How to Solder Copper Pipe: Sweat a Joint That Holds
Plumbing solder seals a copper joint through capillary action: heat the fitting, touch lead-free solder to the seam, and the molten metal wicks into the gap and hardens into a leak-free bond. To solder copper pipe you cut it square, shine the copper bright, brush on flux, heat the fitting (not the solder), feed the wire, and wipe. A dry line and clean metal matter more than a steady hand.
What do you need to solder copper pipe?
Soldering copper pipe takes a torch, lead-free solder, water-soluble flux, and a way to shine the metal bright. The parts that decide whether the joint holds are the flux and the surface prep, not the price of the torch. Buy solder and flux both listed for potable water so the joint is legal on a drinking-water line.
| Item | What to buy | Why it matters |
|---|---|---|
| Solder | Lead-free 95/5 tin-antimony, or a silver-bearing tin wire labeled NSF/ANSI 61 | Legal for drinking water and strong at household water pressure |
| Flux | Water-soluble (self-cleaning) tinning paste rated for potable water | Pulls solder into the joint and rinses off instead of corroding the copper |
| Torch | Propane or MAP-Pro hand torch with a trigger igniter | MAP-Pro burns hotter, so it heats 3/4-inch and larger fittings faster |
| Tube cutter | A rotary copper tube cutter with a reamer | Square, clean cuts that seat fully in the fitting cup |
| Emery cloth and fitting brush | 120-grit sand cloth plus a wire fitting brush | Bright, oxide-free copper is the only surface solder actually bonds to |
| Flux brush | A cheap disposable acid brush | Lays a thin even coat; fingers leave gaps and grease |
| Flame protection | A fiberglass welding blanket or flame pad | Shields framing and drywall when you solder near wood |
A rag and a spray bottle of water round out the kit. For the wrenches that break down a valve or union before you heat it, see our guide to plumbing wrenches, and the wider essential plumbing tools list. Match the fittings to your copper pipe types (Type M, L, or K) so the solder cup fits.
How do you solder a copper pipe step by step?
Solder a copper pipe in a fixed order: cut, clean, flux, assemble, heat, feed, and wipe. Each step sets up the next, and skipping the cleaning or fluxing is what causes most leaks. Work with the line fully drained and the joint dry, because trapped water stops the copper from ever reaching soldering temperature.
- Cut the pipe square with a rotary tube cutter, then ream the inside burr so flow is not restricted.
- Dry-fit the joint and confirm the pipe bottoms out in the fitting cup with no wobble.
- Shine the pipe end and the inside of the fitting socket bright with emery cloth and a fitting brush until the copper is pink and dull-free.
- Brush a thin, even coat of water-soluble flux on the pipe end and inside the fitting. Assemble with a slight twist to spread it. Confirm the line is dry.
- Light the torch and aim the flame at the fitting (the heavier metal), not the seam or the solder itself.
- When the flux bubbles and solder melts the instant it touches the copper, feed the wire to the seam opposite the flame. Capillary action pulls it in and a bright ring appears around the cup.
- Feed roughly the pipe’s diameter in solder around the joint, then pull the heat away.
- Wipe the joint with a damp rag while it is still warm to remove excess flux, then let it cool undisturbed for 30 to 60 seconds.
- Turn the water back on and pressure-test the joint before you close up any wall.
How much solder does one joint need?
A good rule is one pipe diameter of solder wire per joint: about 1/2 inch of solder for a 1/2-inch joint, 3/4 inch for a 3/4-inch joint. That is enough to fill the socket by capillary action without a sloppy blob on the outside. If solder drips down the pipe, the joint is either overfed or facing down and too hot.
Can you solder a copper pipe with water in it?
No, and this is the most common reason a repair leaks. A trickle of water in the line turns to steam under the torch, cools the copper unevenly, and pushes the solder back out of the joint before it can wick in. The fitting will never climb to soldering temperature while water is boiling off inside it.
Drain the system, open the lowest faucet to break the vacuum, and dry the joint. If a slow weep remains, push a wad of white bread about 12 inches upstream to soak up the last drops; it dissolves and flushes out once water is restored. A reusable pipe plug (a Jet Swet style tool) does the same job without leaving anything in the line. When a line simply cannot be dried, it is often faster to install PEX instead with a push-fit or crimp fitting that needs no heat.
What solder and flux are safe for drinking water?
Only lead-free solder is legal on potable water pipe, either 95/5 tin-antimony or a silver-bearing tin wire. The old 50/50 tin-lead solder has been banned for drinking-water use since the 1986 Safe Drinking Water Act amendments, and the 2011 Reduction of Lead in Drinking Water Act tightened “lead free” to a 0.25% weighted average on wetted surfaces.
Look for solder and flux carrying an NSF/ANSI 61 or 372 listing, which certifies them for potable water. Pair the wire with a water-soluble paste flux rather than an acid or petroleum flux, so any residue rinses away instead of eating the copper. Skip electrical rosin-core solder entirely; it is not approved for pipe and often hides a corrosive core that will fail under pressure.
Propane vs MAPP: which torch heats a joint faster?
MAP-Pro (the current stand-in for the discontinued MAPP gas) burns around 3,730°F in air, while propane runs closer to 3,600°F. The hotter flame brings 3/4-inch and larger fittings up to temperature faster and holds heat better on cold days or where nearby metal wicks it away. For 1/2-inch lines, propane is cheaper and heats a joint fine.
| Fuel | Approx. air flame temp | Best for |
|---|---|---|
| Propane | ~3,600°F | 1/2-inch lines, occasional repairs, tight budgets |
| MAP-Pro (MAPP) | ~3,730°F | 3/4-inch and larger, cold conditions, heavy fittings and valves |
Whatever the fuel, keep the flame moving around the fitting so it heats evenly, and use a flame pad to protect studs and drywall. Overheating is its own mistake: too much heat scorches the flux and the solder stops wicking.
Common copper soldering mistakes and how to fix them
Most failed joints trace to a short list of errors, and every one is preventable. The solder itself rarely fails; the prep, the heat placement, or a wet line does. Get the copper bright, heat the fitting instead of the wire, and keep water out, and a sweated joint should outlast the rest of the pipe.
| Mistake | What happens | Fix |
|---|---|---|
| Water left in the line | Steam bubbles the joint and blows solder out | Drain fully, open a low faucet, and use bread or a pipe plug to soak up the trickle |
| Dirty or oxidized copper | Solder beads up and will not flow | Shine the pipe and fitting bright with emery cloth and a brush, then flux immediately |
| Heating the solder, not the fitting | Cold joint that leaks under pressure | Aim the flame at the fitting until solder melts on contact with the copper, not the flame |
| Too little or too much solder | Voids inside, or messy drips outside | Feed about one pipe diameter of wire (roughly 1/2 inch per 1/2-inch joint) |
| Overheating the joint | Flux turns black and solder stops wicking | Back off once the flux bubbles; if it scorched, cool, reclean, reflux, and reheat gently |
| Moving the joint too soon | Cracked seal and a slow leak | Leave it undisturbed 30 to 60 seconds until the bright ring dulls, then wipe |
One more habit separates clean work from callbacks: wipe every joint with a damp rag while it is still warm. Leftover flux keeps working on the copper and leaves a green crust that can etch a pinhole over time. Pressure-test before the walls close, because a joint is easy to redo now and expensive to reach later.
Frequently asked questions
Can you solder a copper pipe with water in it?
No. Even a trickle turns to steam, cools the copper unevenly, and blows the solder back out before it can wick in. Drain the line, open a low faucet, and push a wad of white bread about 12 inches upstream to soak up the last drops. The bread dissolves and flushes out once water is restored. A pipe plug tool does the same job.
Do you need flux to solder copper pipe?
Yes. Flux is a water-soluble paste that strips the last oxide film and pulls molten solder into the joint by capillary action. Without it the solder beads up and will not wick into the socket, leaving a cold joint that leaks. Brush a thin even coat on the cleaned pipe end and inside the fitting, never apply it with a finger.
What kind of solder is used for copper water pipe?
Use lead-free plumbing solder on any pipe carrying drinking water, either 95/5 tin-antimony or a silver-bearing tin wire labeled lead-free and NSF/ANSI 61 or 372 listed. Lead solder (the old 50/50 tin-lead) has been illegal for potable water since the 1986 Safe Drinking Water Act amendments. Electrical solder is not approved for pipe and often hides an acid core.
Why won’t my solder stick to the copper pipe?
Almost always dirty metal or a cold fitting. Solder only bonds to bright, oxide-free copper, so shine the pipe end and the fitting socket with emery cloth and a fitting brush, then flux right away. If the metal is clean, you are likely heating the solder or the pipe instead of the fitting, so the joint never reaches temperature.
How long does a soldered copper joint take to cool?
Give it 30 to 60 seconds undisturbed before you move or stress the pipe, and a few minutes before pressure. Wipe the excess flux with a damp rag while the joint is still warm so it does not leave green corrosion. Moving the pipe while the solder is still bright and molten cracks the seal and causes a slow leak later.
Is MAPP gas better than propane for soldering copper?
MAP-Pro (the modern MAPP replacement) burns near 3,730°F in air versus about 3,600°F for propane, so it brings 3/4-inch and larger fittings up to temperature faster and works better on cold days. Propane is fine and cheaper for 1/2-inch lines. Either seals a joint; the hotter flame just saves time and helps when heat drains into nearby metal.
Last reviewed: September 2026.