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The art or science of conveying stuff around using pipes (also known as tubing) and fittings.

The word can be used as a noun referring to installed pipework and fittings or as a verb referring to the activity of working on pipework and fittings.

The name derives from the Latin plumbum through OFr plomb meaning lead [ref SOED]. Nowadays lead is almost completely absent from pipework, although part of the traditional plumbing craft training is sheet leadwork.

In general plumbing refers to

  • hot and cold water supply
  • waste and drainage
  • wet central heating
  • gas fitting

Some of these activities have more specialised names such as gas fitting and heating. Various regulations apply to these activities.

Plumbing materials and techniques are also used in

  • refrigeration and air-conditioning
  • compressed air systems
  • industrial process systems

although these activities are usually referred to by more specialised names.

Concepts: Pressure and Flow

The terms Pressure and Flow are sometimes confused or used interchangeably. However they refer to quite separate (though connected) concepts:


Flow is the rate at which water (or any other fluid) travels. A familiar example of flow is a river where one can see thousands or millions of gallons of water flowing. Common measures of flow in domestic plumbing are litres per second, litres per minute and gallons per minute. Flow of gas is also measured in cubic feet or cubic metres, per minute or per hour.


Pressure is the force exerted by a fluid on whatever it is contained in. This can be experienced by attempting to stop the flow of water from the cold tap of a kitchen sink: one has to exert a lot of force to prevent the water flowing, and if one fails to stop it the water squirts out with considerable energy. By contrast it is usually easy to stop the flow of hot water from a bath tap upstairs.

In plumbing pressure is measured in bar, pounds per square inch (psi) or as "head" of water expressed in distance e.g. feet of water. The latter indicates how high a body of water would be to exert the measured pressure at its base.

For natural gas and LPG, pressures are usually expressed in millibars or inches of water (or water-gauge).

Note that the pressure at the base of a vessel of water does not depend on the shape of the vessel: the pressure at the bottom of a thin column, a thick column or any sort of unevenly-shaped vessel of water is the same. The reason water storage towers are often Y-shaped rather than cylindrical columns has to do with their structural strength and rate at which the pressure they sustain drops as water is drawn from them (before it can be replenished).


Flow and Pressure are analogous to Current and Voltage in Electricity, and they are similarly related by the resistance of the system in which they exist. The flow through a plumbing system increases with the applied pressure and the ratio of pressure to flow is determined by the resistance of the system. However there are no equivalents to discrete electrical "Resistors" in plumbing: pipework and appliances have resistances which are quite non-linear so they typically behave as low resistances at low flow rates and increase resistance quite dramatically as flow rates increase. Thus there is not a plumbing (or hydrodynamic) equivalent of Ohm's Law: instead tables or graphs of pressure (or head) versus flow must be consulted to determine how a system will operate.


Pipe (also known as 'tube') for plumbing is manufactured in various different materials and to different sizes. Currently sizes are specified in metric measurements representing the nominal outside diameter of the pipe. In UK domestic plumbing the sizes commonly found are 8mm, 10mm, 15mm, 22mm, 28mm, 32mm, 40mm, 50mm, 68mm and 110mm; with sizes up to 28mm being used for hot and cold water supply, heating and gas supply, and larger sizes for waste, rainwater, soil and ventilation. Older installations (generally before the early/mid 1970s) used pipework in Imperial sizes such as ½“, ¾“, 1", 1¼“, 1½“, 2", 3" and 4", where the size referred to the nominal internal diameter of the pipe.


  • Copper pipes are joined by soldering, using a compression fitting, or pushfit connectors. Some fittings for soldering come ready loaded with solder these are known as Yorkshire or Solder ring fittings.
  • Chromium plated copper pipe can connected either by removing the chromium and then soldering or by using compression fittings which are also sold chrome plated.
  • When copper comes into contact with damp cement, concrete or mortar it may be liable to corrode. This would seem to be related to the composition of the concrete etc: the Copper Development Association in an article "Is There A Problem With Embedding Copper Tube In Concrete?" states:

According to the Portland Cement Association the interaction of copper with both dry and wet concrete should not cause a corrosion concern. However, copper should be protected when it comes in contact with concrete mixtures that contain components high in sulfur, such as cinders and fly-ash, which can create an acid that is highly corrosive to most metals including copper.

For this reason it is good practice to wrap the copper in PVC or grease impregnated (aka "Denso") tape. Specific regulations apply to copper pipe used for carrying gas under the Gas Safety (Installation and Use) Regulations (1998), and pipes buried in floor screeds.

  • Long runs of copper pipe in concrete are liable to break due to differential thermal expansion.
  • When cemented or plastered over, "Denso" tape or other flexible wrapping should be used to avoid expansion stresses.

Biological Effects

  • A claimed benefit of the use of copper in Domestic Hot Water (and possibly also Central Heating) systems is that it has antimicrobial effects against, amongst others, the organism that causes Legionella
  • Fishtanks should not be filled with water from newly fitted copper piping, as levels of copper in the water from new piping are generally high enough to kill fish.



Medium Density PolyEthylene is coloured blue for water services or yellow for gas. Black MDPE is also found in older cold water service installations. It is available in inside diameters of 20, 25, 32mm and larger sizes. It is intended to be buried and should not be used for permanent installations where it is exposed to sunlight as UV will degrade the material (although blue MDPE is often run on the surface for temporary installations on building sites etc.) It is usually used for incoming services. It is not suitable for hot water. It is invariably supplied in rolls of various lengths.

PEX and PB

cross-linked polyetheylene (PEX) and Polybutylene (PB) are made to the same outside diameters as copper pipes (but have a smaller inside diameters). It is commonly available in diameters of 15mm and 22mm, with 10mm and 28mm also found.

PB is less springy than PEX, though some makes are more flexible than others.

It can be used for hot and hold water services including central heating.

Barrier pipes have a metallic film sandwiched between inner and outer PB or PEX layers to reduce permeability of the pipe to gases, especially oxygen which is responsible for corrosion in heating systems. See Central Heating for further discussion.

They are usually supplied in a coil but some can be bought in straight lengths.

MDPE, PEX and PB pipes require the use of the correct support stiffeners (some are stainless steel others are plastic) at every end. These keep the pipe from pulling out of fittings.


Acetyl Butyl Styrene and Poly Vinyl Chloride pipes are used in 'solvent weld' systems for overflow, waste and drainage pipes. Waste sizes are nominally 32mm, 40mm and 50mm but are actually larger than nominal and not interchangeable with push-fit (although compression fittings can be used with either type). British Standard (BS) 5255 covers solvent weld waste pipe systems and this standard number printed or impressed on pipe and fittings can be used to distinguish them from push-fit systems.

ABS and PVC are also used for rainwater and soil, vent and drain pipes. Soil, vent and drain sizes are interchangeable with push-fit pipe. Overflow (21.5mm nominal) is sometimes found in slightly larger sizes in old pipework which can make extension of existing overflow systems with new materials difficult.


Polypropylene is not solvent-weldable. It us used in overflow, waste and soil, vent and drainage pipework. PP waste pipes are made to actual sizes of 32mm, 40mm and 50mm, to be joined by push-fit or compression fittings.


  • These are used for compressed air and still used for gas supply pipes, and for heating pipework in larger commercial and industrial systems. They are usually only found in wet pipework in older domestic installations.
  • They are installed by cutting stock tube to length and then threading the ends with a die.
  • A selection of fittings are available for joining the threaded ends together.
  • The threads conform to an international standard that is a metric adoption of the imperial British Standard Pipe.
  • The letter R designates this so R0.75 means a 3/4" BSP thread.
  • The threads can be cut onto a slightly tapered pipe end or parallel.
  • Cast Iron is used for older waste and drainage pipes.

Stainless Steel

This is found in various forms:

  • As a direct substitute for copper pipe (but which can only be joined with compression fittings). This is found in some domestic installations dating from the time of a copper shortage during the late 1970s.
  • Equivalent to mild steel pipe. Used when greater strength or corrosion resistance than steel is required. Used when food grade hygiene standards are needed.
  • As corrugated semi-rigid (bendable) pipes for gas:
    • Short bare stainless steel pipes with pre-attached terminations (known as "Anacondas") for connecting gas meters to incoming gas mains.
    • Continuous roll pipe with a yellow polyethylene sheath (known as "TracPipe") which requires special fittings to join and terminate.

Copper plated Steel

Formerly used as copper pipe substitute in the 70s. The finish is copper but the pipe is magnetic.


Lead piping is no longer installed although some is still found in service. It is found particularly in water pipes in pre-1960 houses, often in the pipe from the street to the main indoor stopcock, and sometimes in tails to taps and WCs etc. It is also sometimes found in connections into and/or out of gas meters.


Water running in lead pipe can leach the metal producing significant levels of lead in the water, causing a heaalth hazard to those ingesting the water (through drinking it or using it for cooking). Naturally occurring chemicals in some sources of water can exacerbate this problem. It used to be common wisdom to run the water for a minute or so first thing in the morning after water had been standing in the pipes overnight, before filling a kettle (or otherwise drawing the water for drinking).

In hard water areas however scale usually forms inside the pipes and prevents water contacting the lead directly, reducing leaching to low and generally not problematic levels.



Used for joining copper pipe (including brass fittings).

Only lead free solder may be used for potable water (including hot water). Solders containing lead may be used for central heating and gas pipework.


These are joined by heating the cleaned, fluxed and assembled pipe and fitting and, when the metal is hot enough, applying solder which melts and is drawn into the joints between pipe and fitting by capillary action.


Also known as "Yorkshire" fittings. These fittings are similar to end-feed but contain a ring of solder which melts when the joint is heated. They are easier to use in awkward locations where it can be difficult to spare an extra hand to apply solder to the joint. However they are also more expensive. Small commonly used fittings are relatively cheap but larger and non-standard fittings can cost many pounds each.


These are practically universal fittings in that they can be used to join copper, chromed and stainless steel pipes as well as plastics. They should only be used in "accessible" locations - which may include underneath floors where the (historic) assumption is that the floor covering can be lifted (although the current popularity of laminate and tiled finishes suggests that this assumption is becoming unrealistic).


There are many systems of push-fit fittings for supply pipework sizes (10mm, 15mm, 22mm and 28mm). Fittings usually contain (amongst other components) an EPDM (synthetic rubber) O-ring which seals the pipe to the fitting and a device with metal teeth which grips the pipe and holds it physically in the fitting.

In general the manufacturers state that their fittings should be used only with their own makes of plastic pipe or with copper tube, and that to join their pipe to other manufacturers' pipe only compression fittings should be used. In practice fittings and pipes are often mixed between different manufacturers' systems.

Demountable fittings

  • Some push-fit fittings (e.g. Hep2O slimline) cannot be removed from pipework and reused.
  • Some fittings can be removed from pipework but usually require replacement of a metal grab ring for re-use.
  • Some fittings can readily be removed and re-used.
    • Some (e.g. Equator, Cuprofit) require used of a tool to demount
    • Others (e.g. Speedfit) can be removed by hand.

For low-pressure larger-bore pipework used for waste and soil, push-fit fittings have a rubber sealing ring which also holds grips the pipe reasonably firmly, although any significant pressure in the pipework can force the joint apart. For this reason pipework should be supported to prevent joints coming apart if the system is being cleaned with a plunger or other pressure-applying device.


PVC and ABS pipe used in overflow, waste and soil systems are joined by applying a solvent gel to pipe and fitting which dissolves the surfaces of the plastics and dries leaving the two parts fused together.


Iron and steel pipe with threaded ends are joined by threaded fittings. These may be brass, iron, steel or stainless steel. Threads may be straight or tapered.

External links

  • Hep2O Installer Guide paper from Hepworth covering design and installation of domestic water supply and heating. Much of this paper is applicable to plumbing systems generally, and to plastic pipework systems beside the company's own "Hep2O".