How do I solder a copper pipe joint?

A soldered joint is a capillary joint: molten solder is drawn into the gap between pipe and fitting by capillary action, and it only happens if both surfaces are properly clean and properly fluxed. That is why nearly every failed joint failed at the preparation stage, and why adding more heat and more solder to a bad joint never rescues it.

Safety

Step by step

1. Deal with the hot work before anything else.Clear flammable material from the area, put a heat-resistant mat behind the joint, have a water spray and an extinguisher within reach, and know what is on the other side of the surface you are heating. Check the area repeatedly for at least an hour after finishing.
2. Drain the pipework completely.A joint with water in it cannot reach soldering temperature — the water absorbs the heat and turns to steam, which blows the solder straight out. Isolate, drain, open a tap or a drain point below, and use a pipe freezing kit or a bread plug only if you understand what you are doing.
3. Cut square and deburr.A pipe slice gives a square cut. Ream the inside edge and clean the outside so the pipe enters the socket fully and evenly. A pipe cut at an angle leaves an uneven capillary gap and the joint will not fill.
4. Clean both surfaces to bright metal.Wire wool or an abrasive strip on the pipe end, and a fitting brush inside the socket. Copper oxidises and solder will not wet an oxidised surface. This is the step that decides whether the joint works, and it is the step people rush.
5. Flux lightly, both surfaces.A thin, even film on the cleaned pipe end and inside the socket. Flux removes the last of the oxide as the joint heats and lets the solder flow. Too much flux runs into the pipe and can corrode it from inside, so light is right.
6. Assemble fully and support the pipe.Push the pipe home to the stop, twist slightly to spread the flux, and make sure nothing will move while it cools. A joint disturbed while the solder is still plastic will leak.
7. Heat the fitting, not the solder.Play the flame on the body of the fitting, moving it around. You are heating the metal so that it melts the solder — touching the flame to the solder itself produces a blob sitting on a cold joint.
8. Know when it is ready.The flux stops bubbling and goes clear, and a bright ring of solder appears right round the mouth of the fitting. On a solder-ring fitting, that ring appearing all the way round is the signal. On an end-feed, touch the solder to the joint mouth and it will be drawn in when the metal is hot enough.
9. Do not overheat.Overheated flux burns and stops working, and the joint then will not take solder at all no matter how much heat you add. If a joint refuses, let it cool, take it apart, clean and flux again.
10. Let it cool undisturbed, then clean up.Do not quench it and do not move it. Once cool, wipe the residual flux off — left on, it corrodes the copper. Then refill slowly and check every joint under pressure before boarding anything in.
11. Know when to use something else.Push-fit and press-fit joints exist precisely because soldering in a confined space, near timber, above a ceiling or on a system that cannot be fully drained is a poor idea. Choosing a different joint is often the professional answer, not a cop-out.

Tips

• Buy a fitting brush. Cleaning the inside of the socket properly is impossible with wire wool folded over a finger, and it is half of the joint.
• Practise on offcuts before the first real joint. Ten minutes of practice is what turns this from a gamble into a routine job.
• Leaded solder is not permitted on drinking water pipework in the UK. Use lead-free solder and a flux suitable for potable water, and check the packaging says so.

Common mistakes

• Soldering with water still in the pipe — The joint cannot reach temperature and steam blows the solder out. It is the single commonest reason a joint will not take, and no amount of extra heat fixes it.
• Heating the solder instead of the fitting — Solder melted by the flame sits on the outside of a cold joint and looks superficially fine. Capillary action only happens when the metal itself is hot enough to melt it.
• Adding more heat to a joint that will not take — Once the flux has burnt it cannot clean the surface, and the joint is done. Cool it, dismantle, clean and reflux — that is faster than persisting.
• Soldering against a joist or plasterboard without a heat mat — Timber and paper facings char and can smoulder for a long time before they ignite. Fires from plumbing hot work characteristically start after the plumber has left.
• Using too much flux — Excess runs into the bore and stays there. Aggressive flux left inside copper pipework causes pinhole corrosion months or years later.

If it doesn't work

Questions people ask