Cisterns and Rainwater Storage

6 min read

Cisterns and Rainwater Storage

Why This Matters

Rain falls when it wants, not when you need it. Springs and wells may drop in dry season. A community cistern bridges the gap — storing thousands of liters collected during wet periods for use during dry ones. In many climates, rainwater harvesting from rooftops alone can supply a community’s drinking water year-round.

How Much Storage Do You Need?

Step 1 — Calculate daily demand. At 20 L/person/day (drinking, cooking, basic hygiene), a group of 20 people needs 400 L/day. For 30 days without rain, that’s 12,000 liters. For a more comfortable 50 L/person/day with a 60-day dry spell: 60,000 liters.

Step 2 — Calculate available rainwater. Use this formula:

Liters collected = Roof area (m2) x Rainfall (mm) x Runoff coefficient

Runoff coefficients:

  • Metal roof: 0.9 (90% of rain collected)
  • Tile roof: 0.8
  • Thatch roof: 0.5 (absorbs a lot)
  • Concrete/plastic ground surface: 0.8

Example: 5 buildings with metal roofs totaling 200m2, in an area with 800mm annual rainfall: 200 x 800 x 0.9 = 144,000 liters per year

That’s plenty for 20 people — but it arrives unevenly. If 600mm falls in 6 wet months and only 200mm in 6 dry months, you need storage to bridge the gap.

Step 3 — Size your tank. For the example above, a 20,000-30,000 liter cistern handles most dry spells. Build bigger if you can — you’ll never regret having too much water storage.

Underground Cisterns

The best option for large volumes (10,000L+). Underground stays cool (slows algae and bacteria growth), doesn’t take up surface space, and is protected from damage.

Shape: A bottle or bell shape (wider at the bottom, narrower neck at the top) is structurally strongest because earth pressure pushes inward equally. A cylinder works well too. Avoid sharp corners — they concentrate stress.

Construction:

  1. Dig the hole. For a 10,000L cistern: roughly 2.5m diameter x 2.5m deep. In stable clay soil, the walls will hold themselves during construction. In sandy soil, you’ll need to line as you go.

  2. Plaster the interior. Apply two coats of cement plaster (1:2 cement:sand ratio) to the walls and floor, each 10-15mm thick. The first coat is rough, the second smooth. Cure each coat for at least 7 days, keeping it damp.

  3. Alternative: plastic liner. If cement is scarce, line the pit with heavy-duty plastic sheeting (pond liner or layered tarp). Overlap seams by 30cm and seal with waterproof tape or heat welding.

  4. Build the roof. A reinforced concrete slab (10cm thick, with rebar mesh) resting on a concrete ring beam around the top edge. Leave a 60cm access hatch with a lockable cover. The roof must support the weight of a person standing on it.

  5. Install pipes. Inlet pipe enters through the wall near the top. Overflow pipe exits at the same level as the inlet. Outlet pipe exits 10cm above the floor (to avoid sucking up sediment). All pipe penetrations through the wall must be sealed watertight.

Warning

Never enter a cistern alone. Confined spaces can accumulate toxic gases. Always have someone at the surface with a rope.

Ferrocement Tanks (Above-Ground)

Ferrocement is a thin shell of cement mortar over wire mesh. It’s strong, waterproof, and uses far less cement than a solid concrete tank.

For a 5,000L cylindrical tank (1.6m diameter x 2.5m tall):

  1. Foundation: Pour a circular concrete pad, 2m diameter, 10cm thick, on a gravel base. Include a drain pipe.

  2. Frame: Bend rebar (6-8mm) into rings matching the tank diameter. Space rings 15cm apart vertically. Wire vertical rebar (6mm) at 15cm spacing around the rings. Wrap the entire frame with chicken wire mesh (2 layers), tying it tightly to the rebar.

  3. Plaster: Apply cement mortar (1:3 cement:sand) from both inside and outside simultaneously (need two people). Build up to 20-25mm total thickness. Smooth the interior surface.

  4. Cure: Keep the tank wet for at least 14 days. Cover with wet burlap or plastic and spray with water twice daily. Rushed curing = cracked tank.

  5. Seal: Apply a final interior coat of neat cement (pure cement mixed with just enough water to make a paste). This fills micro-cracks.

Inlet Filtering

First flush diverter: The first 1-2mm of rain off any roof is filthy — dust, bird droppings, leaves, insects. A first flush diverter is a short length of pipe (typically 50-100mm diameter, 1-2m long, capped at the bottom) that fills with this dirty water before the flow is directed to the tank. After rain stops, a small drip hole in the cap slowly empties the diverter, resetting it.

Calculation: divert the first 1 liter per m2 of roof. For a 40m2 roof: 40L first flush. A 100mm diameter pipe holds about 8L per meter, so you need a 5m diverter pipe.

Leaf screen: A simple mesh screen (2-3mm openings) at the gutter entrance or tank inlet. Clean it monthly.

Settling chamber: A small tank (200-500L) between the inlet and the main cistern, with the inlet at one end and outlet at the other, separated by a baffle. Sediment settles out before water enters the main tank.

Extraction

Never dip buckets into a cistern — every bucket introduces contamination. Instead:

  • Gravity tap: If the tank is above ground or elevated, install a tap 10cm above the bottom. Simplest and best.
  • Hand pump: For underground cisterns, install a suction hand pump (works for cisterns less than 7m deep).
  • Siphon: Start a siphon with a priming pump and gravity does the rest. Only works if the outlet is below the water level.

Common Mistakes

MistakeConsequencePrevention
No first flush diverterContaminated water enters tankInstall diverter sized for 1L per m2 of roof
Bucket dippingIntroduces bacteria, dirt, and debrisUse taps, pumps, or siphons only
Insufficient curing of ferrocementCracks, leaks, tank failure14 days minimum, keep wet the entire time
Tank not covered or sealedMosquito breeding, algae, contaminationSealed lid with fine mesh on all openings
No overflow pipeTank damaged or water backs up into guttersInstall overflow pipe at inlet level, route to soakaway

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