Plumber's Jointing Compound: Which One for Potable Water, Heating, or Gas (and Why It Matters)

A weeping radiator tail discovered three weeks after the plasterer leaves means cutting the radiator off the wall, draining a heating system, redoing the joint, and patching plaster. The fix takes a day. The cause was almost certainly the wrong sealant on the joint, or the right sealant applied badly. Jointing compound is a £10 product that prevents a £400 problem, but only if you pick the right one for the system you're working on. Get it wrong on a cold water pipe and you've also broken the law.

What it is and what it's for

Jointing compound is a paste-form sealant smeared onto threaded plumbing fittings to fill microscopic gaps between the male and female threads as they're tightened. The paste deforms into the thread profile and either stays as a flexible filler (oil-based products) or sets into a gasket-like solid (anaerobic products). Either way, the result is a watertight, gas-tight seal that hand-tightening alone wouldn't achieve.

It comes in tins (typically 200g, 400g, 500g) with a brush built into the lid, in tubes for one-handed application, and as a sealing cord (Loctite 55) which is essentially the same chemistry stretched into a fibre you wrap around the thread instead of brushing on.

You use it on threaded BSP joints. The radiator tail screwing into a radiator valve. A brass adaptor connecting copper pipework to the threaded inlet on a stop tap. A pump union on a heating circuit. The flexible hose connection on a basin tap. Anywhere two threads meet and need to seal under pressure.

You don't use it on push-fit fittings, soldered capillary joints, or flared compression fittings. Push-fit seals on an o-ring that compound interferes with. Solder seals by molten metal capillary action that compound contaminates. Flared fittings seal on the metal-to-metal cone, not the thread.

The approval matrix nobody else publishes clearly

This single table answers the question every beginner asks and every product label dodges. Read it once and the rest of the page makes sense.

WRAS is the Water Regulations Advisory Scheme. A WRAS approval number on a product means it has been independently tested and confirmed safe for contact with water intended for human consumption. The Water Supply (Water Fittings) Regulations 1999 require that any material in contact with potable water in the UK is WRAS-approved or equivalent. Using a non-approved product on a drinking water pipe is not a best-practice issue. It is a regulatory breach.

Boss White vs Water Hawk vs Hawk White: the three tins you'll see in the shop

These three products account for most of the jointing compound sold in UK plumbing aisles. They look interchangeable. They are not. Pick the wrong one and you have either a leak, a regulatory problem, or both.

Boss White is the white paste in the round tin you see on every plumbing merchant's shelf. Around £11 – £12 retail at Toolstation or Wickes for a 400g tin. It seals well. It's the traditional default for central heating work and is the compound most plumbers grew up using. It is not WRAS-approved for potable water. The Toolstation product description states "non-potable duties" outright. If your plumber uses Boss White on a cold mains pipe, the joint will hold, but the installation does not comply with the Water Fittings Regulations. Use Boss White on closed central heating systems and nothing else.

Fernox Water Hawk is silicone-based, non-toxic, and WRAS-approved for hot and cold potable water. Around £10 – £12 for a 400g tin at Wickes or BES. It also covers natural gas, steam, brine and LP gas. This is the universal product. If you only buy one tin of compound for a domestic plumbing job, this is it. The grey-blue tin is the easiest to identify on a shelf next to the white-tinned Boss products.

Fernox Hawk White is linseed-oil based, with a cream-coloured paste. Around £8 – £10 for a 400g tin at Toolstation or Screwfix. It is approved for non-potable water, central heating, natural gas (to 70°C), steam, brine and LP gas. It is not WRAS-approved. The confusing part is that Hawk White and Water Hawk are made by the same manufacturer (Fernox), the names are nearly identical, and the tins live next to each other on the shelf. Hawk White is for heating only. Water Hawk is for drinking water. Read the label every time.

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Gas-rated compound is a separate product, and a separate competence

Domestic gas work in the UK can only legally be carried out by a Gas Safe registered engineer. That's a licensing rule, not a guideline. If you're a homeowner, you don't apply gas compound to a gas joint yourself. You hire someone whose name appears on the Gas Safe register. The reason this section exists is because gas compounds appear on the same merchant shelves as water compounds, and the products are visually similar enough to mix up.

BOSS Gastite (sometimes mislabelled "Boss Gold" in older guides, Gastite is the correct current product name) is a gas-only jointing compound. Around £8 – £10 for a 400g tin. It complies with BS 6956 Parts 5 and 6a. Natural and town gas only. Do not use on water. It is not approved for potable or non-potable water systems.

Calortite Y is the equivalent for LPG (propane and butane) and is sold through gas trade outlets rather than general plumbing merchants. Same rule: gas only.

Flomasta Gas and Water (Screwfix's own brand, around £14 – £15 for 250g) is the exception. It is approved for both gas and cold potable water by Gas Safe registered installers. It is soft-setting and rated -45°C to 260°C. The dual approval is unusual and explains the price premium.

The error pattern that costs registrations: a Gas Safe engineer using gas-rated compound on a copper hot water cylinder fitting because the tin is closer to hand. Some gas compounds contain solvents and PTFE fillers that aren't tested for potable water contact. The reverse error (Water Hawk on a gas joint) is less dangerous because Water Hawk is gas-rated, but a stickler inspector will still flag it because the gas-only product is what the certification expects.

How to apply it: thin smear, second thread inwards

The single most common mistake on jointing compound is using too much. A thick coating doesn't seal better. It squeezes out into the pipework, creates conditions for bacterial biofilm to form on potable water systems, and can clog small-diameter components. The Water Hygiene Centre flags overapplication as a real source of microbiological sample failures, not just an aesthetic issue.

1. Clean the threads first. Both male and female threads should be free of swarf, old PTFE residue, and grit. A wire brush and a clean rag is usually enough. If the joint is being remade after a previous failure, scrape off all old compound before reapplying.

2. Apply only to the male thread. Compound on the female thread gets pushed into the pipe interior as the joint tightens, where it's useless and contaminating. Hold the male fitting and brush a thin layer onto the threads, starting from the second thread back from the end. The first thread should stay visible. The thread profile should still be readable through the paste, not buried in it.

3. Tighten by hand first, then with a spanner. Hand-tightening seats the joint and seats the compound. Then use a spanner to tighten the final turn or two. Stop when firm resistance arrives. Most BSP joints take half a turn to one full turn past hand-tight on a 22mm fitting and slightly less on smaller threads.

4. Wipe excess off the outside of the joint. Whatever paste squeezes out onto the outer face of the fitting can be wiped off with a rag. Whatever pushes inwards stays in the system. Less paste, less to clean up later.

5. Pressure test before boxing in. Standard practice across heating and water installations. Pressurise the system, walk every joint with dry tissue paper, fix any weepers. Most non-setting compounds achieve full seal immediately on assembly. Anaerobic compounds (Loctite 5400) and some setting products want 24 hours before high-pressure testing.

When to combine compound with PTFE tape

Heating installations on radiator tails and pump unions are the standard belt-and-braces use case. The professional consensus across Trade Radiators, BestHeating, and most working plumbers' published guidance is:

1. Wrap PTFE tape around the cleaned male thread (clockwise as you look at the end of the fitting), 10-20 turns depending on how loose the fit is. The tape should bed into the thread profile rather than balloon out.
2. Smear a thin layer of compound (Water Hawk or Hawk White depending on the system) over the PTFE.
3. Tighten as normal.

The PTFE handles bulk filling and lubrication. The compound fills the microscopic gaps the tape leaves and stops the tape unwinding under thermal cycling. On radiator tails specifically, the cyclic loads of repeated heating and cooling can work a tape-only joint loose over years, and a thin layer of compound is cheap insurance against having to drain the system to redo it.

A minority forum claim suggests compound can rot PTFE tape over time. No manufacturer data supports this and the practice is universal in UK heating installation. Ignore the claim and use the belt-and-braces method on radiator and pump connections.

Using compound on a weeping compression olive

This is the borderline use case that every working plumber does and most product datasheets won't endorse. A compression fitting weeps. The olive is slightly scored, or the pipe surface has a flaw the olive can't bed into cleanly. A thin smear of compound on the olive face creates an extra layer that compensates for the imperfection.

The technique:

• Loosen the nut, slide it back, and inspect the olive. If the olive has a clear groove, replace it with a new one before resealing.
• Smear a very thin layer of compound onto the olive face that meets the fitting body, and onto the conical seat inside the body.
• Reassemble and tighten as normal (three-quarters of a turn past hand-tight on most domestic compression sizes).

This is accepted as a remedial fix for weeping olives on existing pipework where cutting back to fit a new fitting isn't practical. It is not a primary sealing method on a fresh joint. A fresh compression joint with a new olive on cleanly cut pipe should seal without compound. If it doesn't, the joint needs investigating, not papering over.

Loctite 55 sealing cord: the modern alternative

Loctite 55 is jointing compound stretched into a fibre. You wrap the cord around the male thread (typically 8-15 wraps depending on thread size), apply a small amount of starter paste from the integrated dispenser, and tighten the joint. The cord and starter together create an immediate full-pressure seal with no waiting time.

Around £16 – £19 for a 160m roll at BES. Smaller 50m consumer rolls run £8 – £10 on Amazon and Toolstation. WRAS-approved for potable water (approval number 1612522). Carries EN751-2 Class ARP for gas. Rated to 85°C. Works on copper, brass, stainless steel, cast iron, galvanised metals, and most engineering plastics.

The case for switching to Loctite 55 over paste compound:

• No mess. The cord doesn't drip, smear, or contaminate adjacent surfaces.
• Single product covers potable water, heating and gas.
• Easier for less experienced installers to get right. Wrap, apply starter, tighten.
• The seal sets immediately at full pressure, no 24-hour cure.
• One roll lasts hundreds of joints, so the per-joint cost is competitive with paste despite the higher purchase price.

The case against:

• Higher upfront cost. £16 – £19 for a roll versus £10 – £12 for a tin of Water Hawk.
• Less familiar to older trade plumbers, so they may default to what they know.
• Maximum temperature 85°C is lower than Water Hawk's 130°C rating, which matters on certain unvented hot water cylinder configurations. Check the temperature rating against the system before specifying.

For a homeowner overseeing a plumbing first fix and second fix, the practical answer is: a 50m roll of Loctite 55 in your spares kit, plus one 400g tin of Fernox Water Hawk for radiator tails and any joint that needs paste. That covers every domestic plumbing thread you'll meet on an extension.

Common mistakes

Too much paste. Already covered above and worth saying twice. The thread profile should still be visible through the paste. Anything more is excess that will end up inside the pipe.

Applying to plain (non-threaded) faces. Compound is for threads. The flat union face on a radiator tail's nut, the cone face on a flared fitting, the rubber washer face inside a tap connector - these are sealing surfaces that work on direct contact. Compound on them either prevents the contact or gets squeezed out into the pipework.

Mixing brand systems on the same joint. A common mistake is using PTFE tape from one brand and a compound from another that contains incompatible solvents. The risk is overstated in forums but worth the simple precaution: stick to one manufacturer's tape and paste pairing on critical heating connections.

Wrong product on potable water. The single most expensive mistake. Boss White or Hawk White on a cold mains, a kitchen tap, or a hot water cylinder is a Water Fittings Regulations breach. Water Hawk, Boss Universal, Loctite 55, or Rectorseal Tru-Blu (NSF) are the WRAS-equivalent options.

Using gas paste on water or vice versa. BOSS Gastite is gas only. Boss White is non-potable water and heating only. Cross-using these is a certification problem on top of being a contamination problem.

Not pressure testing before boxing in. The cardinal sin of plumbing installation. Even an experienced installer should pressurise the entire run and walk every joint with tissue paper before plaster goes on. A weep that's caught on the first day costs nothing. The same weep caught after the kitchen is fitted costs days.

Reusing a thread without cleaning off old compound. Old compound sets, dries, or contaminates with grit. Stripping a joint without cleaning the thread back to bare metal sets the next attempt up to fail.

Alternatives

PTFE tape on its own is sufficient for many BSP threads at low pressure: cold mains takeoffs, garden tap connections, domestic stop valves. Wrap 8-15 turns clockwise, no compound needed. The combination of PTFE plus compound is the upgrade for heating circuits where thermal cycling matters.

Hemp and compound is the traditional pairing for large BSP threads on galvanised pipe and old work. Hemp fills coarse threads better than tape but is non-potable only. Sold as raw flax fibre by the kilo at plumbing merchants.

Anaerobic thread sealants (Loctite 5400, Loctite 577) cure in the absence of air to form a hard set seal. They achieve very high pressure ratings and resist vibration loosening. The trade-off is that the joint is harder to disassemble later and the cure time means you can't pressure test for several hours. Used in industrial and high-pressure applications more than domestic plumbing.

For a domestic kitchen extension with a typical mix of compression fittings, push-fit runs, and threaded connections at appliances, the practical compound shopping list is one tin of Water Hawk (£10 – £12) and a roll of Loctite 55 (£8 – £19 depending on size). Total spend under £30 covers every joint type you'll meet.

Where you'll need this

Jointing compound shows up at two stages of any plumbing first-fix and second-fix work, regardless of project type:

• First fix plumbing - sealing radiator tail threads, manifold connections, valve threads, pump unions, and any threaded BSP connection on copper pipework
• Second fix plumbing - sealing tap connections, isolation valve threads, washing machine valve threads, and remediating weeping olives on existing pipework

These fittings turn up on any extension or renovation that involves new heating circuits, new water supply runs, or kitchen and bathroom plumbing. The product choice rule stays the same across all of them: WRAS-approved compound on anything touching drinking water, gas-rated compound on gas (by a Gas Safe engineer only), and Boss White or Hawk White on closed heating circuits where potable water never reaches.