How much PTFE tape do I need and which way does it go?

Wrap direction, how many turns for which thread, and the joints where PTFE tape has no business at all.

Difficulty: beginner
Time: 10 min
Read: 3 min

Short answer

Wrap clockwise looking at the open end of the male thread, so the tape tightens rather than unwinding as the fitting screws on. Ten to fifteen turns for a tapered thread, fewer for a parallel one. It goes on threads, not on washers, olives or push-fit joints.

PTFE tape fills the tiny spiral gap between two threads. It is not a glue and it is not a substitute for a washer, and most of the trouble with it comes from using it in the wrong place rather than using too little.

What you'll need

PTFE tape — standard white, or the thicker gas-rated yellow where appropriate
A clean dry thread

Which way round PTFE tape goes

Step by step

Work out whether the joint even needs it.A thread that seals against a fibre or rubber washer does not need tape. A thread where the metal seals against metal — a tapered thread — does.
Tell tapered from parallel.A tapered thread narrows visibly towards the end and tightens as it goes in. A parallel thread is the same diameter throughout and relies on a washer at the shoulder.
Clean the thread.Old tape, jointing compound and grit all stop the new tape sitting properly. Pick the old tape off completely.
Start a couple of threads back from the end.Starting right at the tip pushes tape into the pipe bore, where it can end up in a valve or a cartridge.
Wrap in the direction the fitting turns.Looking at the open end of the male thread, wrap clockwise. That way screwing the fitting on tightens the tape rather than peeling it off.
Keep it under tension.Pull gently so the tape stretches slightly and beds into the thread form. Loosely draped tape bunches up and does nothing.
Use the right number of turns.Around ten to fifteen for a standard tapered plumbing thread, more on a coarse or worn thread. It should visibly fill the thread valleys without being a bulky wad.
Use the correct grade.Standard white tape for water. Thicker gas-rated tape exists but gas work is Gas Safe registered work and not a householder job.
Assemble without unwinding.Start the fitting by hand and turn steadily. If it has to be backed off, take the tape off and rewrap — backing off breaks the seal.
Do not use it on a compression olive as the primary seal.A turn or two round an olive can help a marginal joint, but the olive is the seal. Tape is not a substitute for a properly cut pipe end.

Tips

If you have to unscrew a taped fitting to reposition it, rewrap. A backed-off joint has a broken seal even if it looks fine.
Ten to fifteen turns feels like a lot the first time. Too little tape is much more common than too much.
Keep the tape out of the pipe bore — start a couple of threads back from the end.

Common mistakes

Wrapping the tape the wrong way round — Screwing the fitting on then unwinds the tape, bunching it at the end of the thread instead of filling it. The joint leaks and the tape has to come off and be redone.
Using PTFE tape instead of a missing washer — A parallel thread seals at the shoulder against a washer. Tape in the thread does not seal that face at all, so the joint leaks regardless of how much is used.

If it doesn't work

Joint still weeps with plenty of tape

Cause: Parallel thread needing a washer, not tape — Fix: Fit the correct fibre or rubber washer.

Tape bunched at the end of the thread

Cause: Wrapped the wrong way — Fix: Remove all of it and rewrap clockwise looking at the thread end.

Debris in a tap cartridge after work

Cause: Tape pushed into the bore — Fix: Clean the cartridge; start the wrap further back next time.

Fitting will not screw fully home

Cause: Far too much tape on a tapered thread — Fix: Remove and rewrap with fewer turns.

Questions people ask

Which way round does PTFE tape go?

Clockwise when you look at the open end of the male thread, which is the same direction the fitting turns as it tightens. That way assembly presses the tape into the thread instead of peeling it off.

How many wraps of PTFE tape?

Roughly ten to fifteen for a standard tapered water thread. The aim is to fill the thread valleys evenly; a worn or coarse thread needs more.