What to expect
| Typical duration | Restoration 2–5 weeks · new cistern by design |
|---|---|
| Cost band | Figures live in the cost index |
| Permit required | New cisterns yes · restoration usually not |
| Seasonal constraint | Work before the winter rains, not after |
How a cistern actually works
The roof and terrace are the catchment. Runoff is collected and taken to an excavated chamber, and the first portion of each rainfall — which carries the accumulated dust, and this island generates a great deal of it — is diverted rather than stored. What is stored is comparatively clean, cool and, being rainwater, soft.
The chamber is lined with lime plaster gauged with the island's own volcanic earth, which is a natural pozzolan. That combination sets hard, works in permanently damp conditions, and is what makes the cistern watertight. It is the same material tradition that saw Theran earth exported as hydraulic cement in the nineteenth century. See plastering.
Restoring an old cistern
Most traditional properties have one, and a large proportion of them have been abandoned, filled in, or quietly used as something else. Restoring one is worthwhile: it is genuinely useful on an island where water arrives by tanker or desalination, and it costs far less than building new.
- Open, empty and inspect. This includes confined-space precautions — a neglected cistern is not a space to enter casually.
- Assess the structure. Vault or roof condition, wall condition, and whether any part has been undermined or built over.
- Establish why it lost water, if it did. Usually failed plaster, sometimes a crack, occasionally a penetration made by someone routing a service through it.
- Strip failed lining back to substrate and re-plaster in compatible lime and pozzolan, in coats.
- Reinstate or rebuild the catchment: gutters, downpipes, inlet and first-flush arrangement.
- Reinstate a sealed, safe access cover, and screening at the inlet to keep debris and animals out.
- Filtration and, where the water will be used domestically, treatment appropriate to the use.
What the water is good for
Being clean, soft rainwater, cistern water is excellent for garden irrigation, pool top-up, washing and general use, and it saves both money and tanker deliveries. Whether it is suitable for drinking depends on the catchment, the state of the cistern, the screening and the treatment installed — that is a property-specific question and it should be answered with a test rather than an assumption.
Soft water also has a practical advantage worth mentioning: it does not scale appliances the way the island's hard supply does. See plumbing.
New cisterns
On a new build, a cistern is worth serious consideration rather than being treated as a heritage feature. It reduces reliance on tanker delivery, provides irrigation and pool water, and on a property with a large roof and terrace area the catchment is already there.
The design questions are catchment area against expected rainfall, chamber volume against use, structure appropriate to the ground, and how the overflow is routed — because an overflowing cistern discharging against a retaining wall or into a slope creates the water problem you spent money avoiding elsewhere. See excavation and retaining walls.
Questions people actually ask
Is it worth restoring my old cistern?
Usually yes. It reduces tanker deliveries, provides soft water for garden and pool, and it is a genuine asset rather than a decorative one. It also removes a hidden void under your property from the list of unknowns.
Can I drink cistern water?
It depends on the catchment, the condition of the cistern, the screening and the treatment. It is a testable question rather than a general one, and we would rather you tested than assumed either way.
How much water does a cistern hold?
It varies enormously between properties. What matters more is the relationship between your catchment area, the rainfall and your use — a large cistern with a small roof feeding it will not fill.
My cistern loses water. Can it be repaired?
Usually. It is normally failed lining rather than structural failure, and re-plastering in compatible material is the standard repair. Finding a penetration someone made for a pipe is also common.
Can a cistern be built under a new terrace?
Yes, and integrating it at design stage is far cheaper than retrofitting. The structure, access, overflow route and the load above all need designing together.