Johannesburg averages roughly 700mm of rainfall a year, almost all of it in the November-to-March wet season. On a 300 m² roof that's 210,000 litres of free water — if you have the storage and plumbing to capture it. Here's the honest economic and practical case for rainwater harvesting in Northern Joburg.
How much water you'll actually capture
The rule of thumb is: roof area (m²) × annual rainfall (mm) × 0.8 collection efficiency = litres per year.
A 300 m² roof at 700mm × 0.8 = 168,000 litres a year, or roughly 460 litres a day on average. That's enough to flush every toilet, run every washing machine cycle and irrigate the garden for a family of four — with zero municipal cost.
What it costs
- 5,000L rainwater tank with first-flush diverter: R10,500
- Gutter and downpipe diversion plumbing: R4,500 – R7,500
- Pump and pressure system (toilets + irrigation only): R8,500 – R12,500
- Full filtration train (sediment + carbon + UV) for potable use: + R8,500 – R14,000
- Plumbing COC and commissioning: R3,500
So a "toilets and garden" rainwater system lands at roughly R28,000 – R36,000. A full potable system with treatment is R42,000 – R58,000.
The payback maths
Johannesburg Water steps tariffs heavily — over 30 kL/month a household pays around R45/kL. Capturing 14 kL/month off the roof saves roughly R600 – R900 a month in the wet season and less in winter. Realistic payback on a toilets-and-garden system is 4–6 years, with the bonus that it doubles as outage insurance.
Should the rainwater feed the house or just outside?
For most homeowners we recommend a "non-potable" setup — the rainwater feeds toilets, the washing machine, the garden taps and the pool top-up. The kitchen and showers stay on municipal water. This avoids the cost and ongoing maintenance of the UV steriliser and means a failed filter never makes anyone sick.
Combining with municipal backup
The clever play is to size one tank for both jobs — rainwater harvesting in the wet season, municipal backup in winter and during outages. A 5,000L tank with both a roof inlet and a mains top-up float (set to a lower level than the roof inlet) gives you the best of both worlds.
Want to spec a system for your house? Book a free site visit on 082 886 9119. We design and install rainwater systems across Northern Joburg.