The most common DIY plumbing mistakes in UK homes, from not isolating the water and overtightening fittings to the wrong tape, chemical drain cleaners and missing backflow protection, and how to avoid each one.
Most DIY plumbing disasters come from a handful of avoidable errors. Here are the ten we see most often and how to avoid them.
1. Not turning the water off properly
Turning off a tap isn't the same as isolating the supply. Before any job, close the isolation valve on that pipe or the main stopcock, then open the tap to drain residual water. Find and test your stopcock before you need it in an emergency.
2. Overtightening fittings
More force doesn't mean a better seal. Overtightening compression nuts can crush the olive or crack plastic fittings, causing leaks. Tighten until snug, then a further quarter to half turn, and use two spanners so you don't twist the pipe.
3. Using PTFE tape in the wrong place
PTFE tape is for sealing threads. It's not usually used on compression olives, and wrapping it the wrong way round means it unravels as you tighten. Wrap it clockwise when looking at the end of the thread, in the direction the fitting tightens.
4. Mixing incompatible materials
Connecting dissimilar metals, or using the wrong fittings with plastic pipe, can lead to corrosion or leaks. Plastic pipe needs pipe inserts in many fittings. Check the manufacturer's instructions.
5. Pouring chemical drain cleaners down blocked pipes
Harsh chemicals can damage older pipes and seals and are dangerous if they splash back, especially if you later use a plunger. Try a plunger, removing and cleaning the trap, or a drain snake first.
6. Forgetting backflow protection
An outside tap, hose or garden watering system can draw dirty water back into the drinking water supply if pressure drops. Outside taps need a double check valve. This is a requirement of the water fittings regulations, not just good practice.
7. Not checking for leaks afterwards
A joint that seems dry when you finish can weep slowly overnight. Place dry kitchen roll under new joints and check after an hour, the next morning and after a few days.
8. Cutting pipe badly
Pipes cut at an angle or left with burrs don't seal properly, especially in push-fit fittings. Use a proper pipe cutter rather than a hacksaw, and deburr the end.
9. Draining the heating system without care
Draining radiators makes a mess if you don't plan. Turn off the boiler, let the system cool, protect floors, use a hose to an outside drain, and refill slowly, bleeding radiators and checking the boiler pressure. If your system contains inhibitor, add more when refilling.
10. Taking on jobs that need a professional
Gas work must be done by a Gas Safe registered engineer. Unvented hot water cylinders, electric showers and new drainage connections need qualified installers and may need Building Regulations approval.
Before you start any job
- know where your stopcock and isolation valves are;
- have towels, a bucket and a torch ready;
- take photos of how things are connected before dismantling;
- buy the right parts, ideally taking the old one to the shop;
- start early in the day, when merchants are open if you need another part.
Frequently asked questions
Why does my new tap leak at the base?
Often a missing or poorly seated washer or O-ring, or a loose fixing nut.
Is plastic pipe as good as copper?
Plastic pipe is widely used and accepted when installed correctly, but it needs support and can't be used near some heat sources.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.