Compression Fitting Leaking? Why It Happens and How to Fix It

A leaking compression fitting is usually either not quite tight enough or has a damaged olive, the soft brass or copper ring that the nut squeezes onto the pipe to make the seal. The first thing to try is a quarter turn: hold the fitting body still with one spanner and turn the nut with another. If it still weeps, stop tightening, because more force just crushes the olive. The joint needs taking apart and the olive checking, and usually replacing. A leaking push-fit joint can't be tightened at all and has to be remade. This guide explains why these joints fail and how to fix one so it stays dry.

If the leaking joint is on a gas pipe, don't touch it. Turn off the gas at the meter, open the windows and call the National Gas Emergency Service on 0800 111 999.

How a compression joint seals

A compression fitting has three parts: the body, a nut, and an olive. The pipe goes into the body, the olive sits on the pipe, and as the nut is tightened it pushes the olive into a tapered seat inside the body. The olive squeezes down onto the pipe and against the seat, and that metal-to-metal contact is what stops the water. The threads only hold the nut on. They don't seal anything.

You'll find compression joints all over a house: on isolating valves under sinks and toilets, on stopcocks, on tap connectors, on radiator valves, and anywhere a plumber has joined pipe without soldering. They're used where a joint may need taking apart later, which is also why they're the joints most likely to be disturbed.

First try: the quarter turn

1. Dry the joint completely with a cloth or tissue so you can see exactly where the water is coming from. Make sure it's the nut and not a pinhole in the pipe nearby, or a leaking valve spindle.
2. Hold the body of the fitting still with one spanner. This matters: if you only use one spanner, you twist the pipe and can loosen the joint at the other end.
3. With a second spanner, tighten the leaking nut about a quarter of a turn.
4. Dry it again and watch it for a few minutes. On a heating pipe, check it again once the heating has been on and the pipe is hot.

If that stops it, you're done. If it slows but doesn't stop, you can try one more small nip. If it's still weeping after that, don't keep going. The joint needs remaking.

Why compression fittings leak

Cause	What you'll see when you undo the nut
Under-tightened when fitted	Olive barely marked, still loose on the pipe
Overtightened	Olive squashed or out of shape, pipe slightly pinched
Olive sat at an angle	Uneven mark round the olive, heavier on one side
Pipe not pushed fully into the fitting	Pipe end short of the stop inside the body
Damaged pipe	Scratches, dents or an oval pipe under the olive, or a pipe cut at an angle
Old joint disturbed	Joint was dry for years, started weeping after work nearby
Plastic pipe without an insert	Pipe squashed inward where the olive grips it

Movement is behind a lot of leaks. An isolating valve under a sink that gets knocked by the bin, a washing machine hose that pulls on a valve, or heating pipes that expand and contract every day can all loosen a joint over time. In older London houses, joints that have been dry for decades often start weeping after someone works nearby, because moving the pipe disturbs an olive that had settled in place.

Remaking a leaking joint

If tightening hasn't worked, the joint needs taking apart and remaking with a new olive.

1. Turn off the water to that pipe, using the isolating valve if there is one or the main stopcock. On central heating, the system has to be isolated or partly drained, and on a sealed system it will need repressurising afterwards.
2. Undo the nut using two spanners and pull the pipe out of the fitting.
3. Remove the old olive. A squashed olive won't slide off. It can be removed with an olive puller, or the end of the pipe can be cut off behind it if there's enough slack in the pipe.
4. Check the pipe end. It must be round, smooth where the olive sits, cut square and free of burrs. A scratched or oval section needs cutting back.
5. Fit a new olive of the right size and type, push the pipe fully into the fitting, and tighten the nut by hand, then with spanners. Use one spanner to hold the body and don't overtighten.
6. Turn the water back on slowly and check the joint dry, then again an hour later.

On plastic pipe, a metal insert must go inside the pipe end before a compression fitting is used, otherwise the olive squashes the pipe and the joint leaks. Our guide to copper vs plastic pipes covers how the two are joined.

Some plumbers use a thin smear of a jointing compound approved for drinking water on the olive to help it seat. PTFE tape on the threads doesn't help, because the threads aren't where the seal is. Neither will save a crushed olive or a damaged pipe.

Leaking push-fit joints

Push-fit fittings seal with a rubber O-ring inside the fitting, and a ring of teeth grips the pipe. There's nothing to tighten. When one leaks, the usual causes are:

• the pipe isn't pushed all the way in to the stop
• the pipe end was cut at an angle, or left with a burr that nicked the O-ring
• the pipe surface is scratched where it meets the O-ring
• a plastic pipe was fitted without its insert

The fix is to turn off the water, release the pipe (most fittings have a collar that you push in while pulling the pipe out), cut a clean, square end past any damage, fit a new insert on plastic pipe, and push it fully home. Some fittings can't be reused once released, so check before you start. Marking the insertion depth on the pipe with a pen helps you see it's fully in.

When to call a plumber

Call a plumber if a quarter turn hasn't stopped the leak and you'd rather not take the joint apart, if the fitting is on a central heating pipe or a pressurised hot water system, if it's on a stopcock or a valve that won't isolate, if the joint is behind a wall, under a floor or inside a kitchen unit you can't easily reach, or if water has already reached the ceiling below. If the leak is under your kitchen sink, our guide to a leak under the kitchen sink helps you pin down which fitting it is. For a leaking stopcock, see our guide to a leaking stopcock, and for a split or cracked pipe rather than a joint, our guide to a cracked pipe.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Remaking a single accessible joint is usually a short job. Our leak repair service covers pipes, valves and fittings across London, our guide to leaking pipes covers other types of pipe leak, and the pricing page explains how jobs are charged.

Frequently Asked Questions