Rainwater harvesting means collecting rain from your roof through gutters and downpipes into a storage tank. Every millimetre of rain on one square metre of roof gives about one litre of water, so a 150 m² roof in Dar es Salaam, which gets roughly 1,000 to 1,100 mm of rain a year, can collect over 100,000 litres a year after losses. In practice, most households use rainwater to supplement mains or borehole water: for washing, cleaning, gardens and toilets, and for drinking only after proper filtering and treatment. A typical home system with gutters and a 5,000 to 10,000 litre tank costs about TSh 2.5 million to 6 million (about USD 960 to 2,300).
How much rainwater can my roof collect?
The formula is simple:
Water collected (litres) = roof area (m²) x rainfall (mm) x runoff factor
The runoff factor allows for splashing, evaporation and the first flush you throw away. For iron sheets and tiles, use about 0.8.
| Location | Approximate annual rainfall | 100 m² roof | 150 m² roof | 200 m² roof |
|---|---|---|---|---|
| Dar es Salaam | 1,000 to 1,100 mm | about 80,000 to 88,000 L | about 120,000 to 132,000 L | about 160,000 to 176,000 L |
| Arusha | 800 to 900 mm | about 64,000 to 72,000 L | about 96,000 to 108,000 L | about 128,000 to 144,000 L |
| Mwanza | 900 to 1,000 mm | about 72,000 to 80,000 L | about 108,000 to 120,000 L | about 144,000 to 160,000 L |
| Dodoma | 550 to 600 mm | about 44,000 to 48,000 L | about 66,000 to 72,000 L | about 88,000 to 96,000 L |
Rainfall figures are rough long-term averages. Check local records with the Tanzania Meteorological Authority.
Timing is the problem: most of the rain falls in a few months, during the long and short rains. You cannot store a whole year's rain in a domestic tank, so the tank is sized to carry you through part of the dry season, or to top up your main supply.
What size tank do I need?
Think about what the water is for and how long the dry spell is:
- Top-up for washing and cleaning in the rainy season: 2,000 to 5,000 litres.
- Partial supply through the short dry spells: 5,000 to 10,000 litres.
- Main supply for a household in a dry area: 20,000 litres or more, often in an underground concrete tank.
A household using 150 litres a day of rainwater for washing, toilets and cleaning gets about 33 days from a 5,000 litre tank and about 66 days from a 10,000 litre tank, once full.
What are the parts of a rainwater system?
- Roof: iron sheets, tiles or concrete. Avoid roofs with lead flashings or heavily treated timber that can contaminate water.
- Gutters: sized for heavy downpours, sloped about 1 in 200 towards the outlets, with leaf guards where there are trees.
- Downpipes: enough of them, at least one for every 8 to 10 metres of gutter in heavy rain areas.
- First-flush diverter: throws away the first few litres of each storm, which wash dust, bird droppings and leaves off the roof.
- Filter or screen: a mesh at the tank inlet to keep out leaves and insects.
- Tank: plastic, concrete, or ferrocement, on a firm, level base. Covered and screened against mosquitoes.
- Overflow: led safely away from the house foundations, ideally to a soak pit or garden.
- Outlet and pump: a tap at the bottom, or a pump to feed the house plumbing. See plumbing a new house.
Is rainwater safe to drink?
Rain itself is clean, but the roof, gutters and tank are not. Rainwater can carry dust, bird and animal droppings, leaves and bacteria. For drinking:
- Use a first-flush diverter and inlet screen.
- Keep the tank sealed, dark and screened.
- Clean gutters and the tank regularly.
- Filter and boil, or use a proper treatment unit, before drinking.
For washing, cleaning, toilets and gardens, clean stored rainwater is usually fine without treatment.
Plastic, concrete or ferrocement tanks?
| Tank type | Typical sizes | Pros | Cons |
|---|---|---|---|
| Plastic (polyethylene) | 1,000 to 10,000 L | Quick to install, light, cheaper for small sizes | Needs a strong base and shade, can be stolen or damaged |
| Underground concrete | 10,000 to 100,000 L | Large volume, cool water, saves space | Higher cost, needs a pump, must be watertight |
| Ferrocement | 5,000 to 30,000 L | Low cost for large volumes, durable | Needs skilled builders |
How much does a rainwater system cost?
| Item | Example cost (2026) |
|---|---|
| PVC or metal gutters and downpipes, per metre installed | TSh 25,000 to 45,000 (about USD 10 to 17) |
| First-flush diverter and filters | TSh 100,000 to 300,000 (about USD 40 to 115) |
| Plastic tank, 5,000 litres | TSh 900,000 to 1,500,000 (about USD 350 to 580) |
| Plastic tank, 10,000 litres | TSh 1,800,000 to 2,800,000 (about USD 690 to 1,080) |
| Tank base and stand | TSh 300,000 to 1,500,000 (about USD 115 to 580) |
| Underground concrete tank, 20,000 litres | TSh 6,000,000 to 12,000,000 (about USD 2,300 to 4,600) |
Example prices only. USD figures are approximate at roughly TSh 2,600 per USD.
How do I maintain a rainwater system?
- Clean gutters before each rainy season and after storms.
- Check and empty the first-flush diverter after each rain.
- Inspect screens and covers for holes.
- Clean the tank every year or two.
- Check that overflows discharge away from walls and foundations.
Harvesting also reduces flooding around your plot, because less roof water goes straight onto the ground. Our guide to stormwater drainage for homes covers what happens to the rest.
Frequently asked questions
How many litres of water can I collect from my roof?
About 1 litre per square metre of roof for every millimetre of rain, less around 20 per cent for losses. A 100 m² roof in 10 mm of rain gives roughly 800 litres.
Can rainwater replace mains water?
In wetter areas with large storage, it can supply most needs. In most homes, it works best as a top-up alongside mains or borehole water.
Do iron sheet roofs contaminate rainwater?
Painted or galvanised iron sheets are generally fine for collecting rainwater. Avoid lead flashings and roofs with peeling paint or asbestos.
Should I build the tank before or after the house?
Plan it with the house design so gutters, downpipes, tank base and overflows fit properly. Adding it later often means extra cost.
Planning water for a new home?
Vika Construction builds rainwater systems, tanks and drainage as part of new homes and renovations. See our building services or contact us.
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