Why Does a Compression Fitting Leak? The Real Causes and How to Fix Each One

A compression fitting leaks because the olive is not making a continuous, even seal against both the pipe and the inside of the fitting body. That almost always traces back to one of six things: undertightening, overtightening, a badly cut or burred pipe end, the pipe not pushed fully home, a reused olive, or pipe that is scored or oval. Work through them in order and you will find the culprit.

What Actually Seals in a Compression Joint

There are three parts: the fitting body, the nut, and the olive (also called a ferrule) that sits between them. Tightening the nut drives the olive forward into a tapered seat inside the body, squeezing it down onto the pipe and outward against the taper. Two metal-to-metal seals form at once.

Nothing else is sealing. No gasket, no rubber, no thread seal. The threads only supply clamping force, which is why wrapping PTFE around the male thread of a leaking compression fitting almost never fixes anything: you are sealing a surface that was never leaking.

The Six Reasons Compression Joints Weep

1. Not Tightened Enough

The most common cause and the easiest to rule out. Finger tight plus roughly one full turn with a spanner is the usual starting point for 15mm copper, a little more on 22mm. If the joint is weeping slowly rather than spraying, try another quarter turn and watch it for a minute.

2. Tightened Far Too Much

Rarely suspected, because the instinct when something leaks is to keep pulling on the spanner. Push an olive too far into the taper and it work hardens, cracks, or ploughs a ridge into soft copper. After that it will not seal at any torque. If you have already given a fitting two or three full turns and it still weeps, stop. It needs a new olive.

3. A Rough or Out of Square Cut

Pipe cut with a hacksaw leaves a burred, slightly angled end. The burr scores the inside face of the olive as the nut goes on, and an angled cut leaves the olive sitting crooked in the taper. Use a pipe cutter, then deburr inside and out.

4. Pipe Not Pushed Fully Home

Every compression body has an internal shoulder the pipe should bottom out against. If the pipe stops short, the olive sits in the wrong place on the taper and never compresses evenly. Dry assemble first, pencil a mark on the pipe at the nut face, then check the mark has not moved after tightening.

5. A Reused or Damaged Olive

Olives are consumables. Once compressed, an olive has taken the exact shape of that pipe and that taper, and it will rarely reseal on another. Old ones also pick up scale and pitting. If you have taken a joint apart, fit a new olive.

6. Damaged, Oval or Mismatched Pipe

Copper that has been gripped in a vice, kinked at a bend or scratched along its length will leak no matter how good the fitting is. A single lengthwise scratch under the olive gives water a channel straight through the seal. Oval pipe from a badly used bender is the same story. Cut back past the damage.

Diagnosing Which One You Have

1. Dry the joint and confirm the leak really is coming from that fitting and not running down from elsewhere.
2. Note the behaviour. A weep that only appears when a tap is opened points at a marginal seal. A steady drip usually means a damaged olive or pipe.
3. Give it a quarter turn and retest. If that stops it, you were simply undertightened.
4. If it does not stop, do not keep tightening. Isolate, drain down, strip the joint and inspect.

With the nut off, check three things: the olive (cracked, split, deeply grooved?), the pipe end (square, deburred, unscratched?), and the taper inside the body (clean, or packed with old compound and grit?).

Rebuilding the Joint Properly

1. Isolate and drain. A compression joint will not seal against standing water sitting on the sealing faces.
2. Undo the nut and slide it back. If the old olive will not shift, cut the pipe back rather than hacking the olive off with a hacksaw.
3. Cut square with a pipe cutter and deburr inside and out.
4. Wipe the pipe, the new olive and the inside of the body with a clean cloth. Grit is the enemy here.
5. Slide the nut on, then the new olive, then push the pipe fully home until it bottoms out.
6. Hand tighten, then one full turn with a spanner while holding the body still with a second spanner so you are not twisting the pipework.
7. Restore pressure slowly, dry the joint, and recheck after ten minutes and again after an hour.

Does Jointing Compound Help?

A smear of non setting compound or a couple of wraps of PTFE around the olive itself is common practice, particularly on chrome or slightly imperfect pipe. It is a lubricant and gap filler, not a substitute for a good joint. If a fitting only seals when loaded with compound, it will come back. Never put PTFE on the nut threads: it adds friction and gives false feedback on how tight the joint really is.

When to Stop and Call Someone

Compression work on isolated, drained cold supply is normal DIY territory. Step away if the fitting is on a sealed heating system or a gas line, buried in a wall or floor with no access, or if cutting the pipe back just reveals more corroded pipe.