How to Use PTFE Tape Correctly (And When It Will Not Seal Anything)

PTFE tape seals threaded joints by filling the tiny spiral gap between male and female threads while letting them wind together smoothly. To use it correctly, wrap it clockwise around the male thread when viewed from the open end, keep it taut so it sinks into the thread form, and use roughly three to five wraps for a standard tap or fitting. Get the direction wrong and the tape unravels as you tighten, which is the single most common reason a taped joint still weeps.

What PTFE tape actually does

There is a persistent myth that PTFE tape is a glue or a sealant in the way silicone is. It is not. It is a thin ribbon of polytetrafluoroethylene that does two jobs at once. First, it acts as a lubricant so the threads can be wound further together than dry metal on metal would allow, which increases the mechanical contact between the flanks of the thread. Second, it deforms into the leftover helical void so water has no clear path to spiral out along.

That distinction matters because it tells you where tape belongs. It is for tapered threads and thread forms that seal on the thread itself. It does nothing useful on a joint that seals on a rubber washer, an O ring, or a compression olive. Taping the threads of a tap tail or a compression nut is a waste of time and can actually stop the nut pulling up square.

Which joints to tape and which to leave alone

• Tape these: BSPT tapered male threads, iron pipe fittings, radiator valve tails into the radiator body, immersion heater threads, pressure gauge and drain cock threads, hose to fitting adaptors that seal on the thread.
• Do not tape these: compression fittings, tap connectors with a fibre or rubber washer, push fit joints, flexible hose ends with an integral seal, plastic waste pipe connections.

If you can see a washer or an O ring inside the female half, the seal happens there. Adding tape to the thread will not improve it and may hold the faces apart just enough to cause a slow drip.

Step by step

1. Clean and inspect the thread. Wire brush off old tape, hemp, jointing compound and corrosion. Old tape left in place is a classic cause of a joint that will not seal no matter how much fresh tape you add. Check for damaged or flattened thread crests while you are there.
2. Work out the wrap direction. Hold the male fitting so the open end points towards you. You will be wrapping clockwise. The reason is simple: when the fitting screws into the female half it also turns clockwise, so a clockwise wrap gets pushed tighter rather than dragged loose.
3. Start one or two threads back from the end. Leaving the first thread or two bare helps the fitting start straight and stops shredded tape being pushed into the bore where it can foul a valve seat or a shower cartridge.
4. Keep tension on the tape. Pull firmly enough that the tape stretches slightly and you can see the thread pattern coming through it. Loose, baggy wraps just bunch up and get cut through.
5. Overlap as you go. Move along the thread with each turn overlapping the last by about half its width, then come back if you need more layers.
6. Use the right number of wraps. Three to five turns suits most brass and metal fittings. Plastic threads and coarse or worn threads can want more. Very fine threads want fewer.
7. Break the tape and smooth it down. Tear it against the thread and press the tail flat with your thumb so it does not lift as you start the fitting.
8. Assemble by hand first. Wind the joint in by hand until it stops, then bring it up with a spanner. If it goes fully home with almost no resistance, back it out and add another wrap or two.

Choosing the right tape

Tape is colour coded by density and intended service, and the differences are real rather than cosmetic. White is the standard low density tape sold everywhere and is fine for general cold and hot water threads. Pink is a thicker, higher density water tape that fills coarse or slightly damaged threads better and tears less. Yellow is gas rated and is what you should be using on gas threads if you are qualified to work on them. Green is oxygen service and is degreased.

If you are buying one roll for household jobs, pink 12mm is the more forgiving choice. It costs pennies more and shreds far less when you are winding it onto a rough iron thread.

Tape, compound, or both

On difficult joints, plenty of plumbers use PTFE tape plus a smear of a compatible jointing compound over the top, or use PTFE cord instead. Compound helps the tape settle and fills any remaining voids. What you should not do is combine tape with a hemp and compound joint that is already made up, or keep piling tape onto a joint that has failed twice. Two failed attempts usually means the thread is damaged, the fitting is cross threaded, or you are trying to seal a joint that was never meant to seal on its threads.

Common mistakes that cause leaks

• Wrapping the wrong way so the tape peels back on assembly.
• Too few wraps, leaving the thread to bottom out before the seal forms.
• So many wraps that the joint feels tight early and the female half splits, which is a real risk on brass and on plastic.
• Tape left dangling into the pipe bore, later jamming a valve.
• Overtightening to fix a weep. Once a taped joint is home, extra force shears the tape rather than sealing it.

If a joint still weeps, take it apart, strip every scrap of old tape, check the thread for damage, and start again cleanly. Rewinding fresh tape over a partially shredded layer almost never works.