PTFE Tape, Hemp, and Jointing Compounds: Thread Sealants for UK Plumbing and Gas Installations

Thread sealing is one of the most frequently performed tasks in plumbing and gas installation — and one where the wrong material choice can cause failures that are dangerous, costly, or both. PTFE tape, hemp (or linen thread), and jointing compounds each have distinct applications, approval statuses, and compatibility requirements. This guide covers the correct selection and application technique for thread sealants used on water, central heating, and gas pipework in UK domestic and commercial installations.

Why Thread Sealants Are Needed

BSP (British Standard Pipe) threaded connections — the standard in UK plumbing and gas — use parallel threads (BSPP, G-series) or tapered threads (BSPT, R-series). Tapered threads provide mechanical sealing as the male thread is drawn into the female fitting, but are never reliable without an additional sealant — the thread surfaces are machined to a tolerance that allows gas and liquid to pass through even at full engagement. Parallel threads rely entirely on a sealant or an O-ring/fibre washer seated against a flat face.

The purpose of a thread sealant is to fill the microscopic voids between the thread flanks and prevent passage of fluid or gas under operating pressure.

PTFE Tape

What It Is

PTFE (polytetrafluoroethylene) tape is a thin, white film produced specifically for thread sealing. It is inert, non-toxic, non-setting, and chemically compatible with almost all substances encountered in plumbing and gas work. PTFE tape is approved for use on:

• Cold water supply pipework
• Hot water and central heating systems
• Gas (natural gas and LPG) — only if specifically rated for gas use
• Compressed air and industrial gases

Gas-Rated PTFE vs Standard PTFE

Standard white PTFE tape (as sold in most hardware shops and plumbers' merchants) is approved for water but is not gas-approved under most UK gas safety regimes. The reason is that standard PTFE can shred and migrate into a gas valve seat, potentially causing a slow internal leak that is extremely difficult to locate.

For gas pipework, use yellow PTFE tape that carries a Gas Safe or BS 6891 approval marking. Yellow gas-rated PTFE is denser and thicker than standard white tape — it resists shredding during assembly and is approved for use on natural gas and LPG fittings.

How to Apply PTFE Tape Correctly

1. Clean the male thread with a clean dry cloth — any oil, grease, or debris will prevent the tape from gripping.
2. Wrap the tape in the direction of the thread (clockwise on a standard right-hand thread, when viewed from the end of the pipe). Wrapping in the opposite direction causes the tape to unwind and bunch as the fitting is screwed on.
3. Start one or two threads back from the end of the pipe, keeping the tape tight and overlapping each turn by approximately 50%.
4. Apply 3–4 turns for water connections, 5–6 turns for gas connections (more tape compensates for the lower pitch and tighter tolerance required on gas fittings).
5. Tear the tape cleanly at the end and press the tail down against the thread with your thumbnail.
6. Hand-tighten the fitting, then apply final torque with a spanner. PTFE joints should feel progressively stiffer as the fitting is driven home — sudden resistance (clicking or cracking) indicates cross-threading.

Do not over-apply PTFE tape. Excessive layers create a joint that is difficult to drive home and may crack the female fitting on plastics or brass.

When Not to Use PTFE Alone

PTFE tape is not suitable as the sole sealant for:

• Vibrating pipework — the tape can work loose over time in high-vibration environments (generators, pumps, plant rooms).
• Very high pressures above approximately 10 bar — use a compound or liquid sealant for systems above this range.
• Large-diameter threads above 2" BSP — adequate tape coverage is difficult to achieve consistently.

Hemp (Linen Thread)

What It Is

Hemp, also known as linen thread or plumber's hemp, is a natural fibre thread traditionally used as the primary thread sealant in UK plumbing before PTFE became widely available. Today it is almost always used in combination with a jointing compound — the hemp provides mechanical bulk to fill the thread void, while the compound provides chemical sealing and prevents the hemp from becoming porous as it dries.

Hemp is widely preferred by experienced plumbers for larger-diameter connections (3/4" BSP and above), gas meter connections, and any joint where long-term reliability is paramount. Properly applied hemp-and-compound joints are extremely durable and can remain leak-free for decades.

How to Apply Hemp and Compound

1. Pull a length of hemp thread from the ball — approximately 30–50cm depending on the pipe size.
2. Lay the hemp into the thread roots on the male fitting, winding in the direction of the thread and keeping it in the thread grooves rather than on top of the crests.
3. Complete 3–5 turns, building up a layer that fills the thread depth.
4. Apply jointing compound over the hemp using a brush or your finger, working it into the fibres and around the threads.
5. Apply a second light coat of compound to the inside of the female fitting if it is a straight (parallel) fitting.
6. Assemble and torque as normal. Hemp joints may be slightly harder to drive home than PTFE joints — this is expected.

Hemp on Gas Pipework

Hemp used on gas pipework must be used with a gas-approved jointing compound. Gas Safe registered engineers are required to use materials approved under IGEM/G/11 and BS 6891. The jointing compound must be compatible with the gas type (natural gas, LPG) — some compounds are approved for one and not the other. Always check the product approval marking before use on gas fittings.

Jointing Compounds (Thread Sealant Pastes)

Jointing compounds are semi-setting or non-setting pastes applied directly to threads, either alone or with hemp. The most common products in UK plumbing are:

• Boss White — a traditional non-setting white paste approved for water. Not approved for gas. Suitable for cold and hot water systems and central heating where temperatures do not exceed 110°C.
• Fernox LS-X — a modern liquid sealant that is compatible with central heating systems, sealed systems, and unvented cylinders. Contains corrosion inhibitor and is safe for use where Fernox inhibitors are dosed.
• Loctite 577 / 572 (anaerobic thread sealant) — a liquid that cures without air inside the thread engagement zone. Suitable for both water and gas (confirm grade for gas use). Provides a stronger seal than paste compounds and resists vibration well.
• WRAS-approved gas compounds — such as Heldite, Jet Blue, and similar products approved under BS 6891 for gas pipework. These must carry visible BS/IGEM approval marking.

Compound-Only Applications

On small-diameter connections (up to 3/8" BSP) with tapered threads, some plumbers apply compound alone without hemp. This is acceptable for water connections but not recommended for gas — the thread geometry alone is insufficient to prevent gas migration without a fibre fill, particularly on older fittings with slightly worn thread forms.

Liquid Thread Sealants (PTFE-Based)

A third category of thread sealant — liquid PTFE or paste PTFE — bridges the gap between tape and compound. These products are applied to the male thread with a brush and set to a firm but flexible bond. Popular products include Loctite 55 thread sealing cord and Tangit Uni-Lock, both of which are approved for water and gas applications. These are particularly useful for:

• Joints that need to be backed off slightly to align after assembly (PTFE tape will shear; liquid PTFE will flex slightly before fully setting).
• Plastic thread fittings where the torque required for hemp-and-compound risks cracking the fitting body.
• Situations where re-working joints is likely (liquid PTFE joints can often be cracked apart and remade more easily than hemp).

O-Ring and Fibre Washer Connections

Some parallel BSP connections (G-threads) are designed to seal on a flat-face O-ring or fibre washer rather than on the thread form. These include:

• Radiator tail and valve body connections (sealed by a copper, fibre, or EPDM washer).
• Garden tap connections, some ball valves, and gas meter connections with union nuts.
• Compression gauge connections using a solder ring or bonded seal (Dowty washer).

On flat-face connections, do not apply PTFE tape to the threads — the seal is on the face, not the thread, and tape on the threads will prevent the fitting from seating fully on the washer. Instead, confirm the washer is the correct size and material (EPDM for water; PTFE or metallic for gas) and ensure the mating face is clean and flat.

Approval Requirements Summary

Common Mistakes and How to Avoid Them

• Wrapping PTFE in the wrong direction — the tape unwinds as the fitting is driven on, ending up as a pile at the thread root rather than a seal. Always wrap in the thread direction.
• Using standard white PTFE on gas — Gas Safe engineers have received enforcement notices for this. Yellow gas-rated tape costs marginally more and eliminates any compliance risk.
• Applying sealant to the female thread only — sealant must be on the male thread to be drawn into the void as the fitting is assembled. Applying it inside the female socket alone is ineffective.
• Cross-threading and over-torquing — thread sealant does not fix a cross-threaded joint. Always start by hand and feel for smooth engagement. Over-torquing brass fittings in plastic female threads cracks the fitting body — tighten to resistance plus one quarter turn at most on plastic.
• Reusing threaded joints without refreshing the sealant — PTFE tape and hemp are one-time sealants. If a joint is cracked open for any reason, remove all old tape/hemp, clean the thread, and re-seal before reassembly.

Related Products at APM Electricals

• Compression Fittings — 15mm, 22mm, and 28mm brass compression fittings for water supply and heating connections. See our copper compression fittings guide.
• Ball Valves and Stopcocks — 15mm and 22mm ball valves for isolation, available in compression and push-fit ends. See our isolation valves guide.
• Radiator Valves — angled and straight radiator valve sets with 1/2" BSP radiator tail connections. See our radiator valve trade guide.
• Push-Fit Fittings — JG Speedfit and Hep2O plastic push-fit systems for water supply. See our push-fit systems trade guide.
• Copper Press-Fit Fittings — M-Press and ProPress press fittings for fast, no-flame connections. See our press-fit fittings guide.
• Central Heating Inhibitors — Fernox F1 and Sentinel X100 for sealing and protecting central heating circuits. See our guide to boiler system chemicals.

Summary

Thread sealant selection is straightforward once the application is confirmed: yellow gas-rated PTFE or gas-approved hemp compound for gas; standard PTFE, Boss White, or Fernox LS-X for water and heating. Apply PTFE in the thread direction, build hemp into the thread roots before compounding, and never reuse tape or hemp on a cracked-open joint. For flat-face O-ring connections, no thread sealant is needed — the seal is on the face, not the thread. Follow these rules and thread joints will last the life of the installation.

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