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Published by Mo's Plumbing and Heating
Can You Use Plastic Pipe With Compression Fittings?
Yes, and it is completely normal practice. There is one condition, it is not optional, and leaving it out is the single most common cause of a joint that seals perfectly on the day and weeps six months later.
The Short Answer
You can, provided you fit a pipe insert, sometimes called a pipe stiffener, into the end of the plastic pipe before making the joint. The insert is a rigid sleeve that supports the pipe wall from the inside so the olive can crush onto something solid instead of collapsing it. Without one the plastic slowly deforms under the olive and the joint fails later. Use the insert the pipe manufacturer specifies, since they are not universal.
Why the Insert Matters
A compression joint works by crushing a soft brass olive between the nut and the fitting body so that it grips and seals against the pipe. That relies on the pipe being rigid enough to push back.
Copper is. Plastic is not. Tighten an olive onto unsupported plastic and the pipe wall deforms inward under the pressure, so the olive never achieves a proper seal and the pipe is left with a permanent constriction where it was squeezed.
What makes this dangerous as a mistake is that it usually seals at first. The joint is tight, you turn the water on, nothing leaks, and you finish the job satisfied. Then over weeks and months the plastic continues to creep under sustained load, the compression relaxes, and the joint starts weeping, generally after everything has been boxed back in.
The insert prevents all of that by giving the olive something rigid to crush against. It is a few pence and it takes two seconds to fit.
Getting It Right
Short list, and none of it is difficult.
- Use the insert specified for your pipe. Plastic plumbing pipe is not fully standardised between manufacturers, and inserts vary in wall thickness and length to suit. An insert that is loose is doing much less than you think.
- Push the insert fully home so its shoulder sits against the pipe end. A partly inserted stiffener supports the wrong part of the pipe.
- Cut the pipe square with a proper plastic pipe cutter rather than a hacksaw. An angled end will not seat correctly and a ragged one can damage the olive.
- Do not over-tighten. Plastic needs less than copper: hand tight and then roughly a turn, and stop when it firms up. The instinct to give it another half turn because it is only plastic is exactly wrong.
- Check the fitting is suitable. Most standard brass compression fittings accept plastic with an insert, but some manufacturers specify their own fittings for their pipe, and where a system is under warranty that specification matters.
Joining Plastic to Copper
This comes up constantly in older properties, where a repair or an alteration meets existing copper, and it is entirely legitimate.
A standard compression coupler will take copper on one side and plastic on the other, with an insert in the plastic end and nothing in the copper end. That is a normal, permanent joint and there is nothing improvised about it.
Push-fit couplers do the same job and most modern ones accept both materials in the same fitting, again with an insert in the plastic. Check the fitting's own markings, since a few are rated for one material only.
The thing to avoid is trying to solder to plastic, which is not a joke worth making at anyone's expense because people do attempt it. Plastic pipe melts. The transition has to be mechanical.
One practical note: plastic and copper expand at different rates with temperature, so a run mixing both wants clipping that allows a little movement rather than being pinned rigid at every point. That matters more on heating pipework than on cold supply.
Where Plastic Is and Is Not the Right Choice
Worth being even-handed, because plastic gets treated either as a cheap substitute or as obviously superior, and neither is right.
Plastic is good in floor voids and long concealed runs, because it comes in coils with fewer joints, tolerates a degree of movement, and is far more forgiving of freezing, which matters in an unheated void. It also needs no flame, which in an old building is a real advantage rather than a convenience.
Copper is better where the pipe is exposed and visible, where it needs to be rigid and hold its position, where it will be in direct sunlight, and near appliances that get hot. Plastic must be kept away from a boiler's immediate connections, and manufacturers specify a length of copper at the appliance for exactly that reason.
Rodents are the other consideration people forget. Plastic pipe in a void can be gnawed and copper cannot, which is occasionally the deciding factor in an older building with a known problem.
In practice a modern property is usually a mix, chosen for each run, and the presence of both is not a sign anybody cut corners.
The Missing Insert Under a Glasgow Floor
This is a common find in the city's flats, and it is useful to know about because of when it surfaces rather than how often. Plastic pipe arrived in domestic plumbing in bulk at roughly the same time as the wave of flat refurbishments through the 1990s and 2000s, and a good deal of that work was done well. Some of it was done by people meeting plastic pipe for the first time, and the insert is the detail that gets left out.
Because the joint seals initially, nothing announces the mistake for months or years. What eventually appears is a slow weep under a floor or behind boxing, and by then the flooring has been laid, the kitchen has been fitted and the boxing has been tiled. The repair is trivial and reaching it is not, which is the whole cost of the omission.
It is worth a specific look if you have a flat where the plumbing was altered in that period and there is unexplained damp near a run of plastic pipe. It is also worth checking any joint you can already see and reach, since a plastic pipe entering a compression fitting with no visible insert shoulder is a joint on borrowed time and remaking it while it is accessible costs almost nothing.
The freezing point is the other local relevance. Plastic in an unheated tenement void or a top-floor roof space copes with a hard frost considerably better than copper, because it can expand slightly rather than splitting. That is a genuine argument for plastic in exactly the places in these buildings where pipes actually freeze, provided the joints at each end were made properly.
When to Call Someone Out
This is a straightforward job and most people can do it. Worth calling when you find a weeping plastic joint behind boxing or under a floor and want it traced before anything comes up, or where a run mixes materials and you are not sure what is joined to what. Also worth it near a boiler, since manufacturers specify copper at the appliance connections and getting that wrong can affect a warranty. Mo, Kyle or Matt cover this work. If you are checking your own joints and find plastic entering a compression fitting with no insert, that is worth remaking while you can still reach it rather than waiting for it to announce itself.
Leak Detection in GlasgowCommon Questions
It will usually seal on the day, which is what makes it such a persistent mistake. The plastic then creeps under sustained pressure from the olive, the joint relaxes over weeks or months, and it starts weeping later, typically after everything has been closed up. The failure is delayed rather than avoided.
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Read Next
Full guide
How to Repair a Leaky Pipe
Turn the water off first, then find out what the pipe is.
How to Fix a Leaking Pipe Joint
Identify the joint type first.
How to Solder Copper Plumbing
Clean the pipe end and the inside of the fitting until both are bright, flux both, assemble, and heat the fitting rather than the solder until the solder is drawn into the joint by capillary action.
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