A ferrocement tank is a thin cylindrical shell of cement mortar over rebar rings and wire mesh: a 5,000 L tank is 1.6 m in diameter and 2.5 m tall, with walls only 20–25 mm thick, on a 10 cm concrete pad, and it must be kept wet for at least 14 days to cure. The same shell serves as a ground-level cistern fed from a roof, or as an elevated header tank for gravity plumbing, where each metre of height between tank bottom and tap adds about 0.1 bar of pressure.
Part of Cisterns and Rainwater Storage
Ferrocement is a thin shell of cement mortar over wire mesh and rebar, used to build watertight above-ground tanks with far less cement than a solid concrete wall. Two readers arrive here: one storing roof water at ground level, and one raising a tank above a house for gravity-fed plumbing.
A Thin Shell Needs A Fraction Of The Mortar Of A Thick Wall
The strength comes from the steel, not from bulk. Rebar rings carry the sideways pull of the water, the mesh holds the mortar together and spreads any cracking into hairlines, and the mortar does the sealing. That is why a wall of 20–25 mm can hold a standing column of water 2.5 m high.
The cylinder wall area is circumference times height: π × 1.6 m × 2.5 m = 12.6 m². At 20–25 mm thick that is 12.6 × 0.020 to 12.6 × 0.025 = 0.25–0.31 m³ of mortar. A 30–50 mm wall, the thickness quoted on the housing page, needs 12.6 × 0.030 to 12.6 × 0.050 = 0.38–0.63 m³, up to twice as much.
One Shell, Two Jobs: Roof-Water Cistern And Elevated Header Tank
Ground-level cistern. The tank collects roof runoff through a filtered inlet, with an overflow pipe and a tap 10 cm above the bottom. Sizing follows dry-spell bridging.
Elevated header tank. For a gravity plumbing system the tank sits above the highest tap on a hillside or a stand. The housing page gives a 2 m diameter by 2 m tall tank holding about 6,000 L: π × 1 m² × 2 m = 6.28 m³ = 6,280 L, of which about 6,000 L is usable below the overflow. Water alone then weighs about 6,000 kg, a load of 6,000 × 9.81 = 59 kN, before counting the shell. Each metre of height between tank bottom and tap gives about 0.1 bar, so 10 m gives about 1 bar. Put it on firm ground or a stand built for that load, never on an ordinary roof. Minimum height is set by the distribution rule: tank bottom at least 5 m above the highest tap (see tank placement).
What You Need
- Cement, clean sand, clean water — mortar 1:3 cement:sand by volume
- Rebar, 6–8 mm for rings, 6 mm for verticals — and two layers of chicken wire
- Gravel and a concrete pad — 2 m across and 10 cm thick for the 1.6 m tank, with a drain pipe
- Two people — one inside, one outside, plastering at the same time
- Water and wet burlap or plastic — for 14 days of curing
Method: Pad, Frame, Plaster, Cure, Seal
Step 1: Pad. Pour the pad on a gravel base and let it set before building on it.
Step 2: Frame. Rings 15 cm apart, vertical bars at 15 cm around the rings, wrapped in two layers of mesh, tied tight. See frame construction.
Step 3: Plaster from inside and outside together to 20–25 mm total, interior smooth. See plastering technique.
Step 4: Cure wet for at least 14 days, twice-daily spraying under wet burlap or plastic. See curing.
Step 5: Seal the inside with a final coat of neat cement paste, which fills micro-cracks.
Step 6: Test and disinfect. Fill to the overflow and mark the waterline. Damp patches on the outside show where it seeps. Before drinking from it, disinfect with chlorine at 50 mg/L for 24 hours, as for a new well (Well Development). That figure is the dose added, not a residual. Then drain it fully and refill.
Worked Example
Capacity of the 1.6 m tank: π × (0.8 m)² × 2.5 m = π × 0.64 × 2.5 = 5.03 m³ = 5,030 L, rounded to 5,000 L.
Pressure on the wall at the bottom when full: 1,000 kg/m³ × 9.81 m/s² × 2.5 m = 24,500 Pa = 24.5 kPa, about 0.25 bar. Low pressure, but it acts on every part of the wall, which is why the lower rings matter most.
Common Mistakes
- Short curing. Rushed curing cracks the tank. Where the housing page’s 7+ days and this page’s 14 days disagree, use 14 days.
- Thin or patchy mortar at the base. The base takes the highest pressure; do not skimp there.
- Putting a full tank on an unprepared stand. 6,000 L is about 6 tonnes.
- Leaving it unlidded. Mosquitoes and algae enter; fit a lid with fine mesh over every opening.
Safety
A lidded tank is a confined space, and a falling tank is a crushing load
Once a tank is roofed or lidded, its air can be low in oxygen, and carbon dioxide or hydrogen sulphide can pool at the bottom. Nobody enters without ventilation at two openings, a harness tied to a line held by an attendant, and a second person outside who pulls the line and never climbs in; the commonest fatality pattern is a second person entering after the first collapses. Do not test the air with a flame. Wet mortar is alkaline and burns skin and eyes; wear gloves and eye protection.
What’s Next
- Learn next: Frame Construction — the steel skeleton that carries the load
- Combine with: Storage Tanks — how tank size and height fit a distribution system