How do I fit an isolating valve?

Isolating valves are the cheapest plumbing improvement you can make, because every future repair on that fitting becomes a ten-minute job instead of a half-day with the whole house's water off.

Safety

What you'll need

• Isolating valve to match the pipe size — usually 15mm, sometimes 22mm
• Pipe slice or a hacksaw and a deburring tool
• Adjustable spanners, PTFE tape
• Towels, a bowl and a bucket

Step by step

1. Work out why you want one.Under every basin and sink tap, at the toilet cistern, at the washing machine and dishwasher, at the outside tap. Each one turns a future repair from a whole-house shutdown into a local job.
2. Identify the pipe with certainty.Trace it back and confirm it is the cold or hot supply to that fitting, not a heating pipe. Run the tap and feel which pipe changes temperature. Cutting the wrong pipe is the mistake that turns this into an emergency.
3. Measure the pipe.15mm is standard for basins, sinks and toilets; 22mm appears on some supplies. Measure the outside diameter rather than guessing, and buy the matching valve.
4. Choose compression or push-fit.Compression is more compact, works on copper and plastic with an insert, and is the traditional choice. Push-fit is faster and needs no tools but takes more space and needs the pipe end perfectly clean and square.
5. Choose the valve type.A slotted-screw ball valve is standard and fine for occasional use. A lever handle is easier to operate but sticks out. Both work; the screw type is less likely to be knocked accidentally.
6. Turn off the water and drain the pipe.At the stopcock, then open the lowest tap on that circuit to drain down. Expect residual water in the pipe whatever you do, so have towels and a bowl ready.
7. Check the stopcock actually works before starting.Old stopcocks seize or fail to close fully. Turn it and run a tap. Finding out it does not shut off after cutting the pipe is a genuinely bad afternoon.
8. Cut with a pipe slice, not a hacksaw, where you can.A slice gives a square cut with no swarf in the pipe. A hacksaw leaves burrs and filings, which then damage tap washers and valve seats downstream.
9. Cut out enough pipe for the valve body.Offer the valve up and mark both cuts. Cutting one and hoping is how people end up needing a coupler as well.
10. Deburr and clean the pipe ends.Inside and out, then wipe clean. A burr or a scratch on the sealing surface is the main cause of a joint that weeps.
11. Fit the valve the right way round.Most have an arrow indicating flow direction. Fitting it backwards can mean it does not seal properly under pressure.
12. Do not overtighten compression joints.Hand tight then about one full turn with a spanner. Overtightening distorts the olive and makes it leak, and the instinct when it weeps is to tighten more, which makes it worse.
13. Support the pipe while tightening.Two spanners, one holding the valve body. Twisting an unsupported pipe strains other joints along the run, which then leak somewhere you were not working.
14. Turn the water on slowly and check.Gradually, watching the joint. Then dry it, leave kitchen roll wrapped round it and check again in an hour and the next day — slow weeps take time to show.
15. Label it if it is not obvious.A tag saying what it isolates. Under a sink with four pipes, nobody remembers in two years.
16. Exercise it occasionally.Turn it off and on once a year. Valves that are never operated seize, which defeats the point of fitting one.

Tips

• Check the stopcock closes properly before you cut anything. It is the one thing that turns this job into an emergency.
• A pipe slice rather than a hacksaw. No filings in the pipe, and a square cut that seals first time.
• Wrap kitchen roll round the finished joint and check it the next day. Slow weeps do not show immediately.

Common mistakes

• Overtightening a compression joint — It distorts the olive and causes the leak it was meant to prevent. Hand tight plus about one turn is correct, and more tightening makes a weeping joint worse.
• Cutting with a hacksaw and not deburring — Filings travel downstream and damage tap washers and valve seats, and the burr stops the olive sealing. Deburr inside and out every time.

If it doesn't work

Questions people ask