1. The shut-off valve will not close fully

Before any repair, the first test is whether you can actually stop the water. If the isolation valve is seized, weeping or only closes halfway, every job downstream of it becomes a job you cannot undo. That is the point to stop, because the valve itself is now the repair and it needs the mains off to change.

2. You cannot tell where the water is coming from

Water travels along joists, under boards and down the outside of pipes before it appears. A damp patch on a ceiling is rarely directly under its source. If you cannot trace it to a fitting you can see and touch, opening up the nearest surface is guesswork — and usually the wrong surface.

3. The fix involves a joint you will never see again

A compression fitting under a sink can be redone in a minute if it weeps. The same fitting buried in a wall cannot. Any joint that is about to be covered by tile, plaster or screed needs to be made by someone whose work you can hold to account, because testing it properly is a skill in itself.

The cost of getting that one joint wrong is not the joint. It is the tiling, the plastering and the drying-out afterwards.

4. It is a gas appliance

Anything connected to a gas supply — a combi boiler, a gas water heater, a gas hob being moved for a new sink — is not a DIY job in any circumstance, regardless of how straightforward the plumbing side looks. It is also, in most places, illegal to work on without certification.

“The question is never whether you can do the repair. It is what happens if it fails at three in the morning while you are asleep upstairs.”

5. You have already fixed it twice

A repair that needs doing again within a few months was never the real fix. The tap that keeps dripping after two new washers has a damaged seat. The drain that blocks every spring has something structural going on further down the line.

At that point a third attempt costs more than a diagnosis. Someone with a camera and a pressure gauge will find the actual cause in an hour, and you will have spent less than you did on the parts for attempts one and two.