An outside tap looks like a simple fitting on the outside wall, but the pipe that feeds it starts indoors, passes through the wall and is exposed to the cold for part of its length. When something goes wrong, the tap itself may look fine while water leaks inside the wall or behind a kitchen unit. The first sign is often a damp patch on the inside wall, a swollen cabinet base under the sink, or a water meter that moves when nothing is on.
This guide explains why outside taps leak inside the wall, what to shut off, and how the leak is found without taking the wall apart.
How is an outside tap usually plumbed?
A typical outside tap is fed by a branch from the cold mains, often from near the kitchen sink or the stopcock. Along that branch you would normally expect:
- An isolation valve indoors, so the outside tap can be shut off and drained for winter without turning off the house.
- A double check valve, which stops water from a hose flowing back into the drinking water supply. The water fittings regulations guidance calls for backflow protection of this kind on outside taps. It may be a separate fitting on the pipe indoors or built into the tap.
- A pipe through the wall, sometimes in a sleeve, sometimes sealed only with mortar or foam.
- A wall plate or backplate outside, into which the tap is screwed.
Older installations, and some that were added quickly, may have no isolation valve, no check valve, or joints hidden inside the wall. Each of those makes a leak more likely or harder to deal with.
Why does an outside tap leak inside the wall?
| Cause | How it happens | Typical sign |
|---|---|---|
| Frost damage to the pipe | Water left in the section through the wall freezes, expands and splits the pipe or pushes a joint apart | Damp appears as the weather warms up after a cold spell |
| Joint inside the wall or at the backplate | A compression or threaded joint loosens as the tap is used and twisted | Damp directly behind the tap position, sometimes only after the tap has been used |
| Check valve | A leaking body or joint, or a valve damaged by frost | Water around the valve inside, often under the sink |
| Isolation valve | A weeping spindle or connection, especially after being turned for the first time in a while | Drips at the valve, wet cabinet base |
| Cracked tap body | Frost splits the tap, and water tracks back along the pipe into the wall | Water outside as well as damp inside, worse when the tap is used |
| Rain or hose water tracking in | Water runs into a gap around the pipe where it passes through the wall | Damp after rain or after using a hose, dry otherwise |
The last row is not a plumbing leak at all, but it produces the same damp patch, so it is worth keeping in mind.
What should you do if your outside tap is leaking inside?
- Close the outside tap's isolation valve, if it has one. A slotted valve turns with a screwdriver; a valve with a handle turns by hand.
- Open the outside tap to release the water left in the pipe, with a container underneath.
- If there is no isolation valve, close the main stopcock, then open the outside tap to drain what you can.
- Protect the area inside. Clear the cupboard, put down towels and keep water away from sockets and appliances.
- Watch the damp. Mark its edge with a pencil and the date.
- Note what happens next. If the damp stops growing once the branch is isolated, the leak is on the section between the valve and the tap.
If you need the rest of the house back on water before a repair, closing the isolation valve is usually enough. Where there is no valve, our guide to temporary fixes for a leaking pipe explains the options and their limits.
How can you tell where along the pipe it is leaking?
A few observations narrow it down before anything is opened up.
- Damp only after using the tap points to the tap body, the backplate joint or water running back along the outside of the pipe.
- Damp that grows with the tap off but the valve open points to the pressurised section between the valve and the tap, often inside the wall.
- Damp near the valve under the sink points to the valve, the check valve or their joints.
- Damp that stops when the isolation valve is closed confirms the leak is downstream of it.
- Damp that continues with the isolation valve closed means the leak is upstream, on the branch or another pipe, or not from the outside tap at all.
- Damp only after rain points to the gap around the pipe or another route for rainwater through the wall.
A meter test gives a second check. With every tap and appliance off, read the meter, wait at least half an hour and read it again, first with the outside tap's valve open and then with it closed.
How do thermal and moisture checks find a leak inside the cavity?
A leak inside the wall rarely shows exactly where it starts. In a cavity wall, water can run down the inner face of the outer leaf, cross the cavity on a wall tie or through debris, or spread along a pipe. In a solid wall, it soaks into the brickwork and plaster. A survey uses two main tools to find it.
- Moisture mapping. Readings are taken in a grid on the inside wall, around the pipe entry and along the likely pipe route. The wettest area is usually at or below the leak, and the pattern shows whether the water follows the pipe or comes from higher up the wall.
- Thermal imaging. Cold mains water is usually cooler than the room, and wet plaster cools as it evaporates. A thermal camera shows the cool line of the pipe while water is run through it, and a cooler spread where water has soaked into the wall. Our thermal imaging leak detection page explains what the camera can and cannot see.
- Isolation and pressure checks. Capping the branch and holding it under pressure confirms whether that section is losing water.
- A look from outside. Checking the backplate, the seal around the pipe and the brickwork below the tap for staining or salts.
With those results, the repair can be made through one small opening, often from the outside, rather than by chasing along the inside wall. Our guide to the signs of a water leak in a wall covers how damp in walls behaves more generally.
How do you stop an outside tap freezing?
Frost is the most common reason an outside tap leaks inside. Before winter:
- close the indoor isolation valve for the outside tap;
- open the outside tap to drain the pipe, and leave it open so any water left can expand;
- disconnect any hose, as a connected hose holds water against the tap;
- fit an insulated cover over the tap;
- check that the pipe inside is lagged where it runs along a cold external wall.
If your outside tap has no isolation valve, it is worth having one fitted, together with a check valve if there is none. Our leak prevention page covers other winter checks.
What does the repair usually involve?
It depends on where the leak is. A leaking isolation or check valve under the sink can simply be replaced. A leak at the backplate is often fixed from outside by removing the tap and remaking the joint. A pipe split inside the wall usually means opening the wall at the leak point, or rerouting the pipe through a new hole with a proper through-wall kit, which can be easier than repairing it in place. Whatever the repair, the gap around the pipe should be sealed so rain cannot follow it in.
When should you call a leak detection specialist?
- the damp continues with the outside tap's valve closed;
- there is no isolation valve and you cannot tell which pipe is leaking;
- the pipe route is hidden behind fitted units or inside the wall;
- you cannot tell whether the damp is from the pipe or from rain;
- you need the leak documented for an insurer or landlord.
A thermal imaging survey is £150 to £250, and a damp and moisture survey is £250 to £450. Call 020 3912 0200 to describe what you are seeing.
Frequently asked questions
Why is my outside tap leaking inside the house?
Usually because the pipe or a joint inside the wall has been damaged by frost or has loosened, or because the indoor isolation valve or check valve is leaking. Rain running in around the pipe can cause similar damp.
Can a frozen outside tap burst a pipe inside the wall?
Yes. Water left in the pipe section through the wall can freeze, expand and split the pipe or push a joint apart. The leak often shows only when the ice thaws.
Do outside taps need a check valve?
Water fittings regulations guidance calls for backflow protection, normally a double check valve, on outside taps to stop water from a hose getting back into the drinking water supply.
How do I turn off my outside tap for winter?
Close the isolation valve for the outside tap indoors, open the outside tap to drain the pipe, disconnect the hose and fit an insulated cover.
Can the leak be found without opening the wall?
Usually, yes. Isolating the branch, moisture mapping and thermal imaging normally locate the leak closely enough for a single, small opening.
