Pipe Sealing Cord (Loctite 55): Why Plumbers Use It Instead of PTFE Tape

Your plumber finishes connecting a bank of radiator valves, fills the system, pressurises it. One valve points slightly off-vertical. He grips it with an adjustable wrench, backs it off a quarter-turn to straighten the orientation, and walks away. The joint doesn't leak. Try the same trick with PTFE tape and you'll be draining the system, dismantling the valve, scraping shredded tape off the thread, and starting over. That single trick, the ability to back off a joint without losing the seal, is why every plumber over the age of about 35 carries pipe sealing cord on their van. PTFE tape is what beginners use. Cord is what professionals use.

What it is and when you need it

Pipe sealing cord is a thin yarn of multiple synthetic filaments coated in a sticky, non-curing sealing compound. You pull a length off the roll, wrap it around the male thread of a fitting, screw the fitting on, and the cord squeezes into the gaps between the thread peaks and troughs to form a seal. Water can't get past it. Gas can't get past it. Unlike epoxy or thread-locker, the compound never sets hard, so the joint stays serviceable for life.

In the UK trade, "pipe sealing cord" almost universally means Loctite 55, the Henkel-branded product that has dominated the market since the early 2000s. You'll also see Screwfix's own-brand Flomasta cord and the smaller-volume TruTite from MacDonald Plumbing. They're all the same general idea: a polyamide or polyester yarn carrier with a non-hardening sealant. Loctite 55 is the reference product and the one your plumber will reach for unless cost is a serious factor.

You need it for the same job PTFE tape does: sealing male threaded joints on water and gas pipework. Radiator valve tails. Boiler connections. Tap tails on directional fittings. Isolation valves. Any time a brass or steel male thread screws into a female socket and water or gas needs to stay inside the pipe.

What makes cord interesting isn't that it does the same job as tape. It's that it does the job better in specific situations where tape falls apart, and once you've understood those situations you'll struggle to go back.

Why cord beats PTFE tape

There are exactly two advantages, and one of them swallows the other.

You can back off a joint without re-sealing. This is the dominant pro argument. With PTFE tape, the moment you unwind a fitting (even a fraction of a turn) the tape unravels, the seal breaks, and you have to dismantle, clean, re-tape, and remake. With cord, the non-curing sealant flows back into the gap as the threads move. You can tighten, back off a quarter-turn to align an elbow, retighten, back off again. The seal holds throughout. On a bank of radiators where every valve needs to point exactly vertical to the floor for visual neatness, this is the difference between a 30-minute job and a two-hour job.

One product covers water and gas. A standard PTFE tape roll is rated for water only. Gas requires a separate, thicker tape (the yellow-spool kind). Sealing cord that meets BS EN 751-2 is approved for both potable water and 1st, 2nd and 3rd family gases at the same time. One roll on the van, not two.

The standards reference matters because it's where buildwiz.uk corrects a common confusion. BS EN 751-2 is the Part of the standard that covers non-hardening jointing compounds, including pipe sealing cord. BS EN 751-3 covers unsintered PTFE tape (the white tape on a plastic spool). Both parts sit under the same parent standard, but they apply to different products. Loctite 55 carries BS EN 751-2 Class ARp approval, meaning it's certified for high-pressure gas (the most demanding rating). PTFE tape on the same gas joint would be tested against Part 3, not Part 2.

Specifications and limits

Loctite's published numbers for the 55 cord are these.

The pressure ratings tell you why no domestic plumbing application worries you. Your central heating sits at 1.5 bar. Your mains cold tops out around 5 bar. The cord is rated 16 bar for water and tested to ARp for gas, which is the toughest classification BS EN 751-2 defines. Underselling its capability is impossible on a normal extension.

How to use it properly

The technique is simple but the details matter. Get any one of them wrong and the joint weeps.

Wrap clockwise, in the direction of tightening

Hold the male thread with the open end pointing toward you. The cord wraps clockwise, the same direction you'll turn the fitting when you screw it in. If you wrap counter-clockwise, the act of tightening unwinds the cord and tears it off the thread. The joint then leaks immediately.

This rule is identical to the rule for PTFE tape, and identical for the same reason: you're wrapping in the direction of friction so the thread engagement pulls the cord deeper into the gap, not out of it.

Anchor the start with a blade or with the supplied cutter

The cord is slippery. Most rolls come in a yellow plastic dispenser with a small steel blade slot that grips the cord while you anchor the first turn. If you don't use the blade, the first wrap slides off as you start the second.

Pull about six inches of cord off the roll. Lay the loose end across the thread at the second thread from the open end (not the very tip; starting at the tip risks the cord shredding into the pipe interior). Press it down with a thumb. Begin wrapping clockwise, pulling the cord taut against the thread profile.

Build turns according to the roll's instruction table

Loctite 55 packaging includes a guide table that tells you how many turns to use for each thread size. The right number changes with diameter: a 1/2 inch BSP joint needs perhaps 8-10 turns, a 1 inch BSP joint needs 14-16, and a 4 inch joint needs upwards of 30. The guide is printed on the inside of the dispenser lid; read it.

Tear off, no curing, fit immediately

Tear the cord by sharply pulling it tight against the dispenser blade. Don't cut it with scissors; the fibrous break gives a thinner tail that the fitting can swallow without bunching.

Press the tail flat against the thread. Screw the fitting on by hand until you feel resistance, then bring it home with a wrench. Loctite 55 has no cure time, so you can pressurise the system the moment the joint is tightened. Compare with traditional hemp and Boss White, which want at least an hour before you stress them.

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Common mistakes

Three mistakes account for almost every cord failure.

Wrapping the wrong direction. Cord pulled off the roll naturally wants to wrap one way. If you've grabbed the roll in the wrong hand, that direction is anti-clockwise relative to the fitting and the cord shears off as you tighten. Symptom: the joint leaks the moment you pressurise. Fix: dismantle, clean the thread, start again the right way round.

Too few turns. Loctite's table is conservative. If you halve the recommended turn count to save time, the cord doesn't bridge the thread profile fully and water finds a path through. Symptom: a slow weep that appears hours after pressurisation, often once the system has been through a heat cycle. Fix: dismantle, scrape clean, apply the full recommended turn count.

Mixing cord with PTFE tape. Some homeowners read both guides, decide more is better, and apply tape over the cord (or cord over tape). The two products have different compression properties and they fight each other. The fitting won't seat at the right depth, the joint either leaks or cracks the brass casting. Pick one. Don't combine.

A fourth mistake worth flagging separately: using cord on a fitting with a face seal. Pipe sealing cord is for tapered male threads where the seal forms in the thread path. Some imported taps and appliance flexible-hose connections use parallel threads with a fibre or rubber washer at the bottom of the female thread. The seal is the washer, not the threads. Cord on those joints is decorative at best and can deform the washer at worst. If the female end of the connection has a flat-faced washer inside, leave the male thread bare and tighten as instructed.

Gas work caveat

Loctite 55 is approved for natural gas, LPG, and town gas under BS EN 751-2 Class ARp. That doesn't mean a homeowner can use it on gas fittings.

UK domestic gas work is regulated under the Gas Safety (Installation and Use) Regulations 1998. Anyone working on gas pipework, fittings, or appliances on domestic premises must be on the Gas Safe Register. This is a legal requirement, not a guideline, and it applies even if you own the property. The penalty for unregistered gas work is criminal, not civil.

What you can do as a homeowner: ask your Gas Safe engineer which thread sealant they're using and why. If they're still using yellow-spool gas PTFE tape rather than cord, ask whether cord might be a better fit for the joint they're making, particularly if it's one that needs to be backed off for orientation. They may have a good reason (cost, habit, the fitting has a face seal) or they may take the suggestion. You're paying their hourly rate; ask the question.

What to buy

Three suppliers cover the UK market for pipe sealing cord at the homeowner end. The cheapest realistic single roll lands at £5.58 from Bearing King for a 50m Loctite 55 roll. That's enough cord for 40-50 typical 1/2 inch BSP joints, which covers a kitchen extension's plumbing several times over.

If Screwfix is your habitual merchant and you're buying alongside other plumbing kit, the Flomasta own-brand 80m roll runs £12.64. Per-metre, Flomasta works out slightly more expensive than the Bearing King Loctite 55 roll, but you save the separate delivery if Screwfix is your one-stop merchant. Flomasta isn't Loctite 55, but the formulation is BS EN 751-2 approved and reviews from professional plumbers are generally positive.

For a busy plumbing project where you'd otherwise burn through two consumer rolls, the BES or City Plumbing 160m trade roll lands at £15.60–£18.89. Per-metre this is the cheapest you'll find Loctite 55 at retail. The catch is that BES is a trade-counter merchant rather than a consumer DIY shed, so delivery and account setup can take a day or two.

For a homeowner doing the plumbing on a single extension and unlikely to use cord again for years, the Bearing King 50m Loctite 55 roll is the right buy. It's the cheapest Loctite-branded entry, the volume covers your project, and you get the reference product rather than an own-brand alternative.

For a serious DIYer or small-scale landlord with multiple properties, the BES 160m roll is the better buy. The cord stores indefinitely (the dispenser keeps the unused yarn dry; opened rolls remain usable for years) and the per-metre cost almost halves.

Alternatives

PTFE tape is the obvious alternative and the comparison most homeowners will be making. The trade-offs are these.

PTFE tape is cheaper per joint (a 12m roll costs around 35p; you'll use 30cm per joint), simpler to apply for a one-off connection, and more widely available (every DIY shed stocks it). The downsides are that it doesn't tolerate being backed off, water-tape and gas-tape are different products, and the technique is unforgiving of mistakes (wrong direction, too few wraps, slack tape).

Sealing cord is more expensive per joint, requires a few minutes of practice to wrap correctly, and is harder to find on a Sunday afternoon (DIY sheds rarely stock it; merchants do). The upsides are that it's water-and-gas approved as a single product, it forgives the back-off-to-realign manoeuvre that breaks PTFE seals, and the joint can be pressurised immediately.

For a single connection (one tap, one valve), PTFE tape is fine. For a project with multiple threaded joints where any of them might need orientation adjustment, cord pays for itself in time saved.

The other alternative worth knowing about is traditional hemp and jointing compound. You wrap natural hemp fibre around the thread and smear a paste like Boss White or Fernox Hawk White over it. The compound sets semi-hard and the hemp fills the thread path. This is still preferred by experienced plumbers for threads larger than 1 inch BSP and for galvanised steel pipe over 2 inches, where neither tape nor cord reliably fills the deeper thread profile. For standard 1/2 and 3/4 inch domestic connections, cord has largely replaced hemp.

Note that Slic-Tite (sometimes seen on plumbing forums) is a paste, not a cord. It's a different product category and not directly comparable to Loctite 55. Some older Calortite and Hercuseal cord brands also crop up in archive forum threads but no longer have current UK distribution. If you see them recommended, the recommendation is dated.

Storage

Loctite 55 has a stated shelf life of around five years from manufacture in its sealed dispenser. In practice, an opened roll kept dry and reasonably cool (loft, garage, plumber's van) stays usable far longer. The non-curing sealant doesn't degrade in normal storage conditions; the failure mode is the cord drying out if the dispenser cap is left off and air gets to the unused yarn.

Keep the cap closed when you're not using it. Don't store the roll where it can get wet (under a sink, in a leaky shed). Don't expose it to extreme heat (above an unlagged hot water cylinder, on a south-facing dashboard in summer). Beyond that, treat it like any other consumable.

Where you'll need this

• First fix plumbing - sealing every threaded joint as the pipework goes in: tees, isolation valves, mains-fed branch connections
• Second fix plumbing - tap tails, radiator valve connections, boiler hookups, washing machine and dishwasher feeds where final orientation matters

Pipe sealing cord appears during plumbing stages of any extension or renovation project. Anywhere a brass or steel male thread needs sealing, cord is the better choice than tape if you have either to hand.