How to solder copper water pipes and replace a leaking compression joint
This guide walks you through soldering copper water pipes and replacing a leaking compression joint in a clear, step-by-step way. You’ll learn how to cut, clean, flux, solder, and reassemble a compression fitting so the joint becomes watertight. Follow safety tips and take your time—most DIYers finish a simple repair in 60–90 minutes.
Step 1: Turn off water and drain lines
Shut the main or local supply valve and open nearby faucets to relieve pressure; expect 0.5–2 liters to drain from the short section. Place a bucket and towels under the work area to catch residual water and protect the floor. Turning the supply fully off prevents spray when you disconnect the fitting.
[Illustration: shutoff valve with bucket and towels beneath copper pipes]
Step 2: Cut out the damaged section
Mark and cut about 25–50 mm on either side of the leaking joint using a tubing cutter or fine hacksaw. Rotate the cutter with steady pressure or use a mitre box for straight cuts; aim for smooth, square ends to ensure a good fit. Measure twice, cut once—replacing only the necessary length saves time and materials.
[Illustration: close-up of tubing cutter cutting copper pipe with measured marks]
Step 3: Clean pipe ends and fittings
Use 120–220 grit abrasive cloth or a rotary brush to remove oxides and get shiny copper for about 25–35 mm on each end. Also clean inside the compression fitting or 22 mm coupling so flux adheres well. Proper cleaning ensures solder wets the metal and forms a strong joint.
[Illustration: polished copper pipe ends and clean coupling on workbench]
Step 4: Apply flux to bare metal
Brush a thin, even layer of lead-free plumbing flux to the cleaned pipe ends and the inside of the fitting; cover the 8–12 mm that will be inside the joint. Flux prevents oxidation during heating and helps solder flow. Use about a pea-sized amount on brushes for 15–30 mm lengths and spread evenly with the applicator brush.
[Illustration: hand applying flux to copper pipe ends with small brush]
Step 5: Assemble fitting and position
Slide the compression nut and ring (or slip the coupling) into place and push the fitting fully onto the pipe until it seats; align so the joint faces are visible. For compression joints, hand-tighten the nut, then use a wrench to add 1/4 to 1/2 turn—do not overtighten. Proper assembly prevents leaks and ensures even solder distribution if you solder the joint.
[Illustration: hands assembling compression nut and ring on copper pipe aligned under sink]
Step 6: Heat and apply solder
Heat the joint evenly with a propane torch set to medium flame, focusing 20–30 seconds on the fitting body until flux sizzles and the metal is hot (not glowing). Touch 1–2 mm diameter lead-free solder to the joint seam; it should melt and draw into the gap by capillary action—apply about 10–20 grams total for a small 15–22 mm joint. Remove heat and let solder cool naturally for 2–5 minutes; do not disturb the joint while it solidifies.
[Illustration: torch heating a copper joint with solder wire feeding into seam]
Step 7: Test, reassemble, and finish
After cooling, wipe excess flux and visually inspect the solder bead around the joint for full coverage. Open the supply slowly and check for leaks for 10–15 minutes, tightening compression nut up to 1/8 turn if a small drip appears. If dry, insulate or support the pipe and clean up tools; document the repair date and parts used for future maintenance.
[Illustration: person turning water valve and inspecting repaired joint with towel nearby]
- Use a heat shield or wet rag behind the joint to protect nearby plastic or drywall from torch heat.
- Choose lead-free solder rated for potable water; 1–2 mm diameter wire is easiest to control.
- Practice on scrap copper of the same diameter for 10–15 minutes if you are new to soldering.
- Keep a fire extinguisher rated for small fires within 2–3 meters while using an open flame.
- Label valves and parts with a permanent marker and take a photo before disassembly to aid reassembly.
- If working under a sink, remove items and place a stiff board under knees to make a stable working surface.
- Never solder near flammable materials or in a confined space without ventilation; fumes and fire risk are serious hazards.
- Do not attempt soldering on pipes containing gas or unknown contents—verify the line is potable water and isolated.
- Avoid overheating; copper that glows is too hot and can damage fittings or nearby finishes and cause weak solder joints.
- Wear safety goggles and heat-resistant gloves; molten solder and hot metal cause severe burns.
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