Water pipe: which plastic pipe goes where

What water pipe is made of

A water pipe carries water under pressure from a main, a tank or a pump to the taps of a building. Plastic water pipe comes in three families: unplasticised PVC (PVC-U), polypropylene (PP, sold as PPR) and polyethylene (PE). Each is made to an international standard that says what it may carry and at what temperature, and that scope is the quickest way to choose between them.

Waste and soil pipes are covered on the drainage pipes page, and the trap under each appliance on the P traps and bottle traps page.

Three plastic water pipes compared

Three plastic water pipes compared

Material	Standard	What the standard covers	Temperature
PVC-U	ISO 1452	water supply under pressure; drainage and sewerage under pressure	cold water up to 25°C; up to 45°C with reduced pressure
PP (PPR)	ISO 15874	hot and cold water installations within buildings, drinking water or not, and heating	set by class of application
PE	ISO 4427	mains and service pipes, buried or above ground, for drinking and raw water and pressure sewers	rated at 20°C; other temperatures by annex

Sources: BSI’s summary of BS EN ISO 1452-1:2009, the AFNOR summary of ISO 15874-1:2013 and the preview of ISO 4427-1:2019. Rigid PVC pipes and fittings and PPR pipes and fittings have pages of their own.

How water pipe is sized

Plastic water pipe is named by its outside diameter in millimetres, not by the bore: ISO 4427 defines the nominal outside diameter, dn, in millimetres. The wall is given through the standard dimension ratio, SDR, which the same standard defines as the outside diameter divided by the nominal wall thickness. A lower SDR means a thicker wall for the same diameter. A 32 mm pipe at SDR 11, for example, has a wall of about 2.9 mm (our calculation: 32 ÷ 11).

The pressure the pipe may carry is marked as well. ISO 4427 covers PE systems with a maximum allowable operating pressure of up to 25 bar at its 20°C reference temperature. Read the SDR and the pressure class on the printed line along the pipe before buying.

Hot water

Temperature rules out some water pipe straight away. BS EN ISO 1452-1 limits PVC-U pressure pipe to 25°C at full rating, allows it up to 45°C only with the reduced pressures set in part 2 of the standard, and leaves anything hotter to agreement between maker and user. A line from a water heater or a solar tank therefore needs a pipe made for hot water, such as polypropylene to ISO 15874, whose pressures and temperatures are set by class of application.

Joining water pipe

Each material has its own joint. PVC-U pressure pipe is glued in the small sizes and pushed into rubber-ring sockets in the large ones: Plasco, a Dar es Salaam pipe maker, uses solvent cement for pipes from 20 mm to 75 mm and a rubber ring from 75 mm up. The cement and the steps of a glued joint are on the PVC solvent cement page. Polypropylene and polyethylene are joined by their own systems of fittings, described by their makers.

What to check when you buy

• Material and standard. The printed line on the pipe should name the material and its standard: ISO 1452 or EN 1452 for PVC-U, ISO 15874 for PP, ISO 4427 for PE.
• Use. Cold or hot water, inside the building or in the ground.
• Outside diameter. In millimetres, matching the fittings.
• SDR and pressure. The SDR and pressure class for the pressure in your system.
• Temperature. For hot water, a pipe whose standard covers hot water.