Summary

Capillary soldering is the most widely used method for permanently joining copper pipes and fittings in UK domestic and commercial plumbing. The technique relies on capillary action — when the fitting is heated to the correct temperature, molten solder is drawn into the narrow annular space between the pipe and fitting, creating a watertight seal as it cools.

There are two types of capillary fitting: end-feed fittings, which require solder to be fed in by the operative, and integral solder ring (Yorkshire) fittings, which contain a pre-measured ring of solder inside the fitting socket. Both achieve the same result but require slightly different technique.

Correct preparation is everything. The most common cause of a failed joint is contamination — oil, grease, oxide layer, moisture — preventing the solder from bonding to the copper surface. The second most common cause is overheating, which burns off the flux before the solder flows and can cause a porous, leaking joint.

Key Facts

  • Water Fittings Regulations 1999 — all solder used on potable water systems must be lead-free; lead solder is prohibited
  • BS EN 29454 — specification for flux for soft soldering; non-corrosive Type 1.1.2 required for plumbing
  • End-feed fittings — require solder fed externally; give control over solder quantity but require more skill
  • Solder-ring (Yorkshire) fittings — contain pre-loaded solder ring; more consistent, faster for large volumes
  • Pipe preparation — outside of pipe must be cleaned to bright copper with wire wool or emery cloth
  • Deburring — cut ends must be deburred with a reamer or file to remove the internal burr left by pipe cutters
  • Flux application — apply a thin, even coat to the outside of the pipe only; excess flux drips, burns, and causes staining
  • Heat sequence — heat the body of the fitting, not the pipe; the fitting conducts heat to draw solder from the pipe end
  • Lead-free solder — typically 97% tin, 3% copper; slightly higher melting point than leaded solder (solidus approx 227°C)
  • Capillary gap — ideal gap is 0.05–0.2mm; too large a gap prevents reliable solder draw
  • Joint testing — pressure test to 1.5× working pressure before covering; visual inspection alone is insufficient
  • Dissimilar metals — use DZR brass fittings or dielectric unions where copper meets steel to prevent galvanic corrosion
  • Pipe cutters vs hacksaw — pipe cutters give a square, clean cut with minimal swarf
  • Cooling — do not quench with water; allow joints to cool naturally before applying pressure

Quick Reference Table

Need to quote a plumbing job? squote generates accurate quotes from a voice recording.

Try squote free →
Joint Type Flux Required Solder Applied By Relative Cost Skill Level
End-feed fitting Yes Operative externally Lower Higher
Integral solder-ring Yes Pre-loaded in fitting Higher Lower
Push-fit (not soldered) No N/A Moderate Minimal
Compression fitting No N/A Moderate Low
Step Action Common Mistake
1 Cut pipe square Using worn cutter producing oval cut
2 Deburr pipe end internally Skipping — causes turbulence and traps debris
3 Clean pipe OD with wire wool Insufficient — oxide layer remains
4 Clean fitting socket interior Skipping — poor solder adhesion
5 Apply thin flux coat to pipe OD Too much flux — burns and causes voids
6 Push fitting firmly onto pipe Not bottomed out — excessive gap
7 Heat fitting body evenly, not pipe Heating pipe only — solder won't draw in
8 Apply solder (end-feed) or watch ring emerge Overheating — flux burns off first
9 Remove heat when ring visible Continuing heat — porous joint
10 Allow to cool naturally Quenching — can crack solder seal

Detailed Guidance

Pipe Preparation

The gap between the pipe and the fitting socket is typically 0.05–0.2mm. Solder is drawn into this gap by capillary action when the temperature is correct. Any contamination — grease, oxide, or moisture — will prevent bonding and create a void.

Start by cutting the pipe square. A rotary pipe cutter gives the cleanest result; if using a hacksaw, ensure the cut is truly perpendicular and remove all swarf. After cutting, ream the inside of the pipe with the built-in reamer on the cutter, or use a half-round file. The internal burr left by a pipe cutter is small but significant — it causes turbulence at fittings and can trap debris.

Clean the outside of the pipe for approximately 20mm back from the cut end using fine wire wool or emery cloth. Work around the circumference until the copper is uniformly bright — you are removing the thin oxide layer that forms on copper. Clean the inside of the fitting socket in the same way. Do this immediately before assembly; do not leave cleaned pipe sitting in damp or dusty conditions.

Flux Application

Flux serves two functions: it prevents re-oxidation of the copper surface during heating, and it promotes wetting — the ability of molten solder to flow and bond across the copper surface. Without flux, even perfectly clean copper will develop an oxide layer the moment it is heated, and solder will ball up and roll off.

Use a non-corrosive flux conforming to BS EN 29454 Type 1.1.2. Corrosive fluxes (Type 1.1.1 or 1.2.x) can cause long-term corrosion if flux residue is not completely removed — not practical in plumbing.

Apply a thin, uniform coat of flux paste to the outside of the pipe only. You do not need to flux the inside of the fitting socket. Use a small brush or gloved finger. More flux is not better — excess flux will burn during heating, leaving a black residue that can cause joint failure, and it drips onto other surfaces causing staining.

Push the fitting firmly onto the pipe until it bottoms out. Twist slightly to distribute the flux evenly.

Heat Technique

Hold the flame so it plays across the body of the fitting, not the pipe. The fitting has more thermal mass and needs to come up to temperature first; the pipe will heat via conduction. Move the flame constantly — playing it in one spot will overheat that area while leaving others cold.

For an end-feed fitting, watch the flux begin to bubble and turn a clear golden colour — this indicates the temperature is approaching solder flow point. Touch the solder wire to the gap between pipe and fitting at the socket mouth on the side furthest from the flame. Do not apply solder to the area directly being heated. If the temperature is correct, solder will instantly flow into the socket and a bright ring will appear around the full circumference. For a 15mm fitting, approximately 10–15mm of solder wire per joint is required.

For a solder-ring fitting, watch for the pre-loaded ring to emerge as a bright ring at the socket mouth. Once visible, remove the heat immediately.

Avoid overheating. Overheated flux turns black and becomes an insulating barrier. Overheated solder becomes granular and porous. If you overheat a joint, allow it to cool completely, disassemble, clean everything back to bare copper, and start again.

Lead-Free Solder Considerations

Lead-free solder (typically 97% tin, 3% copper) has slightly different handling characteristics compared to the old tin-lead alloys. It requires marginally higher temperatures (approximately 5–10°C more), is less forgiving of incorrect flux application, and has a slightly narrower plastic range — it transitions from liquid to solid more quickly. Technique must be precise: if you apply lead-free solder when the fitting is too cold, it will freeze on the surface rather than flow in.

The prohibition on lead solder for potable water is absolute under the Water Supply (Water Fittings) Regulations 1999. Lead-free solder should be your default for all plumbing work regardless of the application.

Joint Testing

After making all joints, test before burying in walls or ceilings. For domestic hot and cold systems, a pressure test at 1.5 times the maximum working pressure (typically 1.5 × 3 bar = 4.5 bar) held for 30 minutes is standard practice. Use a hydraulic pressure test pump, not compressed air — air carries more energy than water and a joint failure under compressed air can be dangerous.

Common Mistakes

Joint weeps a small amount: Usually flux contamination or a void. Drain, clean, re-flux, and re-solder. Never attempt to solder a joint with live water in the pipe — even a drip will cause steam and prevent solder bonding.

Solder balls up on the surface: Flux burned off before solder applied, or copper not clean enough. Strip back, clean, re-flux.

Solder flows into pipe bore: Fitting not pushed home, leaving an excessive gap. Capillary action works in the narrow gap — too large a gap allows solder to drip into the bore, restricting flow.

Frequently Asked Questions

Can I use an electric heat gun instead of a blowtorch?

No. A heat gun does not generate sufficient concentrated heat for capillary soldering. You need a propane or MAPP gas blowtorch. MAPP gas burns hotter than propane and is useful in cold conditions or for larger fittings. Electric induction soldering tools are available where open flame is not permitted.

How do I solder a joint that keeps weeping?

Drain the pipe completely — even a drip will prevent a sound joint. Use a bread ball (a piece of white bread) pushed into the pipe to temporarily dam residual water while you make the joint; it will dissolve in the flow when refilled. Alternatively, use a freeze-spray kit to create a temporary ice plug.

Is it still legal to use lead solder on older heating systems?

Lead solder must not be used on any system supplying drinking water, hot water, or water used in food preparation. The Water Supply (Water Fittings) Regulations 1999 are absolute on this point. For closed heating circuits with no connection to potable water, lead solder is not specifically prohibited, but lead-free solder should be specified as best practice.

Do I need to flux the inside of the fitting socket?

No. Flux on the outside of the pipe is sufficient. The solder, once molten, will carry flux ahead of it as it flows into the socket. Over-fluxing the socket interior tends to leave heavy flux residue in the bore.

What size gap should there be between pipe and fitting?

The ideal capillary gap is 0.05–0.2mm. Good quality copper fittings are manufactured to give this gap when the pipe is within tolerance. If the gap is excessive (worn or undersize pipe), solder will not be drawn in reliably — use a compression fitting instead.

Regulations & Standards

  • Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) — prohibits lead solder on potable water systems; requires WRAS-approved materials

  • BS EN 29454 — specification for fluxes for soft soldering; Type 1.1.2 (non-corrosive) required for plumbing

  • BS EN ISO 9453 — soft solder alloys; composition requirements for lead-free alloys

  • WRAS (Water Regulations Advisory Scheme) — approval scheme for fittings and materials in contact with potable water

  • Water Regulations Advisory Scheme (WRAS) — guidance on approved materials and fittings

  • Copper Development Association — Jointing Copper Tube — technical guides on copper tube jointing methods

  • HSE — Safe Use of LPG at Work — safe use of blowtorches and LPG on site

  • CIPHE Guidance Notes — installation guidance for plumbing and heating

  • outside tap installation — applying soldering technique to outside tap installations

  • pump selection — heating system components requiring soldered connections

  • compression fittings — alternative jointing method requiring no heat