Copper Compression Fittings: Types, Assembly, Torque Specifications, and Common Failures

Compression fittings are the workhorses of UK plumbing: no heat, no flux, no naked flame — just a nut, an olive, and the correct torque. Done right, a compression joint lasts decades. Done wrong, it drips the moment the system pressurises. This guide covers everything trade plumbers need to know: fitting types, olive selection, assembly torque, dezincification risk, and the most common reasons compression joints leak.

See also: Compression vs Push-Fit vs End-Feed Fittings for a side-by-side comparison of all three jointing methods.

Type A vs Type B Compression Fittings

UK compression fittings fall into two standards:

• Type A (non-manipulative): The most common type in domestic plumbing. The olive compresses against the outside of the pipe as the nut is tightened. No modification to the pipe is needed. Suitable for water supply, central heating, and gas up to 28mm (subject to Gas Safe requirements). Manufactured to BS EN 1254-2.
• Type B (manipulative): The pipe end must be flared or swaged before assembly. The fitting grips the inside of the flared pipe. Used in refrigeration, some gas installations, and specialist applications. Manufactured to BS EN 1254-3. Much less common in domestic work.

For virtually all domestic plumbing and heating, Type A is the correct choice. The term "compression fitting" in a plumbing merchant context almost always means Type A.

Materials: Brass, DZR, and Gunmetal

Standard Brass (CW614N / CZ121)

The majority of compression fittings are made from CW614N leaded brass machined from bar stock. Excellent machinability and good corrosion resistance in neutral water. Not suitable for aggressive water (pH below 7 or high chloride concentrations) without DZR specification.

DZR Brass (Dezincification-Resistant)

In areas with aggressive water — particularly soft-water regions of the UK, coastal areas, and some chalk aquifers — standard brass can suffer dezincification: the zinc leaches out of the alloy, leaving a porous, weak copper sponge. DZR fittings must be used wherever the water authority specifies dezincification-resistant materials, which is a requirement under BS EN 1254 and indirectly under the Water Supply (Water Fittings) Regulations 1999.

DZR fittings are typically marked "DR" or "DZR" and may have a slight colour difference. Common brands include Pegler Yorkshire YorkPress DZR, Conex Bänninger, and Crane. Use DZR in all installations in Wales, South West England, parts of Yorkshire, and Scotland unless local water analysis confirms non-aggressive water.

Gunmetal Fittings

Some flanged and larger fittings are made from gunmetal (bronze) for higher pressure or specialist applications. Rarely needed in domestic work.

Olives: Brass vs Copper

The olive is the compression ring that creates the seal. UK domestic fittings use one of two types:

• Brass olives: Stiffer, less malleable. Standard for most domestic work. Slightly harder to compress evenly on thin-wall pipe.
• Copper olives: Softer and more malleable — they conform better to the pipe surface, giving a tighter seal with less risk of cracking or deforming incorrectly. Preferred by many experienced plumbers for general copper tube work, and often used where joints must be re-made (the softer copper is less likely to score the pipe).

Either works correctly when fitted properly. Avoid mixing olive types on a re-made joint: the existing indent in the pipe may not match the new olive profile, risking a poor seal.

Pipe Sizes and Thread Sizes

Pipe OD (Copper)	Metric designation	Common nut thread	Typical application
10mm	10mm	M16×1.5	Small bore feeds, boiler connections
15mm	15mm	M20×1.5	Cold water, DHW, CH primary (standard domestic)
22mm	22mm	M26×1.5	Main distribution, boiler flow/return, central heating
28mm	28mm	M32×1.5	Primary mains, boiler primaries, large UFH manifold connections
35mm	35mm	M40×1.5	Commercial runs, rising mains
42mm	42mm	M48×1.5	Commercial, district heating

Note: Imperial copper tube (½", ¾", 1") is largely obsolete in UK new-build work but still found extensively in existing properties. Dedicated imperial compression fittings exist for repair work; metric fittings cannot be used on imperial tube of nominally similar size due to OD differences.

How to Make a Correct Compression Joint

Step 1: Cut the Pipe Square

Use a pipe cutter (rotary wheel cutter) or a junior hacksaw with a mitre box. The pipe end must be perfectly square — any angle concentrates the olive compression and causes a leak path. Deburr the internal bore with a pipe reamer to restore full bore and remove swarf.

See: Bending Copper Pipe: Springs, Bending Machines, and Common Mistakes — pipe preparation also applies when bending copper.

Step 2: Thread the Nut and Olive

Slide the nut (threaded end first) onto the pipe, then slide the olive over the pipe end. Insert the pipe fully into the fitting body — the pipe must bottom out inside the fitting. Check by marking the insertion depth on the pipe with a pencil or marker before assembly.

Step 3: Hand-Tighten

Thread the nut onto the fitting body by hand until it is fully finger-tight. The olive should be sitting correctly against the fitting body at this point.

Step 4: Spanner-Tighten

Hold the fitting body with one spanner and turn the nut with a second spanner. The industry rule of thumb for compression joints on copper:

• 15mm and below: ¾ to 1 full turn past hand-tight
• 22mm: ½ to ¾ turn past hand-tight
• 28mm and above: ½ turn past hand-tight

Overtightening is one of the most common causes of compression joint failure. Excessive force deforms the olive, scores the pipe, and can crack the fitting body (particularly plastic-bodied isolating valves with brass compression ends).

Do not use PTFE tape or jointing compound on compression fittings. The seal is mechanical — olive-to-pipe and olive-to-fitting. Sealants interfere with this and can cause the olive to slip rather than compress correctly.

Step 5: Pressure Test

Pressurise the system and check for weeps at the nut. A small drip immediately after pressurising can often be stopped by a quarter-turn tightening. If a joint weeps significantly at normal working pressure, depressurise and remake it — do not overtighten a weeping joint.

See: Pressure Testing Plumbing and Heating Pipework for full test procedures to BS EN 806-4 and BS 6891.

Compression Fittings on Plastic Pipe

Compression fittings are not just for copper. They are widely used on:

• MDPE (blue water service pipe): Use fittings rated for MDPE — typically push-fit or dedicated MDPE compression types. Insert supports (pipe stiffeners) are mandatory to prevent the fitting from crushing the softer plastic. See: MDPE Blue Water Pipe Guide.
• Speedfit (JG / Hep2O polybutylene): These systems use their own dedicated compression fittings, not standard brass compression. See: Push-Fit Plumbing: JG Speedfit and Hep2O.
• CPVC and PVC-C: Rare in UK domestic but found in some industrial applications. Specialist fittings required.

Critical rule: Never use a standard copper compression fitting on plastic pipe without an insert support. The olive will compress the pipe wall inward, permanently deforming it and creating an unreliable seal.

Compression Fittings on Gas

Compression fittings are approved for use in domestic gas installations up to 28mm under IGEM/UP/2 (Edition 3) and BS 6891, subject to the following conditions:

• All gas compression joints must be accessible — buried or concealed gas compression fittings are not permitted
• Joints must be made with approved fittings manufactured to BS EN 1254-2
• Gas Safe Registered engineers must use fittings specified in IGEM/UP/2 and must not use push-fit or solder-ring fittings for gas (end-feed solder joints are acceptable but not preferred for gas in most cases)
• Tightness test must be performed to BS 6891 after any gas compression joint is made

See: Domestic Gas Pipework — Approved Materials, Sizing, and Tightness Testing for full gas pipework requirements.

Common Causes of Compression Joint Failure

1. Pipe Not Fully Inserted

The single most common cause of leaks. If the pipe does not bottom out in the fitting body, the olive sits in the wrong position and cannot compress correctly. Always mark the insertion depth and verify before tightening.

2. Overtightening

More is not better with compression. Over-torquing deforms the olive (particularly soft copper olives), scores the pipe surface, and can split the fitting body. It also makes future dismantling extremely difficult or impossible without cutting the pipe.

3. Olive Scored or Deformed on Reassembly

When a compression joint is remade, the existing olive indent in the copper pipe must be aligned precisely with the new or reused olive. If the olive has been pushed back along the pipe, cut the pipe back past the old olive mark and start fresh. Never reuse a deformed olive.

4. Pipe End Not Square

An angled cut creates an uneven olive contact. Use a proper rotary pipe cutter rather than a hacksaw for critical joints.

5. Dezincification of Fitting Body

In aggressive-water areas, standard brass fittings can dezincify over years, causing the nut threads to fail or the body to become porous. Retrospective dezincification is not repairable — replace with DZR or appropriate alternative.

6. Vibration Loosening

In plant rooms or near circulators, vibration can gradually loosen compression nuts. Use locking compounds sparingly (not thread sealants), or prefer push-fit or solder fittings in high-vibration zones.

7. Frost Damage

A burst pipe from freezing can deform the olive impression. Cut the affected section back and fit new pipe and fittings — do not simply re-compress an olive that has been through a freeze-thaw cycle.

Compression Fittings vs Other Jointing Methods

Method	Speed	Tool requirement	Remakeable?	Best for
Compression (Type A)	Fast	Two spanners	Yes (within limits)	Repairs, tight spaces, no heat zones, gas
Push-fit (Speedfit/Hep2O)	Fastest	Pipe cutter only	Yes	New install, concealed, domestic CH/CW
End-feed solder	Medium	Blowtorch, flux, solder	No (require cutting)	New pipework, visible runs, cost-effective bulk
Press-fit (M-Press, ProPress)	Fast	Press tool (hire/buy)	No	Commercial, high-volume, no-flame zones

See: Copper Press-Fit Fittings: M-Press, ProPress, and Mapress Guide for press-to-connect alternatives.

Working in Confined Spaces

Compression fittings are often chosen because they require no heat source — critical when working near:

• Existing lagging or insulation that could catch fire
• Plastic pipes running nearby
• Timber joists and noggins where fire risk is elevated
• Properties where fire precautions prohibit naked flames (e.g., commercial premises, healthcare facilities)
• Riser cupboards and plant rooms with poor ventilation

In these situations, compression fittings or press-fit are the correct professional choice, even if the rest of the system is soldered.

Approved Fittings for Water Regulations Compliance

All fittings used in domestic water supply must comply with the Water Supply (Water Fittings) Regulations 1999. Compression fittings must:

• Be WRAS-approved (Water Regulations Advisory Scheme)
• Carry appropriate markings (CE, WRAS stamp, BS EN 1254 reference)
• Be used within their specified pressure and temperature ratings

WRAS approval is verified on the WRAS Product Approval Search. Common approved brands include Pegler Yorkshire, Conex Bänninger, Crane Fluid Systems, and John Guest (for plastic). Own-label fittings from reputable merchants are generally WRAS-approved, but verify before use in notifiable works.

Compression Fittings in Heating Systems

Compression fittings in central heating circuits work reliably at normal working pressures (1–3 bar) and temperatures (up to 85°C flow). At higher temperatures, thermal cycling causes expansion and contraction that can gradually loosen nuts — this is more pronounced on 28mm and larger fittings. For high-temperature primary circuits (e.g., above 80°C), consider end-feed solder or press-fit for permanence.

See: Sealed Heating System Pressure: Gauges, Filling Loops, and Repressurisation for working pressure context.

Compression Fittings for Isolation Valves and Service Valves

Many isolation valves — both ¼-turn ball type and slotted-head service valves — use compression ends, making them quick to install without solder and removable for maintenance. Key installation notes:

• Do not overtighten the compression end of a plastic-bodied valve (e.g., a Pegler 751 or Hep2O isolator) — the brass insert can pull away from the plastic body
• Ensure the valve is installed in the correct flow direction if it is a directional type
• Mark installed isolation valves clearly — closed position (90° to pipe) must be identifiable even years later

See: Isolation Valves and Service Valves for a full guide to valve selection.

Key Standards and References

• BS EN 1254-2: Fittings for copper and copper alloy tubes — compression fittings for use with copper tubes (Type A)
• BS EN 1254-3: Compression fittings for use with plastic pipe (Type B)
• Water Supply (Water Fittings) Regulations 1999: Compliance requirement for all fittings in domestic water supply
• IGEM/UP/2 Edition 3: Domestic gas installation specifications — includes approved fitting types for gas compression joints
• BS 6891: Specification for installation of low-pressure gas pipework — tightness test requirements
• WRAS: Water Regulations Advisory Scheme — approval database at wras.co.uk

Summary: Getting Compression Joints Right

A correctly made compression joint is reliable, remakeable, and code-compliant. The rules are simple:

1. Cut square and deburr
2. Insert pipe fully — mark depth before assembly
3. Use the correct fitting for the water type (DZR in aggressive water areas)
4. Tighten to the correct amount — not by feel, but by turn count
5. Never over-tighten — it does not make joints tighter; it makes them weaker
6. Pressure test before concealing
7. Never use thread sealants or PTFE on compression ends

When in doubt, check your local water supplier's specification for dezincification risk — it takes minutes to verify and avoids a callback years later.

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