Short definition
A flared joint is a mechanical copper-tube connection: the tube end is belled into a 45-degree cone with a flaring tool, then a brass nut compresses the flare against a matching seat on the fitting. It’s the standard at gas appliances, icemakers, and dishwasher water lines because it can be disconnected for service.
What it is
Two flare standards exist. SAE J512 (45-degree flare) is the residential standard in the US. JIC (37-degree flare) is industrial and hydraulic — not used in residential plumbing. The two are not interchangeable.
The flare is formed with a yoke-type flaring tool. Hammer-type “flaring tools” (a tapered punch struck with a hammer) produce uneven flares and are not code-acceptable for fuel gas. Yoke tools clamp the tube square in a die, then a screw advances the cone into the tube end.
Flare nut “torque” is misleading. Tapered-thread torque varies; manufacturers specify “flats from finger-tight” instead. Typical 1/4 to 3/8-inch flare nut: 1.25 to 1.5 turns past finger-tight after sealant on the threads.
Gas piping uses a deep-shoulder flare nut with extra thread engagement. Using a regular flare nut on gas piping is a code violation.
Why it matters to a homeowner
You’ll meet flare joints at three spots in a typical home: the gas-range flex line, the icemaker supply, and the dishwasher water inlet. Most homeowner trouble at these joints is mechanical:
- Reused flare from a disconnected appliance. The flare is permanently work-hardened. Cut it off, re-flare. Don’t just re-tighten the old flare — it cracks under the second compression.
- Off-center or cracked flare. The tube wasn’t square in the yoke. Won’t seat evenly. Leaks.
- Wrong flare nut on gas. Regular instead of deep-shoulder. Looks identical at the hardware store; isn’t.
When a contractor’s invoice references “flare nut” replacement on a gas-range hookup, that’s correct work. Don’t try to skip it on a reconnection.
Common variants and not the same as
- Flared vs. compression fitting. Compression uses a brass ferrule bitten into the pipe. Flare uses a permanent shaped pipe end. Compression is more forgiving and serviceable; flare is more reliable on gas.
- 45-degree SAE vs. 37-degree JIC. Incompatible. Don’t mix.
- Flared vs. press / sweated. Flares are mechanical and serviceable. Sweat and press are permanent. Flares are required at most gas appliances because they can be disconnected for service.
Common failure modes
- Cracked flare. Flaring without lubricating the cone. Copper splits.
- Off-center flare. Tube not square in the yoke.
- Over-torqued nut. Soft copper flare elongates and leaks.
- Reused flare. Work-hardened. Re-flare instead.
- Wrong flare nut on gas. Code violation.
Washington note
WA gas permits require a licensed gas fitter for fuel-gas piping work — a separate license tier from general plumbing under WA L&I. A homeowner replacing a flex line on a gas appliance is generally within the homeowner exemption (RCW 18.106) for like-for-like swap, but flaring a new gas-line section requires a permit and a licensed fitter. Yellow gas-rated PTFE tape goes on the threaded portion of the flare-nut union, not on the flare itself. Using regular white PTFE tape on a gas flare-nut union is the single most-cited code violation in WA gas inspections — easy mistake to avoid, and inspectors look for the yellow.