How to Solder Copper Pipe
Updated February 20, 2026
Solder (sweat) copper pipe joints the right way -- cut, clean, flux, heat, and feed solder for watertight connections that last the life of the building.
Overview
Soldering copper (also called sweating) is one of the fundamental plumbing skills. Done right, it creates permanent joints that last the life of the building. The concept is simple: heat the fitting, touch solder to the joint, capillary action pulls molten solder into the gap. In practice, three things have to be right: the pipe and fitting must be thoroughly cleaned, the correct flux must be applied, and the heat goes on the fitting -- not the solder. Get all three right and you get a perfect joint every time. Miss any one and you get a leak.
What You'll Need
Safety First
- Open flame near wood framing, insulation, and combustibles. Always use a flame protector cloth behind the pipe when soldering near anything that can burn. Fire extinguisher within arm's reach. Check the area behind the pipe for 15 minutes after soldering.
- Safety glasses on. Flux splatters when heated and molten solder drips. Both burn skin and eyes on contact. Long sleeves, and do not reach over the torch.
- Lead-free solder only for drinking water plumbing. Lead solder was banned in 1986. Modern plumbing solder is 95/5 tin-antimony or 97/3 tin-copper. Look for 'lead-free' on the label. Lead solder on potable water is a health hazard and code violation.
- The pipe and fitting stay extremely hot for several minutes after soldering. Do not touch anything within 6 inches of the joint until it has fully cooled. Hot copper looks exactly like cool copper.
Step-by-Step Instructions
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Cut the Pipe Cleanly and Deburr
Copper pipe cutter for a clean, square cut. Much cleaner than a hacksaw and no metal filings to contaminate the joint. Place the cutter, tighten until the wheel contacts, rotate around the pipe, tighten slightly each rotation until through. After cutting, ream the inside edge with the built-in reamer (or a deburring tool) to remove the burr. That burr restricts flow and creates turbulence that accelerates corrosion.
Tip: Hacksaw works in a pinch, but file the end smooth and square. An angled or rough cut creates an uneven gap inside the fitting -- weak joint or a leak. Cleaner cut, better joint. -
Clean the Pipe and Fitting Thoroughly
Most critical step. Solder will not bond to dirty or oxidized copper -- it beads up and runs off instead of flowing in. Clean the outside of the pipe with emery cloth, 120-grit sandpaper, or a pipe brush. Bright and shiny for the full depth of the fitting socket (about 3/4 inch for 1/2-inch pipe). Clean the inside of the fitting with a fitting brush or rolled emery cloth -- uniformly bright. After cleaning, do not touch the surfaces. Skin oils prevent bonding.
Tip: Cleaned but not soldering right away? Copper starts oxidizing within 30-60 minutes. More than an hour between cleaning and soldering means you need to re-clean. Fresh, bright copper is non-negotiable. -
Apply Flux to Both Surfaces
Thin, even coat of water-soluble flux on the outside of the pipe and inside of the fitting socket. Cover the cleaned area but do not glob it on -- excess flux drips inside the pipe and corrodes over time. Flux does two things: prevents oxidation during heating and reduces surface tension so solder flows into the joint. Assemble immediately -- push the pipe into the fitting with a slight twist to spread flux evenly. Wipe excess from the outside.
Tip: Water-soluble flux only for plumbing joints. Not petroleum-based, not acid-based. Acid flux corrodes copper over time if not thoroughly cleaned. Look for 'water-soluble' or 'plumbing flux' on the label. -
Heat the Fitting (Not the Pipe, Not the Solder)
Propane torch, medium flame with a defined blue cone. Apply heat to the fitting body, opposite the joint opening. Heat the fitting, not the pipe. The goal: the fitting gets hot enough that solder melts on contact and capillary action draws it into the gap. Move the flame around for even heat. Half-inch fittings take 15-30 seconds, 3/4-inch takes 30-45 seconds. When the flux bubbles and sizzles, you are approaching soldering temperature.
Tip: Most common beginner mistake: heating the solder with the torch. That melts it but does not draw it into the joint -- just blobs on the surface. The fitting must melt the solder by conducted heat alone. Touch solder to the joint (away from the flame) to test. Melts instantly on contact? Ready. -
Feed the Solder into the Joint
Remove the torch (or move it opposite). Touch solder wire to the joint where the pipe enters the fitting. At the right temperature, solder melts instantly and gets drawn into the gap -- you will see it disappear. Feed slowly around the full circumference. Half-inch pipe needs about 3/4 inch of solder wire per joint, 3/4-inch pipe about 1 inch. A bright silver ring around the joint means it is properly filled. Drips forming below? The joint is full, stop feeding.
Tip: Solder beading up instead of flowing in? Three possible causes: not clean enough (most common), flux not applied properly, or fitting not hot enough. Let it cool completely, disassemble, re-clean, re-flux, and try again. You cannot fix a bad joint by adding more heat or solder. -
Wipe and Inspect the Joint
While still warm (wait 15-30 seconds, not hot enough to burn), wipe the joint with a damp rag. Removes flux residue and looks professional. Inspect: continuous silver ring around the full circumference. Gaps or dull spots mean the joint may not be sealed. Let it cool completely (5-10 minutes). Do not quench with water -- rapid cooling cracks the solder. Once cool, turn on the water and check for leaks. Even a tiny seep means the joint failed and needs to be redone.
Tip: A good joint has a smooth, continuous silver ring with no gaps, drips, or blobs. Rough, blobby, or gapped? It will likely leak under pressure. Much easier to redo a joint before the water is on than after.
Pro Tips
- Practice on scrap first. A few feet of 1/2-inch pipe and a handful of couplings ($1 each). Make 5-10 joints until you consistently nail the heat and solder timing. $10 in practice saves frustrating failures on the real job.
- Pipe must be completely dry inside. Even a tiny amount of water turns to steam and prevents the joint from reaching temperature. Repairing an existing pipe? Drain completely and stuff white bread (no crust) upstream of the joint. It absorbs residual water and dissolves when you turn the water back on.
- Hard-to-reach joints or near combustibles? SharkBite push-fit fittings create reliable connections with no flame. See our SharkBite guide.
- MAPP gas (yellow cylinder) burns hotter than propane (blue) and heats fittings faster, especially 3/4-inch and up. For 1/2-inch work, propane is fine.
- Vertical pipe? Solder the bottom of the joint first. Gravity pulls molten solder down, so starting at the bottom lets capillary action draw it up and around. Start at the top and it drips and fills unevenly.
When to Call a Pro
Call a plumber if you need to solder in a tight wall cavity with high fire risk, for whole-house repiping, for joints on a system that cannot be fully drained, or for pipe sizes 1 inch and above that need more heat and experience. Single repair joints in accessible locations? Absolutely learnable with practice.