Leaking Pipes: Finding the Source and Getting It Fixed Properly
A leaking pipe almost never appears where the water shows up.
A leaking pipe almost never appears where the water shows up. Water travels along joists, down the outside of pipework and through ceiling voids before it finds a low point to drip from, which is why chasing the stain rather than the source wastes time and plaster. The job is to isolate the water safely, work out which system the leak belongs to, and then repair the joint or section properly rather than patching it.
Work out which system is leaking
Mains cold pipework leaks continuously and the water is clear and cold. Heating pipework leaks only when the system is warm or pressurised, and the water is often discoloured and slightly metallic-smelling from inhibitor and system sludge. Hot water pipework leaks hot and typically only when the outlet has been used. Knowing which system you are dealing with tells you what to isolate — the stopcock, the heating filling loop and pump, or the cylinder — and saves you draining things that did not need draining.
Common failure points, in the order worth checking
Compression joints that were never quite tight or have been disturbed. Flexible tap connectors, which have a finite life and fail from the inside out. Push-fit fittings that were not fully inserted or lost their O-ring on a burr. Soldered joints stressed by expansion where a pipe passes through a tight hole in a joist. Radiator valve spindles and unions. Pipework buried in screed or chased into a wall is the least likely and the last to investigate, because it is the most disruptive.
Finding a leak you cannot see
Dry everything, then put tissue paper around each candidate joint and check it in an hour — paper shows a slow weep long before your eye does. If the system loses pressure with the boiler off, the leak is on the heating pipework rather than an appliance. If a water meter creeps with every tap closed, the leak is on the mains side. For buried pipework, tracing is a specialist job using thermal imaging or acoustic equipment rather than opening floors speculatively.
Temporary measures and their limits
Repair tape and clamps buy time, nothing more. They can hold a weeping joint overnight so you keep water on until a proper repair, but they should never be treated as a finished job, because the pipe underneath continues to corrode or the joint continues to work loose. Leak-sealer additives poured into heating systems are worse: they can block heat exchangers, filters and pump gaps, converting a small pipework repair into a component replacement.
What a proper repair involves
Isolating and draining the affected section, cutting back to sound pipe, and remaking the joint in the correct material for the pipe run and its location. Where a joint has failed because of movement or stress, the fix includes supporting or re-routing the pipe so the same stress does not return. On heating pipework, the system is refilled, re-pressurised, vented and re-dosed with inhibitor, because inhibitor is lost with the water that drained out.
Reducing the chance of a repeat
Replace flexible connectors on a sensible cycle rather than waiting for them to burst, lag pipework in lofts and unheated spaces, and support long runs properly so they cannot rub or flex. Keep a working stopcock you can actually turn. If your pipework is old imperial copper or the property has had repeated joint failures, a planned section replacement is far less disruptive than an emergency one.
Get a leak traced and repaired
Tell us where the water is showing and whether boiler pressure is dropping, and we will work back to the actual source.
Questions we get asked
Why does my ceiling stain move each time it rains inside?
Water runs along joists and pipework before dripping, so the stain marks the lowest point of the path, not the leak. The source is almost always uphill of it.
Can I use a leak sealer in my heating system?
We would not recommend it. Sealers can block heat exchangers, magnetic filters and pump clearances, turning a straightforward pipework repair into an expensive component failure.
Is a small weep worth fixing now?
Yes. Slow leaks rot joists and plaster quietly for months, and a joint that weeps is a joint that will eventually fail properly, usually at the worst possible time.
Does the heating need re-dosing after a pipe repair?
If water was drained from the system, yes. Inhibitor goes out with the water and the system is left unprotected against corrosion until it is replaced.
Do you have to lift floors to find a leak?
Not as a first step. Pressure testing, isolation and tissue-testing accessible joints narrow the location first, so any floor that does come up comes up in the right place.