For potable water, buy lead-free 95/5 tin-antimony solder and a water-soluble tinning flux, and clean both surfaces to bright copper before you flux them. Flux removes the last oxide and pulls the solder into the gap; it is not a substitute for a brush. Heat the fitting, not the solder.
As an Amazon Associate I earn from qualifying purchases.
Most failed sweat joints aren’t a torch problem or a skill problem — they’re a prep problem, or the wrong flux. The consumables are a small part of the job, and they decide whether the joint holds for thirty years or weeps behind drywall in a month. Here’s what to buy and how to use it.
The short version
For potable water in the US you need lead-free solder — 95/5 tin-antimony is a common lead-free choice. Pair it with a water-soluble tinning flux, which cleans as it heats and rinses away instead of sitting in the line corroding things. And clean both surfaces mechanically first, because flux is not a substitute for a brush.
Lead-free solder: what to buy
Lead solder has been off the table for potable water in the US for decades. The everyday replacement is 95/5 — 95% tin, 5% antimony — which flows well and is widely stocked.
Oatey 22017 95/5 Solder
- Oatey 22017 95/5 solder, 1 lb spool — the bulk roll. If you solder more than one joint a year, buy the pound; the small carded rolls run out mid-job.
Solder marked for electronics is a different alloy with a rosin core and does not belong on a water line. Check that the label says lead-free and is intended for potable plumbing.
Flux: the part people get wrong
Flux does two jobs: it strips the last oxide off the copper as the joint heats, and it lets the molten solder wet and climb into the gap. Water-soluble tinning flux is what I’d hand a beginner — the tinning particles help the solder grab, and being water-soluble, residue rinses out when you flush the line rather than staying behind.
Oatey H-20 95 Water-Soluble Tinning Flux
- Oatey H-20 95 water-soluble tinning flux — the standard small tub. One goes a long way; apply thin, with a brush and not your fingers. The Oatey data sheet says eye or skin contact may cause intense irritation and injury.
The classic mistake is troweling flux on like frosting. A thin, even film on both the cleaned pipe end and the fitting socket is all you want. Excess flux doesn’t improve the joint — Oatey says excess flux turns to acid when heated, travels inside the pipe and causes pitting, and that any excess should be removed before the pipe and fitting go together.
#ad Buying for a shop? Amazon Business (opens in a new tab) accounts are free to create, with business-only prices and bulk quantity discounts.
If this is the kind of thing you want more of, leave your email. No sequence behind it.
Clean the copper — this is the actual secret
Copper grows an oxide layer the moment it’s exposed to air, and solder will not bond to oxide. Both surfaces get cleaned to bright metal, every time:
- 4-in-1 copper fitting brush (not linked: third-party seller only on 10/3/2026) — wire brushes for the inside of fittings plus outside sizes in one handle, so you’re not chasing four separate tools around the truck.
Emery cloth works on pipe ends too. What matters is that you see bright, shiny copper — not brown, not dull — on the pipe end and inside the fitting socket, and that you flux it right away before it can re-oxidize. Which class of tube you are sweating does not change any of this — the outside diameter and the fittings are identical, as Type K vs Type L copper pipe sets out.
The sequence that works
- Cut square and deburr. A ragged or out-of-round cut won’t seat fully in the socket. See the pipe cutter guide.
- Clean both surfaces to bright copper — pipe outside, fitting inside.
- Flux thin and even on both, then assemble immediately.
- Heat the fitting, not the solder. Before you light the torch, follow the safety tips in the Oatey soldering guide: always wear heat-resistant gloves, safety goggles, long sleeves and closed-toe shoes, shield nearby materials (especially wood or anything flammable) with a flame protector cloth, and keep a fire extinguisher nearby. Then play the flame on the fitting body; the joint should pull the solder in on its own.
- Touch the solder to the joint away from the flame. If it melts on contact with the copper, the joint is at temperature and capillary action does the rest. If you have to melt the solder with the flame, it’s not hot enough and you’re about to make a cold joint.
- Let it cool undisturbed, then wipe and flush the line.
Torch picks live in the plumbing torch guide — trigger-start torches make the heating step far more controllable than a striker and a bare bottle.
For the deburring in step 1 — see deburring copper pipe.
When soldering isn’t worth it
Sweating copper takes time and carries risk — open flame in a joist bay, wet lines that won’t heat, tight spaces. That’s exactly the calculation behind press fittings and push-to-connect: no flame, far faster. The honest comparison is in ProPress vs soldering and SharkBite vs ProPress.
One thing this page is not about: brazing. The trade uses “silver solder” and “hard solder” loosely, but the AWS definition is a number — soldering uses filler that melts at or below 840°F, brazing filler melts far above it. Nothing on this page belongs on a refrigeration or medical gas line, which is brazed by specification: see ProPress vs brazing.
What Flux Do You Use for Copper Pipe Soldering?
Use a water-soluble tinning flux for copper pipe soldering. It strips the last oxide off the copper as the joint heats, helps the solder wet and climb into the gap, and rinses out when you flush the line instead of sitting in the pipe. Apply a thin, even film to both surfaces and assemble immediately.
What Is the Best Plumbing Solder?
For potable water in the US the best plumbing solder is a lead-free alloy, and 95/5 tin-antimony is a common choice: it flows well and is widely stocked. Buy the one-pound spool if you solder more than a joint a year. Rosin-core electronics solder is a different alloy and never belongs on a water line.
FAQ
What solder is safe for drinking water?
Lead-free solder intended for potable plumbing — 95/5 tin-antimony is the common choice. Never use leaded or rosin-core electronics solder on a water line.
Do I really need flux if the copper is new?
Yes. Even new copper oxidizes in air, and flux both removes that film and lets the solder wet the surface. Clean mechanically first, then flux — the two do different jobs.
Why won’t my solder flow into the joint?
Usually the joint isn’t hot enough, or the surfaces weren’t cleaned to bright copper. Solder should melt on contact with the heated fitting, not from the flame directly. If it beads up and rolls off, stop and re-clean.
For the plumbers reading this
I send occasional notes on tools and parts that hold up — no fluff, unsubscribe anytime.
Related guides
- Best plumbing torches for soldering copper
- ProPress vs soldering
- Best pipe cutters
- Best copper press fittings
Related: joint hot and the solder still will not draw? The causes, in the order they actually happen: why solder will not flow.
Related: if you cannot keep the joint hot in the first place, start with why your torch keeps going out.