What are expert tips for soldering copper pipe?

The number one rule: the pipe must be completely dry inside. Even a few drops of water in the pipe will prevent the joint from reaching soldering temperature. Stuff bread (yes, bread) into the pipe upstream to absorb residual water -- it dissolves and flushes out when you turn the water back on. If the solder beads up on the surface instead of flowing into the joint, the fitting isn't hot enough or the surfaces weren't properly cleaned. Practice on scrap pieces before working on live plumbing. Five practice joints will build more confidence than any YouTube video.

What mistakes should I avoid with soldering copper pipe?

Not cleaning the pipe and fitting thoroughly -- this is the cause of 90% of failed solder joints. Heating the solder directly with the torch instead of letting the fitting's heat draw it in. Using too much flux, which causes green corrosion over time. Trying to solder with water in the pipe -- even a trickle prevents proper heat.

Soldering Copper Pipe | The Plumbing Directory

Overview

Soldering copper -- or sweating pipe, as plumbers call it -- is one of those skills that looks intimidating but becomes second nature after a few joints. The technique is simple: clean the pipe, apply flux, heat the fitting (not the solder), and let capillary action pull the solder into the joint. Nine times out of ten, a leaking solder joint comes down to one thing -- the pipe wasn't clean and dry enough.

What to Know

Why Copper Still Matters

PEX has taken over a lot of residential plumbing, but copper is still the standard for exposed runs, commercial work, and in areas where code requires it. It's also what you'll find in most homes built before 2010. If you're doing any repair work on an older home, you'll need to know how to sweat a joint.

Lead-Free Solder

Since 1986, plumbing solder must be lead-free for potable water systems. Use 95/5 (tin/antimony) or lead-free silver-bearing solder. Never use 50/50 lead solder on water supply lines. It's still sold for other applications, so check the label.

Tools & Materials

Propane torch with trigger start
Lead-free solder (95/5 or silver-bearing)
Flux (water-soluble paste flux)
Emery cloth or sandpaper (120 grit)
Flux brush
Pipe cutter
Fire-resistant cloth (for working near framing)

Step by Step

Cut and prep the pipe
Cut the pipe square with a pipe cutter. Ream the inside edge to remove the burr -- a burr creates turbulence and restricts flow. Sand the outside of the pipe end with emery cloth until it's bright and shiny, about 1 inch back.
Clean and flux the fitting
Sand the inside of the fitting socket with emery cloth or a fitting brush. Apply a thin, even coat of flux to both the pipe end and the inside of the fitting. Don't glob it on -- excess flux causes corrosion over time.
Assemble and position
Push the pipe into the fitting with a slight twist to spread the flux evenly. If you're working near wood framing, place a fire-resistant cloth behind the joint. Make sure the pipe is fully seated in the fitting.
Heat the fitting
Apply the torch flame to the fitting, not the pipe and not the solder. Heat the thickest part of the fitting (the hub). Move the flame around the fitting evenly. The goal is to heat the fitting enough that it draws solder in by capillary action.
Apply solder
Touch the solder to the joint on the opposite side from the flame. When the fitting is hot enough, the solder will melt on contact and get pulled into the joint by capillary action. Feed solder until you see a bright ring all the way around the joint. Remove the flame immediately.
Wipe and inspect
Quickly wipe the joint with a damp rag while still hot -- this gives a clean, professional finish. Let it cool naturally. Don't spray cold water on it, which can crack the joint. Once cool, turn on the water and check for leaks.

Pro Tips

The number one rule: the pipe must be completely dry inside. Even a few drops of water in the pipe will prevent the joint from reaching soldering temperature. Stuff bread (yes, bread) into the pipe upstream to absorb residual water -- it dissolves and flushes out when you turn the water back on.
If the solder beads up on the surface instead of flowing into the joint, the fitting isn't hot enough or the surfaces weren't properly cleaned.
Practice on scrap pieces before working on live plumbing. Five practice joints will build more confidence than any YouTube video.

Mistakes to Avoid

Not cleaning the pipe and fitting thoroughly -- this is the cause of 90% of failed solder joints.
Heating the solder directly with the torch instead of letting the fitting's heat draw it in.
Using too much flux, which causes green corrosion over time.
Trying to solder with water in the pipe -- even a trickle prevents proper heat.

When to Call a Pro

If you need to solder near gas lines, in tight spaces with limited ventilation, or if the existing copper is corroded and crumbling when you try to clean it -- call a plumber. Also consider a pro if you're doing more than a few joints and want the peace of mind that comes with a warranty on the work.

Bottom Line

Soldering copper is a fundamental plumbing skill worth learning. Clean the pipe, flux it, heat the fitting (not the solder), and let capillary action do the work. Practice on scrap first, and always check for leaks before closing up the wall.

Soldering Copper Pipe