How to Use Plumbers Tape

Wrap PTFE thread seal tape clockwise around the male threads only, three to five wraps, then thread the joint together and tighten. Clockwise means: point the threaded end at you, wrap away from you to the right — the same direction the fitting turns on. Use white tape for water, yellow for fuel gas. Tape goes on tapered pipe threads and nowhere else: never on compression fittings, flare fittings, garden-hose threads or P-trap slip nuts, all of which seal on a ferrule or a washer instead. Two minutes of work, and wrap direction is what makes it hold.

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What plumbers tape actually does

Thread seal tape is polytetrafluoroethylene, PTFE, sold under the genericized name Teflon tape. It is not an adhesive and not a glue — it has no sticky side at all. It works in two ways at once. It fills the tiny helical gap that exists between the crests and roots of mating tapered threads, and it lubricates the joint so the two tapers can pull deeper into each other before friction stops them. That extra engagement is a real part of the seal, which is why a dry threaded joint often weeps even when tightened hard.

The critical implication: tape only works on tapered pipe threads — NPT in the US, the standard on galvanized steel, brass and black iron nipples, and on the threaded ends of most valves and faucet shanks. On any fitting that seals with a rubber washer, an O-ring or a compression ferrule, the threads are just a clamp, and tape is at best useless.

Which tape for which job

For a homeowner, one roll of white and one of yellow covers everything. A roll costs $2 to $5 and appears in most starter kits — see best plumbing tools.

Step by step: wrapping a threaded joint

1. Clean the threads. Wipe off old tape shreds, dried compound and grit with a rag. Old tape left in the root of the thread is the second most common cause of a leak after wrap direction. A wire brush is fine on steel and brass; do not use one on plastic.
2. Start one to two threads back from the end. Leaving the first thread or two bare stops tape shredding off into the water line — a shred that ends up in a faucet cartridge or a fill valve causes a fault that is genuinely hard to diagnose later.
3. Wrap clockwise, viewed from the open end. Hold the tape taut against the threads with your thumb and rotate the pipe or the roll so the tape lays away from you to the right. Keep tension so the tape stretches slightly and conforms into the thread roots rather than bridging over them.
4. Overlap each pass by roughly half its width and work toward the body of the fitting. Three wraps on 1/2 and 3/4-inch threads; four or five on 1-inch, on coarse threads, or with a thin economy roll.
5. Tear and smooth. Pull sharply against the thread edge to break the tape, then press the tail down with your thumb. You should be able to see the thread profile through the tape — if the threads have disappeared under a smooth white sleeve, you have used too much.
6. Assemble hand-tight, then one to two turns with a wrench. NPT joints seal on interference, not on brute force. Two turns past hand-tight is the common working figure for household sizes. Stop at the first sign of the fitting getting hard to turn.

Where tape must never go

• Compression fittings — supply stops, angle valves, 1/4-inch icemaker lines. The ferrule seals; tape on the nut threads reduces clamping load.
• Garden hose and washing machine hose threads (GHT). These seal on a rubber washer. If a hose bibb connection drips, replace the washer.
• P-trap and tailpiece slip nuts. Sealed by a nylon or rubber slip washer. See how to replace a P-trap.
• Faucet supply line couplings with a built-in cone or gasket, including most braided stainless connectors.
• Flare fittings on gas and refrigeration lines — the flared tube face is the seal.

Why the joint still leaks after taping

Work through this list in order before adding more tape, which almost never helps:

• Wrong wrap direction. Disassemble; if the tape is bunched into a rope ahead of the fitting, this was it.
• Debris in the threads, or old tape not fully removed.
• A cracked female fitting, usually plastic, usually over-tightened. Tape acts as a wedge and a hairline crack often only weeps under pressure.
• Damaged or cross-threaded male threads. If the fitting went on crooked for the first turn, no amount of tape saves it.
• It is not a tapered thread at all. Check whether the joint was supposed to have a washer that is missing or flattened.

Gas and when to call a licensed plumber

Yellow gas-rated tape exists because fuel gas connections are held to a different standard, not because the color is prettier. Even so, threaded gas work is the point where DIY stops for most homeowners. Fuel gas piping, appliance connections and any alteration to a gas train are permitted work in the large majority of US jurisdictions, and requirements come from your state and local adoption of the fuel gas code — check with your local building department rather than relying on any national rule. A leak you cannot smell can still accumulate, and a bad connection near an appliance flue is a carbon monoxide risk.

Call a licensed plumber or gas fitter for anything on a gas line, for water heater connections, and for work inside a wall. Rate expectations are in how much a plumber charges per hour.

Related reading: how to replace a shower head (the classic three-wrap job), how to fix a leaky faucet, what tools you need to replace a faucet, and how to replace a shut-off valve.