Joining two pipes so they stay tight for decades is the very foundation of the plumbing trade. Here we cover the four major joining techniques — hard soldering and soft soldering of copper, press fittings, threaded joints for steel pipe and welding — plus material selection against galvanic corrosion and temperature/pressure, and the safety rules that come with hot work.
Introduction
No matter how well a system is designed, it stands or falls with its connections. A leaky solder joint or a poorly cut threaded connection costs more than all the other work combined once the wall is closed. Therefore, pipe assembly is what an apprentice practices first and a journeyman never stops refining. The four dominant techniques are soldering, pressing, threading and welding — each with its own material, its own tools and its own pitfalls.
Material choice — what determines pipe type
Before you choose a joining method, you choose a material. Copper is the classic for heating and domestic water; stainless steel and chrome-plated steel are used for pressure systems; galvanised and black steel for threads and heating installations; PEX, PE-RT and PP for distribution pipes and underfloor heating. The choice is governed by the medium (drinking water, heating, gas, drainage), temperature, pressure, corrosion conditions and whether the pipe is concealed or exposed.
- 01Copper (DS/EN 1057) — hot water and heating; soldered or pressed; tolerates high temperature but is sensitive to erosion corrosion at high water velocity (see "Water velocity in pipes").
- 02Stainless / chrome-plated steel — press systems for utilities and heating; corrosion-resistant and hygienic.
- 03Galvanised and black steel (DS/EN 10255) — threaded pipes for heat and gas; black steel must not be used for drinking water.
- 04PEX/PE-RT/PP — flexible plastic pipes for distribution and underfloor heating; joined with press, sliding coupling or welding; limited by pressure/temperature class.
Soldering of copper — soft and hard
Soldering joins copper pipes by capillary action: the molten solder is drawn into the narrow gap between pipe and fitting and solidifies to form a tight, strong joint. The boundary between the two types is at the solder's melting point: soft soldering occurs below 450 °C, hard soldering (also called brazing) above 450 °C. The definition follows EN ISO 857-2.
| Property | Soft soldering | Hard soldering (brazing) |
|---|---|---|
| Temperature | under 450 °C | over 450 °C (ofte 600–750 °C) |
| Solder metal | lead-free tin alloy (e.g. tin-copper, tin-silver) | copper-phosphor (CuP) or silver-bearing solder |
| Strength | lower — to smaller dimensions and lower pressure | high — also larger dimensions and higher loads |
| Soldering flux | always necessary | CuP on copper-copper can do without flux; on brass/bronze flux + silver solder is required |
| Typical use | service water in smaller copper dimensions | heat, larger dimensions, mechanically exposed joints |
Regardless of type, the preparation is the same and critical: the pipe end is cut square (with a pipe cutter, not a hacksaw, to avoid burrs), deburred inside and outside, and both pipe and fitting are polished bright with steel wool or emery cloth. Flux is applied in an even layer, pipe and fitting are assembled, and heat is applied evenly all around until the solder flows into the joint of its own accord. Excess flux is wiped off afterwards — flux residue is acidic and can corrode the copper.
- 01Lead-free solder is a legal requirement for drinking water — never use old lead-containing solder on utilities.
- 02Too much heat burns off the flux before the solder melts, and the joint becomes leaky — heat controllably.
- 03The solder must be drawn in by capillary action, not applied from outside like a weld.
- 04When soldering close to valves with rubber/seals: remove the contents or cool, so heat doesn't damage them.
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