4 Compression Fitting Mistakes That Cause Leaks
Compression fittings are the most common way to join copper pipe without soldering. They're simple, reliable, and require no heat. But they leak more often than they should — almost always because of one of four mistakes made during installation. Here's what goes wrong and how to get it right.
Quick Recap: How Compression Fittings Seal
A compression fitting has three parts: the body, the nut, and the olive. The olive is a small brass or copper ring that sits around the pipe. Inside both the fitting body and the nut, there are tapered seats. When you tighten the nut, those two tapers squeeze the olive from both ends, compressing it onto the copper pipe. That compression is what creates the watertight seal — not the thread, not the nut, not the body. It's the olive.
Understanding that makes the four mistakes below obvious. Everything that goes wrong with a compression fitting is about the olive not sealing properly.
Mistake 1: No Jointing Compound
This is the most common mistake and the easiest to fix. A compression fitting will often seal without jointing compound — but "often" isn't "always", and on a joint that's going to be hidden behind a wall or under a floor for the next 20 years, "often" isn't good enough.
Jointing compound does two things:
- It lubricates the olive. When you tighten a dry olive, it grabs and squeaks against the brass tapers. That friction means the olive compresses unevenly — it bites hard on one side and barely touches the other. Jointing compound lets the olive slide smoothly into position and compress evenly all the way around.
- It fills microscopic gaps. Even a well-tightened olive can have tiny imperfections in the seal. Jointing compound sets to a firm paste that fills those gaps. If you've slightly undertightened the fitting, the compound covers for you.
How to apply it
Use a multi-purpose jointing compound that's approved for potable water (and gas if you're working on gas pipework). Apply a thin layer to the olive and to the tapered seats inside the fitting body and nut. You don't need much — a light coating is enough. Then assemble and tighten as normal.
The squeak test
If you're tightening a compression fitting and it squeaks, that's metal grinding on dry metal. The olive is compressing unevenly. Take it apart, apply jointing compound, and reassemble. A properly lubricated fitting tightens smoothly and firmly with no squeaking.
Mistake 2: Overtightening
The instinct is understandable: tighter must be better. More force means a tighter seal. But compression fittings have a sweet spot, and going past it makes things worse, not better.
When you overtighten:
- The olive deforms past its useful range. It squashes flat instead of compressing evenly. A deformed olive can't seal properly — it creates gaps instead of closing them.
- The pipe pulls out of alignment. Excessive force drags the pipe into the fitting, putting it at an angle. An angled pipe means the olive isn't sitting square on the seat.
- The olive starts appearing behind the nut. If you can see the olive poking out the back of the nut, you've gone too far. At this point, you've used up all the available compression and there's no thread left to adjust with.
- You can't fix it without cutting. An overtightened olive is permanently deformed onto the pipe. You can't reuse it. You'll need to remove it (see the bonus tip below) or cut back the pipe and start fresh.
How tight is right?
Hand-tighten the nut first, then use a spanner to turn it roughly one and a quarter turns further. That's it. With jointing compound on the olive, that's more than enough to create a solid seal. If it leaks at that point, the answer is not "tighten more" — it's "take apart and investigate".
Mistake 3: Undertightening
The opposite problem, and potentially worse because it can catch you out years later. An undertightened fitting might not leak immediately — the jointing compound or just the friction of the olive sitting loosely on the pipe can hold water pressure for months or even years. Then one day — often when you're on holiday and nobody's using the taps — the pressure shifts slightly and the fitting lets go.
Undertightening usually happens for one of two reasons:
- You got distracted. The phone rang, someone asked a question, you moved on to the next fitting and forgot to go back and tighten this one. It happens to everyone, including plumbers with decades of experience.
- You were too cautious. Having heard that overtightening is bad, you erred too far the other way and left the fitting finger-tight. Finger-tight is not tight enough.
The pen mark trick
Once you've tightened a fitting properly, draw a small mark across the nut and the body with a felt-tip pen. When you do your final check before turning the water on, every fitting should have a mark. If one doesn't, it hasn't been tightened. Simple, effective, and it's saved more ceilings than any other trick in plumbing.
Mistake 4: PTFE Tape in the Wrong Place
This is the mistake that looks right but does absolutely nothing. Someone has a leaking compression fitting, so they wrap PTFE tape around the thread of the nut and retighten it. The tape sits between the thread of the nut and the thread of the body. It feels like you're doing something useful. You're not.
Here's why: the thread doesn't seal anything. The thread's only job is to pull the nut towards the body, which compresses the olive. Water doesn't pass through the thread — it passes through the olive. Wrapping PTFE on the thread is like putting a plaster on the wrong arm.
Where PTFE tape actually goes
If you're using PTFE tape to fix a weeping compression fitting, it goes around the olive, not the thread. Take the fitting apart, wrap 5–7 turns of PTFE around the olive (in a clockwise direction when looking at the end of the pipe), and reassemble. The tape fills the gaps between the olive and the tapered seats where the actual sealing happens.
PTFE tape direction matters
Always wrap in the direction the nut tightens (clockwise when looking at the fitting end-on). If you wrap it the wrong way, the nut unravels the tape as you tighten it and you end up with a wad of loose PTFE inside the fitting doing nothing. Clockwise wrapping means the nut pulls the tape tighter as it turns.
For a more detailed guide on fixing a leaking compression fitting with PTFE tape and olive replacement, see our full compression fitting repair guide.
Bonus: How to Remove a Stuck Olive
If you've overtightened a fitting, damaged an olive, or simply need to reposition a joint, the olive will be stuck on the pipe. It's been compressed onto the copper — it won't slide off by hand. There are two ways to remove it:
Method 1: Olive cutter tool
An olive removal tool (sometimes called an olive cutter or olive splitter) clamps around the pipe and cuts through the olive without damaging the copper underneath. Slide it on, tighten the cutting blade against the olive, and rotate. The olive splits and slides off. These cost a few pounds and are worth having if you do any amount of plumbing work.
Method 2: Junior hacksaw and screwdriver
If you don't have the tool, a junior hacksaw and a flat-head screwdriver will do the same job. Make a careful diagonal cut across the olive with the hacksaw — cut into the olive, not the pipe. You only need to go about three-quarters through. Then wedge the screwdriver into the cut and twist. The olive will snap and slide off.
Once the old olive is off, check the pipe underneath. If it's scored or marked from the removal, you'll need to cut back to clean copper before fitting a new olive. A new olive needs a smooth, round surface to seal against.