Complete Guide: How to Solder Copper Pipe: Clean, Flux, Heat, Fill

Learning how to solder copper pipe lets you join water lines with strong, leak-resistant connections. The method, called sweat soldering, melts a metal filler into a joint that has been cleaned and fluxed. Done well, the joint lasts for decades. Done poorly, it leaks or fails under pressure, so the preparation matters as much as the flame.

What soldering does to a copper joint

Sweat soldering heats a copper fitting and pipe until the solder melts and is drawn into the gap between them by capillary action. The solder then cools and locks the two parts together. The joint needs to be clean, dry and at the right temperature, because solder will not flow into a dirty or cold joint. A joint that looks finished may still leak if the solder did not reach the full depth of the fitting.

Quick facts: how to solder copper pipe

Quick facts
Time needed	About 15 to 20 minutes per joint, including cooling
Tools	Torch, flux, lead-free solder, emery cloth or wire brush, rag, heat shield
Key rule	Keep the joint dry and clean; solder flows to heat, not to the flame
Call a plumber if	The line is pressurized, the joint is hard to reach, or a leak returns

Step 1: shut off and drain the line

Turn off the water supply and open a faucet at the low point to drain the pipe. A line that still holds water will not heat properly, and steam can blow solder out of the joint. Dry the pipe and fitting completely before you start.

Step 2: clean the pipe and the fitting

Sand the outside of the pipe end and the inside of the fitting with emery cloth or a wire brush until the metal shines. Clean all the way around, because any oxide or grease prevents the solder from bonding. Wipe the surfaces clean with a dry rag.

Step 3: apply flux and assemble

Brush a thin, even layer of flux on the cleaned pipe end and the fitting. Push the fitting onto the pipe until it seats fully, then twist slightly to spread the flux. Wipe away any flux that squeezes out of the joint, since excess flux can carry corrosion into the line.

Step 4: heat and feed the solder

Heat the fitting, not the solder, with the torch moving around the joint. Keep the torch moving so the heat spreads evenly, since how to solder copper pipe well depends on even heating. When the flux bubbles and clears, touch the solder to the joint where the pipe meets the fitting. The solder should melt and be drawn into the gap. Remove the flame as soon as the solder flows fully around the joint.

Safety when soldering copper pipe

A torch can start a fire, so clear combustible materials from the work area and use a heat shield on nearby wood or insulation. Keep a fire extinguisher close, and never solder a pipe that holds water or gas. Use lead-free solder for drinking water lines, since it is the standard for potable water.

What to do if the joint leaks

Let the joint cool and dry, then clean the area and try again if the leak is small and the fitting is undamaged. Repeated leaks often point to a dirty joint, too much heat, or an overfilled gap, so review each step. A leak at a damaged fitting needs the fitting replaced.

When to call a professional

Call a plumber if the line cannot be drained fully, if the joint is in a tight space you cannot protect from heat, or if the pipe is old and brittle. A plumber can also check the whole line for pressure and leaks before you turn the water back on.

Keeping copper joints sound

Make sure each joint is clean and dry before soldering, and use the right amount of heat for the pipe size. Test every joint with water pressure before closing the walls or ceiling.

How to solder copper pipe: keeping the joint clean

A clean joint is the most important part of how to solder copper pipe well. Dirt, oil and oxide on the surfaces stop the solder from flowing evenly, so clean the pipe end and the inside of the fitting with emery cloth or a fitting brush until the metal shines. Apply a thin layer of flux before you assemble the joint. Clean joints are the first step in how to solder copper pipe at any size.

Heat the fitting evenly with the torch, moving the flame around the joint rather than focusing on one spot. When the flux bubbles and dries, touch the solder to the joint. The solder should melt and flow into the gap by capillary action. Remove the flame as soon as the solder has flowed around the full circle.

Let the joint cool without moving it, and wipe away excess flux once it is cool. Learning how to solder copper pipe with patience gives a joint that holds pressure for years. Protect nearby wood, insulation and plastic from the flame, keep a fire extinguisher close, and work in a well-ventilated area, since flux fumes are unpleasant. Practice how to solder copper pipe on a scrap length first, so the flame and the solder feel familiar before you work on a live line.

Related reading

• Simple Guide: How to Cut PVC Pipe Square
• Practical Guide: How to Turn Off Water to Toilet Before a Repair
• Simple Guide: How to Check Heating Element in Water Heater Safely
• Complete Guide: How to Use a Drain Snake Without Damaging the Pipe

Browse all plumbing guides, or find a plumber in your city from the plumbing directory.