Copper, plastic and MDPE: the water pipe types in UK homes

Open a cupboard or lift a floorboard in a UK home and you might find shiny or tarnished copper, white or grey plastic, a blue plastic pipe coming up through the floor, or in an older house, dull grey lead. Each behaves differently and is joined differently.

This guide is about what the pipe is made of and what it suits. How to join it is a separate subject, covered in our guide to push-fit, compression and soldered joints.

Sizes you will see

UK domestic water and heating pipe is sized by its outside diameter in millimetres:

• 15mm: the most common size, used to most taps, toilets, appliances and radiators;
• 22mm: main runs, bath taps, hot water from the cylinder and heating flow and return near the boiler;
• 10mm and 8mm: microbore, used on some heating systems, usually to individual radiators;
• 28mm and larger: bigger systems and some older homes.

The size matters when you buy fittings. A 15mm fitting will not seal on 22mm pipe. Older homes may have imperial copper pipe, sized in inches, which is close to but not exactly the metric sizes. Some imperial sizes need adaptors or different olives. If a metric fitting does not sit right on an old pipe, stop and check the size.

Copper

Copper has been the standard for indoor water and heating pipe in the UK for decades.

How to recognise it: metal, orange-brown when new, darkening to brown or green with age. It is rigid and rings when tapped.

Good for:

• visible runs, such as boxed pipe or pipe to radiators, where a neat, rigid finish matters;
• hot and cold water and central heating;
• runs near the boiler or cylinder, where the maker's instructions may call for metal pipe;
• outdoors and in places where plastic could be damaged by sunlight or knocks.

Less good for:

• long runs through floors and joists, where it has to be cut into lengths and joined;
• places where frost is likely, as frozen copper splits more readily than flexible pipe;
• DIY repairs if you do not want to use heat. Copper can be joined with compression and push-fit fittings as well as solder.

Copper bends with the right tool, conducts heat well and lasts a long time in most water. In some soft or aggressive water areas it can corrode from the inside, which shows as pinhole leaks.

Plastic: polybutylene and PEX

Plastic plumbing pipe is now very common, especially for new runs under floors.

• Polybutylene (PB) is soft and very flexible. It is usually grey or white. Hep2O is a well-known PB system.
• PEX (cross-linked polyethylene) is a little stiffer than PB. It is usually white. JG Speedfit sells PEX pipe.
• Barrier pipe has an extra layer that limits oxygen getting through the pipe wall. It is the type to use on central heating, where oxygen entering the water speeds up corrosion inside the radiators and boiler. Many plastic pipes sold today are barrier pipe; check the label.

How to recognise it: printed lettering along the length with the brand, size and type, and it is light and flexible.

Good for:

• long runs under floors and through joists, as it comes in coils and bends round corners;
• places where fewer joints are better, because joints are where leaks start;
• reducing the noise of water hammer and some frost damage, compared with rigid pipe.

Less good for:

• visible runs where it can sag or look untidy;
• near heat sources, in direct sunlight, or where it can be crushed or chewed;
• connecting directly to a boiler, where the boiler maker's instructions often say how close plastic may come. The installer must follow them.

Plastic pipe usually needs a pipe insert in the end when joined with compression or most push-fit fittings. Use the inserts and fittings the pipe maker says are compatible. Mixing brands of push-fit fitting and pipe is a common cause of leaks.

Plastic cannot be used for earth bonding the way copper often was. If you replace a length of copper with plastic, an electrician may need to check the earth bonding is still continuous.

Blue MDPE: the incoming supply

The pipe that brings water from the street to your stopcock is usually blue MDPE (medium-density polyethylene) in newer homes, laid underground. In older homes it may be lead, iron or copper.

You normally see it only where it comes up through the floor to the internal stopcock. It is not a pipe for general indoor plumbing. It is joined with its own fittings, and work on the underground supply is usually a job for a plumber or groundworker, sometimes with the water company involved.

Water Regs UK's page on installing pipework says all pipework must be watertight, suitable and appropriate for the environment it is installed in, and protected from freezing and undue warming. For a supply pipe entering a building, it says the pipe should enter at a depth of 750 mm, and that pipework passing through walls or laid shallower than that should be ducted. If you are replacing or moving the incoming supply, read that page and talk to your water company. Our guide to stopcocks and water meters explains where your responsibility starts.

Lead and iron

Grey, soft pipe with bulging joints is probably lead. It should be replaced. Our guide to lead pipe replacement explains why and who is responsible for which part.

Old galvanised iron pipe is rare in homes now but still turns up. It corrodes inside, restricting flow. Replacing it is usually the answer.

Choosing pipe for a job

Ask:

1. What is it carrying? Cold, hot, or heating? For heating, plastic should be barrier pipe.
2. Where will it run? Hidden under floors suits plastic; visible or near the boiler often suits copper.
3. How will it be joined? Make sure the fittings suit the pipe material and size.
4. Is it approved? Water fittings should be suitable for drinking water. Look for evidence of compliance on the packaging.

Threaded fittings such as tap tails and some valves may need sealing. Our guide to thread sealing tape explains where PTFE is and is not used.

For the tools that cut and prepare each material, see our plumbing tool kit. A plastic pipe cutter suits plastic; a rotary pipe cutter suits copper.

This guide covers water pipe only. Any gas work is for a Gas Safe registered engineer; see when to call a Gas Safe engineer.

Example pipe at Wickes

We haven't tested this. It is an example, with what the retailer's page says.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Is copper better than plastic pipe?

Neither is better everywhere. Copper suits visible runs and work near the boiler. Plastic suits long, hidden runs with fewer joints. Many homes use both.

Can I connect plastic pipe to copper?

Yes, with fittings designed for both, such as push-fit or compression fittings rated for the two materials. Plastic pipe usually needs an insert at the joint. Check the fitting and pipe makers' instructions.

What is the blue pipe coming up through my floor?

Usually the MDPE supply pipe bringing mains water from the street to your stopcock. It runs underground and is joined with its own fittings. Work on it is usually for a plumber, and the water company may be involved.