How to solder copper pipe correctly

Being able to solder copper pipe is probably a plumber’s most important and most useful skill. It’s not difficult, but there are a couple of key things that you must do every time to ensure a watertight joint.

Before we begin, be sure to check out our other popular articles on copper pipe: how to cut copper pipe and how to fix leaking copper pipe.

How to solder copper pipe

The most important thing to consider when soldering copper is where you are soldering. For example, you can’t solder too close to plastic plumbing fittings, such as water tanks and push-fit connections. Most of these products will have minimum soldering distances. Check which the manufacturer to find out how far away a solder fitting must be.

Next, you’ll need to have the right kind of solder. Lead-based solder can only really be used on central heating system pipework. You can’t use solder which contains lead if the pipes carry water for drinking, washing, or bathing.

And of course, you should consider the area where you are soldering and take appropriate safety measures to prevent the risk of fire.

Solder copper pipe kit

For this task, you’ll need:

  • A blow torch
  • Gas
  • Solder – leaded or lead-free depending on what pipes you’re soldering
  • Water-soluble flux
  • Heat mat
  • Emory cloth or steel wool
  • An old towel

Dry the pipe

There must be absolutely no water in the pipe that you are soldering, so you’ll need to drain it down.

If the pipe in question runs horizontally, you can remove any residual water with a towel or a wet-dry vac.

Clean the pipe and fitting

Using your emory cloth or steel wool, clean both ends of the pipe. This is probably the most time-consuming part of the process. The ends should be bright shiny copper, without any signs of tarnish. If you have just cut pipe, make sure the cut is level and free from any burrs or debris.

You will also need to clean the inside of the fitting.

Apply the flux

Next, you will need to apply flux to the pipe using a suitable brush or flux applicator.

Don’t apply flux to the inside of the fitting, as you will end up with flux inside the pipe. Ideally, you should use water-soluble flux because if the non-soluble kind does get inside, it will damage your pipework and may cause leaks.

Avoid the temptation to apply the flux with your finger. You won’t apply it as evenly, and there’s the risk of cutting your finger on the edge of the pipe if it’s sharp. For the same reason, use a cloth to wipe away any flux you get on the inside of the pipe.

What is flux actually for? Flux stops oxides from forming on the metal when it’s heated up. That’s important because solder prefers pure, clean metal, without any oxides. It’s really difficult to solder without any flux – the solder simply won’t want to flow into the joint.

Use a cloth to wipe away any excess flux. A rule of thumb is: don’t have any flux where you don’t want any solder.

Apply heat

Once you’ve slotted everything together and put your heat mat or mats in place, it’s now time to solder. Get the solder wire ready so you don’t have to put your blowtorch down after lighting it.

There is no need for a roaring flame with the gas on full tilt. You should be looking for a modest, steady blue flame, with no traces of yellow, so that you don’t introduce soot and other impurities into the joint.

The flame itself generally consists of two flames, an out one and an inner one. The hottest point will be the tip of the inner flame, so you’ll really only need to use the final third of the whole flame itself.

Apply heat to the area of the pipe and joint where you want the solder to be drawn into. It’s generally good practice to heat from below and apply solder from the top. That’s because heat rises, so the joint will heat up faster and more evenly. If the pipe does not heat up evenly, the solder will cool down and drip off without actually going inside the joint.

As for how long to heat copper pipe, it will take around 15 seconds, but don’t rush it. Give it plenty of time. You can check if the joint is hot enough by lightly dabbing the solder wire into it and seeing if it melts.

Solder copper pipe – get soldering!

Once the joint is hot enough, dab and run the solder wire along and into the joint so that a little bit melts off each time. Don’t simply push it in until it drips out.

The solder should flow into the joint by capillary action. Capillary action refers to a fluid’s ability to flow into narrow spaces via its surface tension, usually without the help of gravity and often against it.

Once you believe a good seal has been made, allow the joint to cool. If it doesn’t look watertight, simply brush a bit of flux over the area, heat it up, and apply more solder.

Dry and clean

Let the joint cool by itself. Don’t cool it down with water or a wet cloth, as sudden drops in temperature may cause cracks in the solder and create leaks.

Once it’s cool, clean it with a damp rag or towel, and turn the water back on. Well done!