Why PTFE tape
does not stop a leak —
and what actually does
Wrapping PTFE tape around a parallel thread and retightening is a common repeated mistake. The tape goes on the thread. The seal forms on the face. These are two different places. The tape never reaches the leak.
Two types of thread — two completely different sealing methods
The reason PTFE tape works on some joints and does nothing on others is not about tape quality or how many wraps you apply. It is about thread geometry. There are two fundamentally different types of threaded connection, and they seal in completely different ways.
BSPP — Parallel thread
The thread diameter is constant along its full length. The thread does not seal — it only clamps the two fitting faces together. The seal forms between a flat gasket compressed between the flat end face of the male fitting and the flat seat of the female socket.
Found across many common UK heating and plumbing fittings — boiler connections, pump unions, radiator valves and tap connectors.
BSPT — Tapered thread
The thread diameter reduces along its length. As the male thread drives into the female thread, the taper creates an interference fit that seals on the thread flanks themselves. PTFE tape fills the gaps in the thread flanks and helps form this seal.
Less common in UK domestic systems — found on some gas meter connections, certain industrial applications and imported equipment.
Putting PTFE tape on a parallel thread does not seal the joint because the tape never reaches the sealing face. The flat gasket between the two flat faces is where the seal forms. Tape on the thread builds up thickness on the thread flanks and may prevent the faces from closing correctly and can make the leak harder to diagnose or, in some cases, worse.
Where the seal actually forms on a parallel BSP joint
On a parallel BSP flat face connection, unscrew the fitting and look at the end face of the male connector. There is a flat machined surface — the gasket seat. A rubber or fibre gasket sits in a recess inside the female fitting, and when the connection is tightened, this gasket is compressed between the two flat faces.
The usual failure point is the gasket or washer inside the fitting, rather than the thread itself. The flat rubber or fibre ring compressed between the two faces has hardened, cracked or permanently compressed. Adding tape to the thread does not touch the gasket — it is in a completely different location on the fitting.
Common scenarios — and why tape fails in each
Boiler or pump union dripping
Pump unions, boiler flow and return connections, heat exchanger ports — all parallel BSP flat face. The union nut clamps the faces together. The gasket between them seals. When the gasket fails, adding tape to the union thread achieves nothing. The gasket needs replacing.
PTFE on thread → does not reach the sealShower hose or washing machine hose leaking
Shower hose connections and appliance inlet hoses use parallel flat face fittings. The washer inside the fitting is the seal. Wrapping tape around the hose thread and retightening does not fix the washer — it may temporarily reduce dripping but does not address the failed seal.
PTFE on thread → washer still failedOutdoor tap or garden hose adapter dripping
Most UK outdoor tap connections use parallel BSP thread. The tap adapter seals on a flat washer at the face. The quick connect body seals on an O-ring. PTFE tape on the tap thread does not replace either of these seals.
PTFE on thread → washer or O-ring still degradedGas meter connection or some industrial fittings
Some gas meter and industrial fittings use tapered BSP thread, where PTFE tape on the thread may be the correct sealing method if the fitting is designed to seal on the thread. Confirm the thread type and service requirement first.
PTFE on tapered thread → correct applicationWhat actually fixes a leaking parallel BSP joint
The fix depends on which type of seal the fitting uses:
- Flat gasket or fibre washer: disassemble the joint, remove the old gasket from the recess inside the fitting, fit a new gasket of the correct size and material. Reassemble without PTFE tape on the thread. The new gasket seals the joint — the thread clamps it.
- Rubber washer in a hose fitting: same principle. The washer sits inside the hose fitting end. Remove it, replace it with the correct size. No tape needed on the thread.
- O-ring in a quick connect body: the O-ring sits inside the female quick connect body in a groove. It needs to be hooked out and replaced with the correct size O-ring. Tape on the thread does not reach it.
How to confirm you have a flat face connection: unscrew the fitting. Look at the end face of the male part — is there a flat machined surface? Look inside the female part — is there a flat seat or a recess with a gasket in it? If yes, the joint seals on a flat face gasket, so the correct next step is gasket replacement rather than tape on the thread.
If the thread narrows visibly along its length — like a tapered plug — it is a taper thread. PTFE tape on the thread is correct here.
Material still matters — once you know it is a flat face joint
Once the joint is identified as a flat face gasketed connection, the replacement material still needs to match the service. Heating water, potable water, gas, temperature, pressure and face condition all affect which grade is appropriate. Use the gasket material selection guides in the Technical Library or the relevant technical datasheet rather than choosing by appearance alone.
PTFE tape belongs on tapered threads. Many common UK plumbing connections seal on a flat face gasket. These are different things in different places on the fitting.
If a joint is dripping and tape has not fixed it, one common reason is that the joint is a parallel flat face connection sealing on a gasket rather than on the thread. The gasket or washer inside the fitting has failed. Replacing it — with the correct material for the duty — is what stops the leak. The thread needs no tape and no compound. It just needs to clamp two flat faces together with a good gasket between them.
FAQ
Why does PTFE tape not stop a leak on a plumbing joint?
PTFE tape is intended for tapered thread connections — where the thread itself narrows along its length and creates an interference fit with the mating thread. On these joints, PTFE fills the gaps in the thread flanks and contributes to the seal. Many common UK plumbing connections — including boiler fittings, pump unions, radiator valves and tap connectors — use parallel BSP thread. On many parallel-thread unions the thread only provides clamping, while sealing occurs at a separate element such as a flat gasket, O-ring, bonded washer, cone or metal seat. Identify the actual joint design before deciding where sealant belongs. Wrapping PTFE tape on a parallel thread does not reach the flat face where the seal actually forms, so it usually does not fix the leak source.
How do I know if my joint has a tapered or parallel thread?
The most reliable method is to use a thread gauge. Visually, a tapered thread is noticeably narrower at the tip than at the base — you can often see this by looking along the male thread. A parallel thread looks the same diameter all the way along. Many BSP flat face fittings also have a visible machined flat face at the end of the male fitting — the gasket seating surface — which is typically absent on plain taper thread fittings. If in doubt, use a gauge rather than guessing.
What should I use instead of PTFE tape on a leaking BSP joint?
For a parallel BSP flat face connection, the correct fix is to replace the flat gasket or washer inside the fitting. The gasket sits between the flat end face of the male fitting and the flat seat of the female socket or union body. It is compressed between these faces when the fitting is tightened. When it hardens or cracks, the joint leaks. Replacing it — not adding tape to the thread — is what stops the leak. PTFE tape belongs on tapered thread connections where the thread itself seals.