Compression Fittings: How They Work, How to Use Them, and When to Choose Them Over Push-Fit

A plumber connects 30 or 40 joints during first-fix plumbing on a typical extension. One that weeps after the system is pressurised means draining down, cutting out, and redoing the joint, plus whatever damage the leak caused to plaster, insulation, or flooring before anyone noticed it. If you're overseeing the plumbing on your project, or doing any pipework yourself, understanding how compression fittings work (and what makes them fail) saves you from being the person who discovers a damp patch three weeks after the plumber has moved on.

What it is and what it's for

A compression fitting is a brass connector that joins lengths of copper pipe without soldering or heat. It works by squeezing a soft metal ring (called an olive or ferrule) onto the outside of the pipe as you tighten a nut. The olive deforms against both the pipe surface and the inside of the fitting body, creating a watertight seal. Three components: the body (the shaped brass piece, could be an elbow, tee, or straight coupler), the olive, and the nut.

No flame, no flux, no solder. That's the appeal. You can make a compression joint in a confined space with two spanners and nothing else. The joint is also demountable. Undo the nut, slide the olive off (or cut behind it), and the pipe is free. This is why boiler manufacturers require compression fittings on boiler connections: when the boiler needs servicing or replacing, the engineer can disconnect pipes without cutting.

Compression fittings comply with BS EN 1254-2:2021, which defines two types. Type A fittings are non-manipulative (the pipe goes in as-is) and are the standard for all domestic above-ground plumbing. Type B fittings require the pipe end to be flared before assembly and are designed for below-ground or high-pressure work. You'll only encounter Type A in a house.

The governing UK regulation for all domestic water fittings is the Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148). One rule catches people out.

Types, sizes, and what each one does

Compression fittings come in every shape you'd need to route pipework through a building. The common domestic sizes are 15mm (for hot and cold supply pipes, central heating flow and return), 22mm (for main runs, bath feeds, and boiler connections), and 28mm (for larger heating circuits, less common in domestic work).

Chrome-plated fittings cost a small premium (roughly 10-20% more) and are used on exposed pipework where appearance matters, like visible pipe runs under a basin or behind a toilet.

Brass vs chrome-plated

Standard brass fittings are used everywhere pipe is concealed behind walls, under floors, or in ceiling voids (provided the joints themselves remain accessible per the regulations). Chrome-plated fittings are the same brass body with a decorative chrome finish for exposed pipe runs. The chrome adds nothing to performance. It just looks better.

How to make a leak-free compression joint

This is the section that separates a good guide from a product listing. Most leaking compression joints fail for one of four reasons: the pipe wasn't cut square, the pipe wasn't fully inserted, the olive was damaged, or the nut was over-tightened. Get these four things right and the joint won't leak.

1. Cut the pipe square. Use a pipe cutter, not a hacksaw. A hacksaw leaves a rough, angled edge that prevents the pipe from seating fully into the fitting body. For tight spaces, a pipeslice (a compact rotary cutter that fits around the pipe) works where a full-size cutter won't. Rotate the cutter around the pipe, tightening slightly after each revolution, until it cuts through.

2. Deburr inside and outside. The pipe cutter leaves a raised lip of copper on the inside edge and sometimes a slight burr on the outside. The internal burr restricts water flow. The external burr can score the olive as it slides over the pipe, preventing a proper seal. Use the deburring blade built into most pipe cutters, or a small round file.

3. Assemble in the right order. Slide the nut onto the pipe first (thread facing the fitting end), then slide the olive on. Push the pipe into the fitting body until it hits the internal backstop. You'll feel it stop. This seating depth is critical. If the pipe isn't fully inserted, the olive won't grip at the correct position and the joint will weep under pressure.

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4. Tighten correctly. Thread the nut onto the fitting body by hand until it's finger-tight. Then use two spanners: one to hold the fitting body still, one to turn the nut. Tighten the nut by three-quarters of a turn to one full turn past hand-tight. No more.

5. Pressure test before boxing in. Once all joints are made, pressurise the system and inspect every fitting with dry tissue paper. A weeping joint will show as a damp mark on the tissue before it's visible to the eye. Fix any weepers now, not after the plasterer has been.

The PTFE and jointing compound debate

This topic generates more argument in plumbing forums than almost anything else. Here's the definitive position.

PTFE tape does not go on the olive. Manufacturer guidance from Flowflex (a major UK compression fitting manufacturer) is explicit: wrapping PTFE around the olive interrupts the metal-to-metal compression that creates the seal. It can actually cause leaks. PTFE on the threads of the nut is harmless and acts as a lubricant, making the nut easier to tighten evenly. But it's not doing any sealing there either.

Jointing compound is insurance, not a requirement. A new fitting, new olive, and clean pipe assembled correctly will seal without any compound. Many plumbers apply a thin smear of approved jointing compound (Hawk White for central heating systems, Water Hawk for potable water) to the olive faces as a precaution. On cheaper fittings with looser manufacturing tolerances, this can make the difference between a dry joint and a weeper. A tin lasts dozens of joints and is a cheap investment if you're less experienced.

Copper olives vs brass olives

The olive that comes in the bag with most fittings is brass. It works. But some plumbers swap it for a copper olive on every joint, and there are reasons for that.

Copper is softer than brass. It deforms more readily against the pipe surface, which means it creates a tighter initial seal with less tightening torque. On hot water systems where pipes expand and contract through heating cycles, a copper olive maintains contact better because it yields slightly with the pipe movement.

Brass olives are harder, hold their shape better, and resist corrosion marginally more than copper. They're the default because they're cheaper to manufacture. For most domestic joints on copper pipe, either works.

Two exceptions where the choice matters. First, if you're connecting to plastic pipe (PEX or Speedfit) using a compression fitting with a TSM insert, use a copper olive. Brass is too hard and can crack or deform the plastic pipe wall. Second, if the installation involves LPG gas (not common in extensions, but relevant for some boiler work), regulations require copper olives. Brass must not be used on LPG.

Cost and where to buy

Compression fittings are sold individually and in multi-packs at every plumbing merchant and hardware retailer in the UK. Prices are consistent across the major chains.

For a typical first-fix plumbing job on an extension (hot, cold, and central heating pipework), you'll need a mix of elbows, tees, couplers, and reducers. A rough estimate: 20-40 fittings across all sizes, depending on the complexity of the pipe routes. Buying multi-packs brings the per-fitting cost down noticeably.

The main UK brands are Flomasta (Screwfix's own label), Primaflow (Wickes), and Made4Trade (Toolstation). All are WRAS-approved and comply with BS EN 1254-2. For quality-sensitive joints (boiler connections, exposed runs), plumbers often specify Prestex or Conex Banninger. These are professional-grade fittings with tighter manufacturing tolerances and better olive-to-body fit. They cost marginally more but the consistency means fewer weepers.

Compression vs push-fit vs solder: when to use each

This is the decision most homeowner guides skip, and it's the one that actually matters.

Use compression for boiler connections (manufacturers require it for warranty and serviceability), exposed pipe runs where appearance matters (chrome-plated fittings), and any joint that may need to be disassembled in future. Compression is also the safest option for DIYers since there's no flame involved.

Use push-fit for speed on long runs through joists and stud walls, where you're joining plastic pipe (Speedfit, Hep2O), or in tight spaces where getting two spanners in is difficult. Push-fit costs more per fitting but saves time, and time is what your plumber bills for.

Use solder for concealed pipe runs inside walls and floors where the Water Fittings Regulations require joints that don't need access. Solder fittings are the cheapest per joint but require proper technique and carry a fire risk from the blowtorch. Your plumber will use solder for runs that get buried and compression for everything that stays accessible.

Industry reliability data shows compression joints have a 0.8% failure rate within 12 months versus 1.4% for push-fit. Of the compression failures, 42% are from insufficient tightening and 31% from damaged olives. Both preventable.

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Common mistakes

Over-tightening. This is the most common cause of compression joint leaks, mentioned in 9 out of 11 plumbing forum threads analysed. Cranking the nut as hard as you can doesn't make a tighter seal. It deforms the copper pipe itself, which means the olive can no longer grip a round surface. The pipe wall collapses slightly under the olive, and the joint weeps. Once you've over-tightened, that section of pipe is ruined and must be cut out. A fresh piece of pipe, a new olive, and correct tightening will fix it.

Pipe not fully inserted. If the pipe doesn't reach the internal backstop inside the fitting body, the olive sits in the wrong position. The joint will hold initially but fail under pressure or after thermal cycling. Push the pipe in firmly and check that it's seated before tightening. Mark the insertion depth on the pipe with a felt-tip pen if you want visual confirmation.

Reusing olives. When you take a compression fitting apart, the olive stays on the pipe, deformed to the exact profile of that pipe at that position. Sliding it onto a different pipe, or reassembling the same joint with the same olive after repositioning, will not seal. Always use a new olive. Cut behind the old one with a pipeslice if it won't slide off.

Mixing imperial and metric. Old houses sometimes have imperial copper pipe (commonly 1/2" which is 12.7mm, not 15mm). Fitting a 15mm compression fitting onto imperial pipe leaves enough clearance that the olive can't grip properly. At best you get a slow weep; at worst, the pipe blows out under pressure. If you're connecting into existing pipework in an older property, check the pipe diameter with a vernier calliper before buying fittings. Adaptors exist for imperial-to-metric transitions.

Using standard fittings where DZR is required. If a compression fitting must be installed in a location that's not easily accessible (for instance in a service duct with a removable panel), the Water Fittings Regulations require the fitting to be made from dezincification-resistant (DZR) brass. Standard brass can corrode over time through a process where zinc leaches out of the alloy, leaving a weak, porous copper structure. DZR fittings are marked "CR" (corrosion resistant) or "DZR" and cost slightly more. Your plumber should know this. Check if they don't.

Alternatives

Push-fit fittings (Speedfit, Hep2O, Tectite) do the same job faster and with no tools beyond a pipe cutter. The trade-off is cost per fitting versus labour time. On a job with 40 joints, the speed advantage of push-fit adds up to over an hour of your plumber's labour, which can more than offset the higher material cost. If your plumber charges by the day rather than a fixed price, push-fit on the bulk runs and compression on the boiler connections is a sensible split.

Push-fit has a lower maximum operating temperature (65-95°C depending on brand) which makes it unsuitable for connections directly off a boiler where flow temperatures can exceed 80°C. Push-fit also cannot be used for gas.

For concealed pipework inside walls and solid floors, neither compression nor push-fit is appropriate. Soldered capillary joints are the correct choice for any pipe run that will be permanently inaccessible.

Where you'll need this

Compression fittings appear at two stages of any extension or renovation involving plumbing:

• First fix plumbing - joining copper pipe runs for hot, cold, and central heating circuits; the bulk of compression fittings are used here
• Second fix plumbing - final connections to taps, valves, cylinders, and appliances; adaptor fittings (compression to BSP thread) are common at this stage

Compression fittings are a small part of total project cost but a disproportionate source of callbacks if installed badly. Understanding how they work puts you in a position to check your plumber's work at pressure-test stage, before everything gets boxed in and plastered over.