Soldered (sweat) joints

Entry in Joining methods · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

A sweat fitting is a copper or copper-alloy fitting with a smooth socket that slips over copper tube and is joined by soldering, the filler metal being drawn into the narrow gap by capillary action. ASTM B828 is the standard practice for making these capillary solder joints on copper tube and fittings.^[1]

Sweat joints are a common way to join rigid copper water tube in houses. This entry explains how they work and what governs them; it does not give soldering instructions, which are a trade skill and, in most places, permitted work.

1 How a solder joint works

The fitting socket is sized so the tube slides in with a thin, even clearance. When the joint is heated, molten solder is pulled into that clearance by capillary action, the same effect that draws liquid into a narrow gap. A joint-gap range of about 0.002 to 0.005 in gives the best results in capillary joining.^[2] Flux on the mating surfaces keeps them clean of oxide while they heat so the solder can wet the metal.

The American Welding Society definition separates soldering from brazing by temperature: soldering uses a filler metal with a liquidus below 840°F (450°C), and brazing uses one above 840°F but below the melting range of the base metals.^[2] Both rely on capillary flow; neither melts the copper itself.

2 Lead-free solder

The 1986 Safe Drinking Water Act amendments prohibited solder and flux that were not "lead free" in plumbing that supplies water for human consumption, and set the limit for solder and flux at 0.2% lead.^[3] ASTM B828 likewise states that only lead-free solders may be used on potable water systems, some of which are described in ASTM B32.^[1] State codes repeat the rule; Massachusetts, for example, requires lead-free solder filler metals for potable water joints.^[5]

Older homes can still have 50/50 tin-lead solder on supply lines installed before the ban. The entry on lead-free solder covers the alloys and how they are marked.

3 Flux

Flux is applied to the tube end and fitting socket before heating. Makers sell water-soluble paste fluxes that are designed to be flushed from the system after soldering, and burn-resistant fluxes that stay active at the higher temperatures lead-free solders need.^[4] The flux maker's instructions on flushing the new piping apply.

4 Limits and alternatives

A sweat joint needs a dry joint and an open flame or other heat source, which is why soldering near framing, in occupied homes or on wet lines is skilled work. Flame-free options for copper include copper press fittings, compression fittings and push-fit fittings, each with its own listing and code limits. Where a copper line meets galvanized steel, a dielectric union or other approved fitting is used rather than a direct joint.