Line a cistern pit with heavy-duty plastic sheeting, such as pond liner or layered tarp, when cement is scarce: overlap seams by 30 cm and seal them with waterproof tape or heat welding, and protect the sheet from stones and sun. Use salvaged plastic for stored drinking water only if you know what it held and what it is made of, because plastic of unknown history can leach chemicals, and the liner holds the water in without holding the pit wall up.
Part of Cisterns and Rainwater Storage
A plastic liner is a waterproof sheet laid inside an excavated pit to hold water, in place of a cement-rendered lining. It is faster than plastering but fails differently: by puncture, by seam failure and by uplift.
A Liner Is A Membrane, Not A Wall
A liner keeps water in. It does not carry the load of the soil. The pit must stand on its own: stable clay, or sand that has been lined with a supporting wall before the sheet goes in. The sheet then lies against a smooth, firm surface and is supported by the water it holds.
What You Need
- Heavy plastic sheeting — pond liner or layered tarp
- An underlay — sand, fine soil, or old carpet or cloth, to protect the sheet from sharp stones
- Waterproof tape, or a heat-welding tool — the cisterns page names both for seams
- Soil or stones for an anchor trench — and a cover for the finished pit
- Knowledge of what the plastic held before
Method: Prepare, Line, Seam, Anchor
Step 1: Smooth the pit. Remove every stone and root. Pad the surface with the underlay.
Step 2: Lay the sheet. Work from the floor up. Fold, do not stretch; a sheet pulled tight is stressed when it fills.
Step 3: Overlap seams by 30 cm and seal them with waterproof tape or heat welding. A tape seam is only as good as the surface under it: clean, dry and pressed down firmly.
Step 4: Anchor the top edge in a trench around the rim, backfilled, so the sheet cannot slide in as it fills.
Step 5: Protect from sun. Plastic degrades in sunlight; the roof (Cistern Roof) shades it.
Step 6: Fill slowly and watch the sheet settle into the pit. Smooth wrinkles from inside only with a rope, a harness and an attendant.
Worked Example
A liner for the 2.5 m by 2.5 m pit covers the wall 2π × 1.25 × 2.5 = 19.6 m² and the floor π × 1.25² = 4.9 m², a total of 24.5 m². Add an anchor allowance: with a 0.5 m strip all round the 7.85 m rim (π × 2.5 m) that is 7.85 × 0.5 = 3.9 m², plus a margin for folds. So order at least 28.4 m² of sheet, assuming no waste. The 0.5 m strip is an example allowance, not a standard.
Which Plastics Are Fit For Drinking Water
Plastic can leach, and sunlight and heat speed it up. A liner sold for fish ponds or drinking-water storage is a better bet than industrial or agricultural sheet. Do not use plastic that held fuel, pesticide, fertiliser or chemicals, or roofing membrane with unknown additives. If you cannot establish what it is, use the cistern for garden, animal or washing water, and treat water for drinking (Water Purification).
Common Mistakes
- Using a sheet with a hole in it. A single puncture drains the cistern.
- Leaving sharp stones against the sheet. Water pressure presses it onto them.
- Taping a dirty seam. The tape peels in weeks.
- Digging in a high water table. Groundwater can lift an empty liner; see Excavation Design.
- Leaving the liner in the sun. It cracks and fails.
Safety
Entering a cistern can kill, and the rescuer dies second
A pit lined with plastic is a confined space once roofed, and can be short of oxygen, with hydrogen sulphide or carbon dioxide pooling at the bottom. The commonest fatality pattern is a second person climbing in after the first collapses. Enter only with 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. Do not use a flame to test the air. Heat welding gives off fumes and can ignite the plastic; weld in the open air, away from anything that burns.
What’s Next
- Learn next: Cistern Roof — the cover that also shades the liner
- Combine with: Plastic Lining — the same sheeting used on open channels