How do I tell what my water pipes are made of and what size they are?
Copper, plastic, steel and lead all look different once you know what to look for, and pipe size in the UK is measured in a way that catches people out.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
Copper is warm-toned and rings when tapped. Plastic is grey, white or blue and dulls a tap. Old galvanised steel is grey, magnetic and rusty at the threads. Lead is dull grey, very soft, scratches bright silver with a coin and is not magnetic. UK copper is sized by outside diameter — 15mm and 22mm are the common domestic sizes.
Every plumbing purchase, from a compression fitting to a new tap, asks what your pipe is and what size. Getting it wrong means a second trip and sometimes a joint that will not seal. There is also a good reason to look beyond the shopping list: identifying lead pipework in a house matters for health, and it is easy to spot once you know what you are looking at.
Safety
Step by step
- Identify copper.The commonest material in UK domestic plumbing. Warm reddish-brown when clean, dulling to brown or green with age. Rigid, rings clearly when tapped with a coin, not magnetic. Joined by soldered fittings, compression fittings with an olive, or push-fit.
- Identify plastic.Grey, white or occasionally blue. Slightly flexible, dull and dead when tapped, warm to the touch compared with copper carrying the same water. Joined by push-fit or crimped fittings and usually with a stiffening insert pushed into the pipe end. Common in newer houses and rewired sections.
- Identify galvanised steel.Older installations. Grey, dull, heavy, strongly magnetic, and joined with screwed threaded fittings rather than compression or solder. Usually rusty at the threads. Increasingly rare and generally worth replacing where it is found, as the bore scales and corrodes.
- Identify lead.Dull grey, thick-walled and noticeably soft. Scratch it lightly with a coin and it shows bright silver underneath. Not magnetic. Joints are lumpy wiped joints rather than fittings, and the pipe often has gentle bends rather than elbows because it was bent by hand. Most likely on the incoming supply in houses built before the 1970s.
- Deal with lead correctly.Do not work on it. Run the tap for a while before drinking after the water has stood, particularly first thing in the morning. Contact your water company: they will usually test your water for lead free, and many operate schemes to replace their side of a lead supply pipe when you replace yours.
- Measure copper by outside diameter.UK copper is sized by outside diameter in millimetres. 15mm serves individual outlets, 22mm serves main runs and baths, 28mm appears on larger heating circuits and supply pipes. Measure across the outside of the pipe, not the inside.
- Watch for imperial pipe.Houses plumbed before metrication have imperial copper: half inch, three quarter inch and one inch, which are close to but not the same as 15mm, 22mm and 28mm. Metric compression fittings will not seal properly on imperial pipe. Imperial-to-metric adaptors and imperial olives exist and are what you need — take a sample or a measurement to the merchant.
- Take a photograph and a measurement to the shop.A photo of the existing joint and a measurement across the pipe with a vernier or a ruler solves almost every buying problem. Guessing between 15mm and half inch is the classic wasted trip.
Tips
- Keep a fridge magnet and a coin with the plumbing bits. Between them they identify almost every pipe material in thirty seconds.
- A vernier caliper is inexpensive and settles the 15mm versus half inch question definitively. A ruler across the outside will usually do.
- Where copper and plastic meet, check what fittings the installer used. Mixed systems are entirely normal but each joint has its own requirements.
- If you find lead, it is worth knowing that the pipe from the boundary is yours and the communication pipe is the water company's — replacing both together is the sensible approach.
If it doesn't work
New compression fitting will not seal however hard it is tightened
Cause: Imperial pipe with a metric olive, or the wrong olive material — Fix: Measure the pipe accurately. Imperial half inch is slightly larger than 15mm. Use an imperial olive or an adaptor rather than tightening harder, which just deforms the fitting.
Push-fit joint blows off or weeps
Cause: No stiffening insert in a plastic pipe, or the pipe end was not cut square and deburred — Fix: Every plastic pipe in a push-fit joint needs the correct insert. Cut square, deburr, insert, and push fully home.
Pipe is magnetic
Cause: Galvanised steel, or a steel component in the run — Fix: Copper, plastic and lead are all non-magnetic. A magnet is the fastest single test there is.
Grey pipe that scratches bright and is very soft
Cause: Lead — Fix: Do not cut, file or heat it. Contact the water company about testing and about their lead replacement scheme.
Pipe under the floor is a different size from the visible one
Cause: The house has been partly replumbed at some point — Fix: Common. Assume nothing from one visible length — check at the point you are actually working.
Questions people ask
Is lead pipework dangerous?
Lead dissolves slowly into water that has stood in the pipe, and there is no safe level for children. It is a genuine reason to replace lead supply pipework, and water companies will test your water for it free and advise on their replacement scheme.
Can I join plastic to copper?
Yes, routinely — push-fit fittings are designed to accept both, provided a stiffening insert is used in the plastic side. Compression fittings can also be used with the correct insert.
Why does pipe size matter so much?
Because flow rate depends heavily on bore. A 15mm run feeding a bath and two other outlets is a bottleneck that no fitting will fix. Identifying what size runs where is the first step in diagnosing most flow problems.