Four things it can be, and only two are ours
A lower-ground or excavated basement in Kensington is below the water table for at least part of the year. That single fact changes the diagnosis, because in a first-floor flat any water is a leak by default, and down here it genuinely is not.
| Cause | Characteristic | Whose job |
|---|
| Plumbing leak | Clean water, unrelated to weather, responds to isolating the supply or draining the heating | Ours |
| Groundwater through failed waterproofing | Follows rainfall by hours or days, often at floor-to-wall junctions, sometimes with mineral staining | Waterproofing specialist |
| Condensation | Worst on cold surfaces, worse in humid weather and with poor ventilation, no discrete source | Ventilation, not plumbing |
| Drainage backup or a failed pump | Foul smell, correlates with usage upstairs or with storms, often sudden | Drainage contractor, sometimes us |
Getting this wrong is expensive in both directions. People spend thousands on tanking to cure a weeping supply pipe, and people spend thousands looking for a pipe that does not exist when the real answer is that the tanking failed.
The tests that separate them, in order
1. Isolate and watch the meter
Turn off the internal stop valve so no water can enter the property, and watch the meter for an hour. If the meter is still turning, there is a loss on the pressurised side between the boundary and the valve, which puts it in the supply pipe, potentially under the lightwell or the front vault. If the meter stops dead and the water keeps coming, the pressurised supply is not the source. That is a twenty-minute test and it eliminates the most expensive possibility or confirms it.
2. Drop the heating pressure
If there is a sealed heating system, note its pressure, then check it again after twenty-four hours with the water isolated. A falling gauge with no other explanation points at heating pipework, which in a basement conversion frequently runs in or under the new floor slab.
3. Correlate with rain
Keep a note for a week: when water appears, how much, and what the weather did. Groundwater ingress lags rainfall, typically by hours to a couple of days as the surrounding ground saturates. Plumbing does not care what the weather is doing. This is the single most informative thing a homeowner can do before anyone attends, and it is free.
4. Look at where it enters
Groundwater arrives at junctions and weaknesses: the joint between floor slab and wall, around service penetrations, at construction joints, through the back of a lightwell. A plumbing leak usually enters somewhere structurally arbitrary, because it is following a pipe, not a crack.
The supply pipe under the lightwell
This is the overlap case and it is common. In a converted Kensington lower ground floor the incoming supply frequently runs under the front lightwell or through the coal vault before reaching the internal stop valve. A leak on that section produces water in a basement that behaves almost exactly like groundwater, because it is soaking into the same ground and arriving through the same wall junction.
What separates them is the meter test above, plus the fact that a supply pipe leak is constant rather than weather-driven. It is worth being clear about the ownership line: Thames Water are responsible for the water mains and the communication pipe linking your supply pipe to those mains, while as a homeowner you are responsible for the supply pipe running from the boundary of your property into your home, and for internal pipes and fittings. In a Kensington terrace with a vaulted pavement, the boundary can be in an unintuitive place.
If a leak has been reported to you by the water company, note that under section 75 of the Water Industry Act 1991 an undertaker can serve notice requiring steps to be taken within a period of not less than seven days, and if those steps are not taken can carry them out itself and recover its reasonable expenses. Our Thames Water pages go through what that notice means in practice.
Why a failed sump pump looks like a burst pipe
Most excavated basements in this part of London rely on a drained cavity system: a membrane behind the finishes, a perimeter drainage channel, and a sump with one or more pumps that lift the water out. It works well and it is completely dependent on the pumps.
When a pump fails, or the float switch sticks, or the power trips, water that was being managed invisibly appears very quickly and in volume. It presents as a sudden flood with no obvious source, which is exactly what a burst pipe presents as. The distinguishing features: it comes up from the floor rather than down from anywhere, the sump chamber is full, and it usually coincides with heavy rain or a power event.
Before assuming a pipe has burst, lift the sump lid and look. If the chamber is full and the pump is not running, that is your answer and it is an electrician or a pump specialist, not us.
What we do and what we decline
We locate and repair plumbing leaks, including supply pipe and heating leaks under and around basement slabs, and we reinstate the finishes we have to open. Detection is charged at £150 an hour, agreed before we attend, and if we do not find the leak you do not pay for the time we spent looking.
We do not install or repair tanking, cavity drainage membranes or sump systems. That is a structural waterproofing discipline with its own design responsibility and its own insurance, and a plumber patching a waterproofing failure is doing you harm. If the diagnosis lands there, we will say so, give you what we found in writing so you are not paying someone else to establish it again, and step back.
We also decline the case where the basement has never been dry and the owner wants a plumbing explanation for it. If it was damp before the bathroom was installed, the bathroom is not the cause.
If you are seeing a wet patch on a wall or floor and cannot place it, that page walks through the same reasoning for above-ground rooms.
