Make an underground cistern watertight with two coats of cement plaster mixed 1 part cement to 2 parts sand by volume, each coat 10–15 mm thick: a rough first coat, a smooth second, and at least 7 days of damp curing after each. For the 2.5 m by 2.5 m pit the walls and floor cover about 24.5 m², so the two coats come to about 0.49–0.74 m³ of placed plaster.
Part of Cisterns and Rainwater Storage
Cement plaster is a rendered interior lining of cement and sand that makes the walls and floor of an underground cistern hold water. Its weak points are bond to the wall, cracking as it dries, and cold joints.
Two Thin Coats Work Better Than One Thick One
The cisterns page specifies two coats of 1:2 cement:sand, 10–15 mm each, the first rough and the second smooth, applied to walls and floor, each cured for at least 7 days and kept damp. That mix is richer in cement than the 1:3 used for the ferrocement shell. The rough first coat gives the second something to key into, and two coats mean a defect in one is covered by the other.
What You Need
- Cement that is dry and lump-free, and clean, sharp sand free of clay, silt, salt and plant matter
- Clean water — drinkable quality
- Steel trowel, wooden float, bucket, brush for wetting and scoring
- A mixing board or tub, measured by volume with the same bucket for cement and sand
- Gloves, eye protection and a rope for the person inside
Method: Rough Coat, Cure, Smooth Coat, Cure
Step 1: Prepare the surface. Remove loose soil and flaking. Wet the wall so it does not suck the water from the plaster.
Step 2: Mix 1:2 by volume. One bucket of cement to two of sand, mixed dry until the colour is even, then water added slowly. It should be stiff, holding its shape on the trowel.
Step 3: First coat, 10–15 mm, rough. Press it on firmly and score or rake the surface. A coat laid on without pressing leaves hollow patches behind it.
Step 4: Round the wall-to-floor angle. A square corner concentrates stress. Fill it with a rounded cove.
Step 5: Cure at least 7 days, keeping it damp (Curing).
Step 6: Second coat, 10–15 mm, smoothed. Dampen the first coat, apply, trowel smooth, and cure again for at least 7 days.
Step 7: Seal pipe penetrations, where the inlet, overflow and outlet pass through the wall, with extra care.
Worked Example
Area of the pit: wall = 2π × 1.25 m × 2.5 m = 19.6 m²; floor = π × 1.25² = 4.9 m²; total 24.5 m².
Placed volume: 24.5 × 0.020 = 0.49 m³ for two coats at 10 mm, and 24.5 × 0.030 = 0.74 m³ for two coats at 15 mm. So 0.49–0.74 m³ goes on the wall. The dry materials to buy exceed this, because loose cement and sand mixed with water settle into less volume than the sum.
Diagnosing Failures
- Hollow sound when tapped. The plaster is not bonded; hack it off and redo it.
- Fine crazing cracks. Dried too fast; keep it wet longer.
- A crack that runs the full depth. The pit wall moved or the plaster was too thick; patch only after finding the cause.
- Dark damp patches on a filled tank. A weak spot or a missing seal at a pipe.
Common Mistakes
- Skipping the 7 days. The plaster cracks and the tank leaks.
- Dirty sand. Clay and silt weaken the bond.
- Adding water to a stiffening batch. It weakens the mix.
- Working a day’s batch into the next day’s joint without wetting it. A cold joint leaks.
- Plastering a damp, loose wall. The coat slides off.
Safety
Entering a cistern can kill, and the rescuer dies second
A pit or tank can hold air that is short of oxygen, and may also hold hydrogen sulphide or carbon dioxide, which is heavier than air and pools at the bottom. The commonest fatality pattern is a second person climbing in after the first collapses. The entrant wears a harness tied to a line held by an attendant at the surface, the tank is ventilated at two openings, and a second person stays outside to pull the line and never goes in. Never run a petrol or diesel mixer or pump near the pit: its exhaust is carbon monoxide. Do not test the air with a flame.
Wet cement burns
Wet cement is strongly alkaline and causes chemical burns to skin and eyes. Wear gloves and eye protection, wash splashes off at once, and do not kneel in wet mortar. Dry cement and sand dust is harmful to breathe; mix with the wind behind you.
What’s Next
- Learn next: Cistern Roof — covering the finished lining
- Combine with: Plastic Liner — the alternative where cement is scarce