a household chlorine dose adds a final margin of safety.
Storage: from oil drums to ferrocement
Storage is the cost centre, so match it to your purse and purpose:
- Drums and jars (100–500 L): cheap, movable, fine for kitchen supply. Cluster several under one downpipe with linking overflows.
- Plastic tanks (1,000–5,000 L): quick to install, long-lasting if shaded from direct sun.
- Ferrocement tanks (2,000–20,000 L): the smallholder classic — a cage of mesh and wire plastered inside and out with rich cement mortar, built on site by a trained mason for a fraction of the equivalent plastic price at larger sizes. Cure it damp for two weeks and it outlives its builder.
- Underground cisterns: the cheapest litres of all at large volumes, best for garden water; need a pump or bucket-and-windlass and careful child-safe covering.
Site any tank on a firm, raised base close to the downpipe, with the overflow directed somewhere useful — a banana clump or a soak pit, never against the house foundation.
Water at the door repays its cost twice: once in the hours and backs saved from carrying, and again in what those hours grow, cook and earn instead.
Beyond the tank: harvesting into the soil
Roof water is only half the craft. The same storm that fills your tank also sheets across your land and away. Field rainwater harvesting slows and sinks it: contour bunds of earth or stone that hold runoff on the slope, infiltration pits and half-moon basins around fruit trees, mulch everywhere. A mango or banana planted at the outlet of a tank overflow or a bath-water drain grows through the dry season on water that was officially wasted. The principle is one sentence long: make every drop that falls on your land either soak in or get stored — and give none of it away to the road.