Water outages and rising tariffs have made backup water common in South African homes. The usual pieces are a tank, fed by the roof, the mains or a borehole, and often a pump. This guide shows how to size each from your own numbers.
Rainwater from the roof
One millimetre of rain on one square metre of roof is one litre of water. JoJo’s advice is to allow about 15% for wastage: splashing, evaporation and the first flush that washes the roof. So:
Formula: litres collected = roof area (m², on plan) × rainfall (mm) × 0.85
Use the plan area of the roof that drains to the tank: length × width as seen from above, not along the slope. A steep roof doesn’t catch more rain than a flat one of the same plan size.
Worked example: a 100 m² roof in a 25 mm storm collects 100 × 25 × 0.85 = 2,125 litres.
Choosing a tank
JoJo’s vertical tanks come in 260, 500, 1,000, 1,500, 1,800, 2,000, 2,400, 2,700, 4,750, 5,250, 5,500, 10,000, 15,000 and 20,000 litre sizes. For the storm above, a 2,400 L tank holds the lot. Size the tank for a heavy storm or a wet spell rather than the yearly total, unless you use the water steadily; a tank that fills and overflows after one storm wastes the rest of the season’s rain. The rainwater tank calculator picks the tank and shows how many days the water lasts at your daily use.
Set the tank on a level, solid base or a proper stand. A full 2,400 L tank holds about 2.4 tonnes of water, so the base matters. The tank volume calculator works out the capacity of tanks and sumps of other shapes.
Backup from the mains
A tank filled from the municipal supply works the same way: tank size = water used per day × the days of backup you want, rounded up to a tank size. A household using 400 litres a day that wants two days of backup needs at least 800 litres, so a 1,000 L tank.
Boreholes
A borehole pump must lift water from below ground to the tank, and must not pump more than the borehole can supply. Start with the borehole’s yield test certificate, which gives its sustainable yield. The Borehole Water Association stresses that the true sustained yield must be established by an accepted yield test, and that yields change with rainfall and with other boreholes nearby.
South African pump suppliers size a pump from two numbers:
- Flow: the water needed per day ÷ the hours the pump may run. It must not exceed the tested yield, or the pump will draw the water down until it runs dry.
- Total dynamic head: static water level + drawdown while pumping + height from the borehole to the tank + friction loss in the pipe + any pressure needed at the outlet (1 bar is about 10.2 m).
Worked example: 1,000 litres a day, pumped over 6 hours, is about 167 litres an hour, well under a tested yield of 1,500 L/h. With water at 40 m, 10 m of drawdown, a 5 m lift to the tank and 5 m of pipe friction, the head is 60 m. The supplier then picks a pump from its performance curve for 167 L/h at 60 m. The borehole pump and tank calculator does these sums and sizes the storage tank.
Tips
- Fit leaf screens and a first-flush diverter so the dirtiest water bypasses the tank.
- Test the water before drinking it, from the roof or a borehole.
- Fit a float switch in the tank and dry-run protection on a borehole pump.
- Have the plumbing connected by a registered plumber; see geysers, PIRB CoCs and the 50% solar rule.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.