How to solder copper pipe with a blowtorch, step by step

A capillary joint is not glued, it is drawn. You heat the fitting until lead-free solder flows - roughly 230-250 °C - and capillary action pulls the molten solder into the gap between pipe and cup. Done right, the solder rings the whole joint and you never see a gap again.

Every step below serves that one mechanism. Cleaning gives the solder a surface it can wet. Flux keeps the copper from oxidising while you heat it. The flame brings the fitting - not the pipe - up to the temperature where solder flows on contact. Skip any of them and the joint tells you, usually at 2 bar on a Friday night.

Lay it all out before you light anything: the torch, lead-free solder wire, flux with a small brush, cleaning strips or wire wool, a heat mat, a damp rag and a spare gas cartridge. On a cartridge torch the whole kit fits in one hand and a jacket pocket, which is half the reason plumbers default to it.

The torch wants a steady, aimable flame rather than a fierce one - the 1300°C EN417 cartridge head lights on piezo with one thumb and holds its flame in a draught, which is what you need under a floorboard. Whatever torch you run, read the blowtorch safety checklist before the first joint of the day, not after the first scorch mark.

Cut the pipe square, take the burr off the inside, then clean the outside of the pipe end and the inside of the fitting until both are bright metal. Bright, not just clean-looking - the grey film you can barely see is oxide, and solder will not wet oxide.

Clean a little further than the fitting is deep, so no dirty band sits at the mouth of the joint. Once it is bright, touch it with the flux brush and nothing else - fingerprints carry grease, and grease is another thing solder refuses to stick to.

Flux has one job: keep the air off the bright copper while you heat it, so the surface stays wettable until the solder arrives. A thin, even film on the pipe end and inside the cup is all that is needed.

More flux is not safer flux. A great cold slug of it burns, chars and can block the gap you are trying to fill. If it is dripping, there is too much.

Push the pipe fully home into the fitting - feel it bottom out - and give it a small twist as you push to spread the flux. A joint that is half-seated is half a capillary gap, and half a gap holds half as well.

On anything with an orientation - an elbow, a tee branch - mark the alignment with a pencil before heating. Once the flame is on and the fitting is hot is the wrong time to discover the elbow points at the joist.

Play the flame on the body of the fitting and keep it moving around it, so the heat soaks in evenly from every side. The fitting is the part that must reach soldering temperature; the pipe gets there by conduction from the fitting. A flame parked on the pipe just burns flux off the one spot you need it.

How long depends on the mass: 15 mm tube comes up fast - seconds, not half-minutes - so hold the torch back and stay moving. 22 mm wants longer and steadier. Watch the flux, not the clock: when it turns from paste to a glassy, quiet film, the joint is ready for solder.

Pull the flame back or off, and touch the end of the solder wire to the mouth of the joint. If the joint is at temperature, the solder melts on contact and runs round the ring by itself - you feed, capillary action does the distribution. Keep feeding until a bright ring of solder shows all the way round, then stop.

If the solder just sits and softens without flowing, the joint is not hot enough - take the solder away, bring the flame back to the fitting, and try again in a few seconds. Melting the solder in the flame and letting it drip onto the joint is the one move that guarantees a cold, unreliable joint.

Let the joint set without touching it - a joint nudged while the solder is half-frozen can crack through, invisibly. When the shine dulls, it is set. Wipe the flux residue off with the damp rag while the joint is still warm; flux left on copper keeps working, slowly, for years.

Then read the joint. A continuous bright ring at the mouth and a little solder visible in the gap is what done looks like. Dull, grainy or lumpy means it moved or never flowed - that joint gets reheated, refluxed and redone now, not after the water goes on.

First, dirty pipe - solder will not wet oxide, no matter how long you heat it. Second, heating the pipe instead of the fitting - the flux burns off before the fitting reaches temperature. Third, moving the joint while the solder sets - the hidden crack that weeps a month later. Fourth, flooding the joint with solder - a fat bead outside can hide a gap inside. Fifth, scorched flux - blackened flux means the flame was too fierce or stayed too long, and the joint under it is suspect.

Four of the five are heat control, and heat control is mostly flame size and patience. If your joints keep failing the same way, the fix is usually in this list rather than in the torch.

On 15 and 22 mm domestic pipework, a cartridge blowtorch is genuinely enough - the failures on those sizes are technique, not equipment. The equipment question returns at 28 mm and above, on heavy brass fittings, and outdoors in winter, where the fitting is big enough to shed heat as fast as a small flame supplies it.

If the flame has been on the fitting for a minute and nothing flows, that is the boundary - and past it the job is brazing, not soldering. Which blowtorch for soldering copper pipe maps the flame sizes to the pipe sizes, and soldering vs brazing explains where one process hands over to the other.