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How to Solder Copper Plumbing
Soldering is not difficult and it is unforgiving of two things: a wet pipe and a dirty joint. Get those right and the joint will outlast you. Get either wrong and it will look perfect and weep in a fortnight. The harder question in an old building is whether to use a flame there at all.
The Short Answer
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. You are heating the fitting so that it melts the solder, not melting the solder with the flame. The pipe must be completely dry, because water in the pipe carries heat away faster than a blowtorch can put it in and the joint will never reach temperature.
What You Need
Not much, and the cheap version of each is fine except the cutter.
- A pipe slice or a proper pipe cutter. A hacksaw works and leaves a ragged end that has to be filed square, and a joint that is not square is a joint that leaks.
- Wire wool or an abrasive cleaning strip, and an internal brush for the fitting. Both surfaces have to be bright, not merely clean looking.
- Flux, and use a proper plumbing flux rather than an aggressive self-cleaning paste. Excess aggressive flux left in a joint keeps attacking the copper afterwards.
- Lead-free solder. Leaded solder must not be used on drinking water pipework and has not been acceptable for that for a long time.
- A blowtorch, and a heatproof mat. The mat is not optional in an old building for reasons covered below.
- End feed or solder ring fittings. Solder ring have the solder built into them and are more forgiving for a beginner; end feed take solder from the reel and are cheaper and neater.
The Method
Order matters here and every step is doing something specific rather than being ritual.
- Drain the pipe completely. Not mostly. Water in the pipe conducts heat away so effectively that the joint will not reach soldering temperature, and this is the single commonest reason a first attempt fails.
- Cut square, then deburr the inside of the cut with a file or the reamer on the cutter. A burr disturbs flow and stops the pipe seating fully.
- Clean the outside of the pipe end and the inside of the fitting until both are bright copper. Do this even on new fittings, which carry an oxide film from the factory.
- Flux both surfaces thinly. More flux is not better and excess simply runs into the pipe.
- Assemble, twisting slightly to spread the flux, and make sure the pipe is fully home in the fitting.
- Heat the FITTING, moving the flame around it rather than holding it in one place. Do not put the flame on the solder. When the fitting is hot enough, touching the solder to the joint mouth will draw it right round by capillary action.
- Stop as soon as a bright ring shows all the way round. Overheating burns the flux, which leaves a joint that looks soldered and is not.
- Let it cool on its own. Quenching with water stresses the joint. Then wipe the flux residue off with a damp cloth, because flux left on the outside corrodes the copper over time.
Why Joints Fail
Four reasons, and three of them happen before the flame comes out.
Water in the pipe. It cannot be overstated because it is by far the most common. If a pipe will not drain because there is no low point or no drain-off, that is a different problem with its own techniques and its own guide in this cluster.
An unclean surface. Solder will not bond to oxide or grease. If a joint looks like the solder sat on the outside as a blob and did not draw in, that is nearly always cleanliness rather than heat.
Overheating. Burnt flux goes black and stops working, and further heat then does nothing useful. The tell is a joint that took much longer than it should and has dark residue around it.
And movement while cooling. A joint disturbed while the solder is still plastic will have a hairline path through it that may not show until the system is under pressure. Support the pipework so nothing shifts.
When a joint does fail, remaking it means taking it fully apart, cleaning both surfaces back to bright, and starting again. Adding more solder to a bad joint does not work and wastes an afternoon.
When Not to Use a Flame at All
This is the judgement that matters more than the technique, and it applies more often in this city's housing than in a modern house.
A blowtorch in a floor void or against a joist is a real fire risk, and the risk is not mainly the flame. It is that heat conducts along the pipe and into whatever the pipe passes through, and that timber which has been drying in a building for a hundred and twenty years ignites at a lower temperature and smoulders rather than flaming. A smouldering joist can sit unnoticed for hours before it becomes a fire, which is why it is the scenario that causes real damage.
So: if the joint is in a confined space, against timber, near insulation, or anywhere you cannot see all round it, use a compression or push-fit fitting instead. That is not a compromise. A correctly made compression joint is a permanent repair and there is no prize for having soldered it.
Where a flame genuinely is the right call in an awkward spot, a heatproof mat behind the joint is the minimum, and having a water spray and a working smoke alarm nearby is sensible. Check the area again twenty minutes after finishing and again before going to bed, because that is the window in which a smoulder announces itself.
And never use a flame near a gas pipe or where you can smell gas. If there is any gas smell at all, stop, get out, and ring 0800 111 999.
The Honest Alternative
For a homeowner doing one repair, compression is usually the better answer and it is worth saying plainly rather than treating soldering as the proper way and everything else as second best.
Compression needs no heat, no fire risk, tolerates a slightly damp pipe, and can be undone and remade if it weeps. Its downsides are bulk, which matters in a tight space, and that it wants a spanner on both sides and reasonable access all round.
Push-fit is easier still and is permanent when the pipe is cut square, deburred and pushed fully home, with an insert in plastic pipe. The professional preference for soldered joints is largely about neatness, compactness and cost at volume rather than about reliability.
Soldering earns its place when you are doing several joints, when space is tight, or when the pipework will be buried and a compact joint matters. For a single repair under a sink, there is no reason to introduce a flame to the situation.
Flame, Old Timber and Tenement Floor Voids
The fire argument is sharper in Glasgow's older housing than the general advice suggests, and it is worth understanding rather than treated as boilerplate. Traditional tenement construction puts pipework in deep floor voids between substantial joists, often with the underside of the floor forming the ceiling of the flat below, and frequently with a century or more of dry dust and debris sitting in the void.
That combination is the problem. Very dry old softwood, an enclosed space with poor visibility, accumulated dust, and a ceiling below that you cannot inspect from where you are working. A smoulder started by conducted heat in that environment is both easy to start and hard to notice, and by the time it is obvious it is in someone else's property as well as yours.
So the practical rule for this housing stock is to prefer a mechanical joint anywhere you cannot see completely around the work, and to reserve the torch for joints in the open where you can watch what is happening. That is not caution for its own sake; it is the reason a lot of professional work in these buildings uses press fittings and compression in voids and keeps soldering for exposed runs.
There is one more local wrinkle worth knowing. Where a repair is being made onto original pipework in an older flat, check what you are joining to before assuming it will take solder. Successive eras of work leave copper, plastic and sometimes lead in one property, and a length that looks like dull copper in poor light can be lead. Solder will not join it, and heat applied to lead does something quite different from what you are expecting.
When to Call Someone Out
Worth calling rather than reaching for a torch when the joint is in a floor void, against a joist, or anywhere you cannot see all round it, and worth calling when the pipe will not drain enough to solder. Also when the pipework you are joining onto turns out to be lead, which will not take a soldered fitting at all. Mo, Kyle or Matt cover this work and generally use press or compression fittings in enclosed spaces for exactly the reasons above. If a joint you have made keeps weeping after two attempts, that is usually a cleanliness or a squareness problem rather than a technique one, and it is a quick fix for somebody with the right tools.
Leak Detection in GlasgowCommon Questions
Almost always water in the pipe or an unclean surface. Water conducts heat away faster than a torch can supply it, so the joint never reaches temperature. If it is dry and still will not take, the copper or the fitting is not bright enough. Solder does not bond to oxide, however hot it gets.
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