Sweating a Copper Joint Is Mostly About Getting the Water Out

Quick answer: Shut off the water, drain the line completely, and scrub both pieces until they shine. Heat the fitting, not the solder, and touch the solder to the side opposite the flame. If the solder balls up and won't flow, there's still water in the pipe.

You can hold a propane torch on a half-inch fitting for five minutes and never get it hot enough if a teaspoon of water is sitting in the line behind it. The water boils, the steam carries the heat away, and the copper sits at 212°F while your solder melts at about 450°F. You'll hear it hiss. You'll see the solder bead up and drip onto your shoe. Nothing is wrong with your torch. The pipe is wet.

Fix that and the rest of this job is very learnable.

Make sure it's a joint you should sweat

Dry everything with a towel, then watch for a few minutes with a flashlight. Where the water shows up first tells you what you're dealing with.

• Wet ring where the pipe enters a fitting: a failed solder joint. That's this job.
• A bead from the middle of a straight run: a pinhole in the pipe itself. You can cut out that section and sweat in a coupling, but one pinhole often means the pipe is thinning elsewhere. Have a plumber look at the run.
• A drip off a valve stem: usually the packing nut. Snug it about a quarter turn with a wrench. No torch.
• A compression nut under a sink: snug it a quarter turn. No torch.
• Green crust on a dry joint: old flux residue that never got wiped off. Clean it and keep an eye on it. It isn't necessarily leaking.

What you need, roughly

Prices vary by store and region, so treat these as ballpark.

• Propane torch with a self-igniting head: about $40 to $60. Propane is plenty for half-inch and three-quarter-inch pipe. MAP-Pro gas runs hotter and faster, which also means it's easier to overheat a joint while you're learning.
• Lead-free solder: about $15 to $30 a spool. Federal law has required lead-free solder on drinking-water lines since 1986. Don't use the old 50/50 stuff on anything that feeds a tap.
• Water-soluble flux paste and a brush: under $10.
• Tubing cutter: about $15 to $25. A hacksaw leaves a ragged, out-of-square end. Buy the cutter.
• Emery cloth or plumber's sanding cloth, plus a fitting brush sized to your pipe: about $5 to $10 total. The four-in-one cleaning tools that do pipe and fitting both are fine.
• Fittings: usually a few dollars each. Buy two of whatever you need. You'll ruin one.
• Flame-protector cloth: around $15. Not optional near wood framing.
• A fire extinguisher and a spray bottle of water, within reach.

Shut it off and drain it dry

Close the main shutoff, or the closest valve that actually stops the flow. Then open a faucet at the lowest point in the house and one at the highest. The top one lets air in so the line can drain.

If you're working on the hot side, deal with the water heater first. On an electric heater, cut the breaker so the elements don't burn out in an empty tank. On gas, set it to pilot or vacation. If you're not sure how, stop and find out before you go further.

Leave a faucet open while you solder. Steam needs somewhere to go, and pressure building in a closed line is how a joint blows out while it's still soft.

If the line still trickles after it's drained (an old shutoff that doesn't fully seal is the usual culprit), plumbers use a trick that sounds like a joke. Pull the crust off a slice of white bread, roll it into a plug, and push it a few inches up the pipe. It holds back the trickle long enough to solder. Afterward it dissolves, and you take the aerators off the faucets and flush it out.

Cut, ream, clean

Clamp the tubing cutter on, spin it a full revolution, then tighten it a quarter turn. Repeat. Crank it down hard all at once and you'll crimp the pipe out of round.

The cutter leaves a lip on the inside edge. Ream it off with the cutter's built-in blade. That lip causes turbulence, and turbulence wears through copper over the years.

Now clean. Sand the outside of the pipe end about an inch back until it's bright, even, salmon-colored copper. Run the fitting brush inside the socket until it shines the same way. Then don't touch the cleaned surfaces with your fingers. Skin oil is enough to keep solder from sticking.

Most leaking joints you'll ever cut out failed right here. Somebody gave it two lazy passes with the sandpaper and moved on.

Flux it and dry fit

Brush a thin, even coat of flux on the pipe end and inside the fitting. Thin. A glob of flux doesn't help, and the excess burns and gets in the way.

Push the pipe all the way into the fitting until it bottoms out, and give it a slight twist to spread the flux. Wipe off what squeezes out. If the fitting is a tee or elbow with several joints, flux and assemble all of them before you light the torch.

Heat the fitting, feed the solder

Hang the flame-protector cloth behind the work. Light the torch and put the tip of the inner blue cone on the body of the fitting, not on the pipe and not on the seam. The fitting is thicker. It needs the heat more.

On half-inch pipe that's usually somewhere around ten to twenty seconds with propane. The flux will sizzle and then go quiet and glossy. Pull the flame slightly and touch the end of the solder to the joint on the side opposite the torch.

If the solder melts on contact with the copper, the joint is ready. If you have to put it in the flame to melt it, keep heating.

Once it flows, run the solder around the seam. Capillary action pulls it into the gap toward the heat, which is why you feed from the far side. For half-inch pipe, roughly half an inch to three-quarters of an inch of solder wire is about right. You're done when a thin silver ring shows all the way around. More isn't better. Extra just drips inside the pipe or onto the floor.

Soldering a valve: open a ball valve all the way before you heat it. On valves with rubber parts, check the instructions. Some need the stem pulled first so the heat doesn't cook the seals.

Several joints close together: wrap a wet rag around any joint you've already finished so the heat from the next one doesn't melt it loose.

Let it cool, then test

Let it cool on its own. Dousing a hot joint with water can stress it. While it's still warm enough to handle with a rag, wipe off the leftover flux. Flux is mildly corrosive, and that green crust you saw on old joints is what happens when nobody wipes it.

Close the faucets, then open the main slowly. Watch every joint you touched for a few minutes with a dry paper towel against it. Then take the aerators off the faucets and run water for a minute to flush out any flux, bits of copper or bread.

When it won't take

• Solder balls up and spits: water in the line. Drain it again.
• Solder sits on the surface and won't wick in: dirty copper, or not enough heat.
• Flux turns black and the copper goes dark: overheated. Solder won't bond to oxidized metal.

You can't fix a bad joint by piling on more solder. Heat it, pull it apart with pliers while it's hot, let it cool, then clean, re-flux and use a new fitting. That's why you bought two.

Stand fire watch

Wood framing and old insulation can smolder for a while after the flame is gone, and you won't see it at first. Before you leave, feel the framing near the joint and look closely. Then come back in thirty minutes and check again. Keep the spray bottle handy the whole time.

When to put the torch down

Call a licensed plumber when:

• It's a gas line. Don't sweat it, don't patch it, don't experiment. Shut off the gas and call.
• The leak is on the main before your shutoff, or you can't get the water to stop.
• The joint is jammed against joists or subfloor where you can't keep the flame off wood.
• You're adding new runs, not just repairing. Many areas require a permit for that, and codes vary by town and county, so check yours.

One fair concession. Push-to-connect fittings like SharkBite cost more per fitting and don't need a torch. In a tight spot next to framing, or for an emergency fix at midnight, they're a sensible call. Some local codes limit where they can go, especially inside walls, so check before you bury one.

If you opened drywall to reach the pipe, the patch goes faster when you don't make it worse. And if the leak was under the kitchen sink, that's a good time to deal with a disposal that hums but won't spin while you've got the cabinet cleared out.

Before you close it up, slide foam insulation over the new section and any bare copper nearby. It costs a couple of dollars and saves the next person a frozen pipe. Sweep up the copper filings and the scraps of solder. Then wipe the flux off your tools so they're ready next time.