The first two valves, in this order
A cold water storage cistern is a tank that is always full and always connected to the mains. Two things have to stop before the damage stops.
- Stop it filling. There should be a small isolating valve on the inlet pipe right next to the tank, usually a slotted screw you turn a quarter-turn with a flathead. Turn it so the slot sits across the pipe. If you cannot find one, turn off the main stopcock instead — normally under the kitchen sink or by the front door.
- Stop it emptying. There should be a second valve on the outlet, the pipe that feeds your taps and your hot water cylinder. Closing it holds the stored water in the tank rather than letting the whole lot find the leak.
- Open the cold taps downstairs if the tank is cracked below the water line and you want it down fast. Bath and basin cold taps in a vented house are fed from the tank, not the mains, so running them drains it into the drain instead of into your ceiling.
- Do not touch the feed-and-expansion tank next to it, if there is one — the small tank is the heating system's. Leave it alone unless that is the one leaking.
Those valves are not optional extras. The Water Supply (Water Fittings) Regulations 1999 require a servicing valve on the inlet pipe adjacent to the cistern, and one on the outlet of every storage cistern except a cistern feeding the primary circuit of a heating system. Tanks installed before 1999, and plenty installed after by people in a hurry, do not have them. If yours does not, the stopcock is your only inlet control and there is no outlet control at all.
Why this one is worse than a burst pipe
A burst 15mm pipe is dramatic and it is usually found within minutes, because someone hears it or sees it. A storage tank is the opposite on both counts.
It is the largest single body of water in a domestic building. A common cistern is nominally fifty gallons — roughly 227 litres, so roughly 227kg — sitting on bearers across the ceiling joists, directly above bedrooms. And it is in the one room nobody enters. Lofts get visited at Christmas and when something breaks. A weep that starts in March is found in December.
The water does not fall straight down, either. It runs along a joist, soaks the insulation, tracks to the lowest point of the ceiling and comes through somewhere that tells you nothing about where it started. By the time you see a stain, the board above it has been wet for a while and the insulation is saturated and doing nothing.
Getting into the loft without going through the ceiling
This is the part people skip. A loft floor is usually not a floor. It is joists, with plasterboard nailed to the underside and insulation laid between them, and the insulation makes the joists invisible. Plasterboard carries its own weight and nothing else. Step between joists and you go through, from ceiling height, headfirst.
The fragile-surface principle is well established in the trade for exactly this reason: a surface that is not designed to be walked on is treated as a hazard, not as a floor. In a loft that means:
- Take a torch. Do not rely on the loft light — the water may be near the wiring, and a wet loft is a reason to leave the lights off, not to switch them on.
- Stay on the boarded area, or on crawl boards laid across the joists. Do not put a foot anywhere you cannot see timber.
- Assume the insulation is wet and heavy, and that the boards near the tank may be soft.
- Never carry water down a loft ladder.
- If the ceiling below is already bulging or sagging, stay out and stay from under it. A sagging ceiling holding trapped water comes down as one piece.
If getting up there safely is not realistic — a hatch with no ladder, no boarding, a bad back, on your own in the house — that is a fair reason to call someone rather than a failure of nerve.
Where these tanks actually leak
The connectors and grommets
Most of them. Where the outlet pipe passes through the tank wall there is a tank connector with a washer, and on older installations a rubber grommet. Plastic hardens and takes a set, rubber perishes, and the nut was tightened once in 1986. The result is a bead of water on the outside of the tank wall that runs down and off the bottom edge, which is why the wet patch on the ceiling is often not under the tank at all. The inlet side does the same thing where the float valve tail passes through.
Stress cracks in plastic tanks
Early polyethylene cisterns and cheap replacements go brittle with age and UV exposure through a felted roof. The failure is a hairline crack, usually radiating from a connector boss or from a corner where the tank was sat on uneven bearers or straight onto joists without a supported base. It will weep for a long time before it opens up. A cracked plastic tank is not repairable in any way worth paying for — glue, tape and sealant on a polyethylene wall under a 200-litre head of water do not hold.
Galvanised steel tanks
If your tank is metal, it is old. Galvanised cisterns rust from the inside, and the first sign is usually orange staining in the bath when the cold tap runs, or a brown tidemark inside the tank. Once it is perforating, it is at the end of its life. These are also heavy, which matters for both the replacement job and for what the ceiling below has been carrying.
The float valve, which is not really a leak
If the tank is overflowing rather than leaking, the ball valve washer has failed or the float has taken on water and sunk. The valve never shuts and the surplus goes out of the overflow. The regulations require a valve capable of shutting off below the overflowing level, and an overflow pipe with a means of warning you it is running — a pipe meant to discharge somewhere conspicuous outside, so you notice. Plenty were run into a soffit, into the gutter, or into nothing at all. Then the tank overflows into the loft for months with nothing to warn you.
A dripping ball valve is a washer or a new valve. Short plumbing job, not a detection job, and you should not be charged for detection to find it.
Is it the tank, or is it the roof?
Water in a loft is not automatically a tank. The alternatives get ruled out first: a slipped slate, a failed flashing at a chimney or party wall, a gutter backing up under the eaves, or condensation in a loft whose eaves vents are blocked by insulation. A roof leak appears after rain and dries between. A tank leak does not care about the weather. If it is the roof, you want a roofer, and we will say so rather than sell you a tank.
Repair, or replace
| What it is | Realistic outcome |
|---|
| Weeping tank connector or grommet | Drain below the connector, renew the washers and connector, refill. Repairable. |
| Failed float valve or sunk float | New valve or washer. Short job. Check where the overflow discharges while you are there. |
| Hairline crack in a plastic tank | Replace the tank. Patching buys weeks, not years. |
| Rusted galvanised tank | Replace. Usually with a plastic cistern, a proper supported base and both servicing valves fitted. |
| Tank sound, everything wet anyway | Look above, not below. Roof, flashing or condensation. |
A replacement is the moment to fix what made this expensive: a base that spreads the load across the joists, a lid that excludes light and insects, insulation, and an overflow that discharges where you can see it. All requirements for a storage cistern, and all commonly missing on a tank that has just failed.
The ceiling, afterwards
The leak is the short part. What is left is a stained or bulging ceiling, sodden insulation that has to come out, and plaster that keeps moving until it dries. We do that side in-house — tank fixed or replaced, ceiling cut back, boarded, skimmed and decorated, by the same firm. The usual experience is a plumber who stops at the valve and leaves you to find a plasterer in January.
If you are claiming, the detection visit comes with a written report of what was found and where, which is what a trace and access claim is assessed on. The insurer decides what the policy covers; the report is there so the decision is made on evidence rather than on your description over the phone. See ceiling leak repair for how the source and the ceiling are handled together, and leak damage repair and reinstatement for the making-good side.
What it costs
We work at an hourly rate, agreed before we attend, with no call-out fee, and if we do not find the leak you do not pay for the time we spent looking. A tank that is visibly leaking rarely needs detection at all — you can see it, and the job is the repair. Detection earns its keep when the ceiling is wet and the tank looks fine, which is the case where guessing costs more than measuring. Full figures are on pricing.
