Rain falls on every roof, and for an off-grid home it is the cheapest water source you will ever own. A basic rainwater harvesting setup can cover drinking water, washing, and the garden for much of the year. The system is simple in principle, but the details decide whether the water is clean and reliable or a source of endless trouble.

Every setup has the same four parts: a catchment surface, gutters and pipes, a filter, and storage. The roof is the catchment, and a metal roof is ideal because it sheds water cleanly and does not leach chemicals. Asphalt shingles work too, but they need a first-flush diverter and a good filter, because they shed grit and residue into the water.

Gutters carry water to a first-flush diverter, a simple device that discards the first few litres of each rainfall, washing away dust, bird droppings, and leaves before the water reaches the tank. This one device does more for water quality than any filter downstream, and it costs very little. After the diverter, water passes through a mesh filter to catch debris, then into the tank.

Size the tank to your roof and your rainfall. A simple estimate helps: one millimetre of rain on one square metre of roof yields about one litre of water, minus losses. A hundred square metres of roof in a place that gets a millimetre of rain a day collects roughly a hundred litres a day, so a tank of a few thousand litres covers a household through dry spells.

Place the tank on level, compacted ground, close to the house but where overflow can drain away safely. Dark, opaque tanks are best because light grows algae. Keep the lid sealed, the inlet screened against mosquitoes, and the overflow pipe fitted with mesh. A tank that breeds mosquitoes is worse than no tank at all.

Rainwater from a clean roof is usually safe for washing and the garden with only basic filtering. For drinking, add treatment: a sediment filter followed by a carbon filter, and then either ultraviolet light or a small amount of chlorine, depending on your preference. Test the water at least once a year, and after any roof work or long dry spell.

Maintain the system on a simple schedule: clean gutters each season, check the first-flush diverter after storms, and inspect the tank before winter. A harvested-water home that is maintained is a home that never worries about a dry tap. The rain does the work; your job is keeping the path from sky to tap clean.

Not all harvested water needs the same treatment, and treating everything to drinking standard wastes money and filters. A simple two-tier system works better: one line from the tank for the garden and washing, filtered only through a mesh screen, and a second line for drinking and cooking that passes through a sediment filter, a carbon filter, and ultraviolet treatment. The garden line can even bypass the tank in a storm, feeding beds directly from the overflow.

Maintain both lines on a schedule. Clean the mesh filter monthly during the rainy season, replace carbon filters as the manufacturer recommends, and check the ultraviolet lamp yearly, because a lamp that has dimmed with age looks fine and sterilises nothing. Test drinking water at least once a year, and always after roof work, a long dry spell, or a storm that dropped debris on the roof. Water you trust is water you maintain.

Label every valve and pipe while you remember what it does. Six months later, during a leak, labels save you from guessing which valve feeds the house and which drains the tank. A ten-minute labelling job now prevents a flooded floor later. Keep a sketch of the system with the manual, so anyone helping you can find the valve fast.