The leaks that cost the most are the ones nobody can see. Water runs along a joist, a pipe or the back of a plasterboard sheet and comes out somewhere else entirely, which is why the stain on the ceiling is hardly ever underneath the hole.

The evidence before you see any water

• Boiler pressure dropping and needing topping up. That is the heating pipework, not the boiler.

• The water meter still creeping with every tap and appliance turned off.

• A warm patch on a floor, meaning a hot pipe under screed or in a solid floor.

• Blown plaster, paint lifting, a skirting board darkening at one end, a musty cupboard.

• A ceiling mark that keeps coming back after every repaint.

Narrowing it down before opening anything up

The first decision is which system the water is coming from, because that halves the search straight away. A mains leak runs constantly. A waste leak only appears when something is used, so each bath, shower, basin and appliance gets run in turn and watched underneath. A heating leak shows up as pressure loss, and often behaves differently hot and cold as the pipework expands. After that it is isolation: valving off circuits, capping runs and seeing whether the loss stops, so the pipe is found by elimination rather than by opening up the room the damp happened to appear in. When boards do have to come up, they come up along the run, not at the stain.

The ones that pretend to be something else

• A failed shower tray seal or a cracked bed under the tray, which only shows when the shower is used.

• Waste joints under a bath, almost always blamed on a supply pipe first.

• A screw or a nail driven through a pipe when flooring went down, weeping years later.

• An overflow or discharge pipe outside that everybody reads as rainwater.

Then the repair

The failed section is cut out and remade properly. A repair clamp holds a burst until that can happen, but it is not the finished job. Where a pipe runs in a solid floor, rerouting it on a new path is normally cheaper and far less destructive than digging the floor out. And where the same run has pinholed two or three times, patching it only moves the problem along the pipe, so that run wants replacing. Escape of water damage, including trace and access, is often covered by home insurance, so it is worth reading your policy. Fixed price agreed first, no call-out fee. Call 0161 515 9977.

Common questions

How do I tell whether the leak is on the mains or the heating?

Start with the boiler pressure gauge. If it drops over days or weeks and needs topping up, water is escaping from heating pipework somewhere. If the pressure holds but the water meter keeps moving with everything switched off, it is on the mains or a supply pipe. If it only appears after somebody has had a shower or run the washing machine, you are looking at waste rather than either of them.

Will the ceiling have to come down?

Rarely, and it is the last thing we want to do. Most hidden leaks get located by isolating sections and watching what happens, which means we can usually take up a board or cut a small access on the pipe run instead of opening a whole ceiling. If an opening does need making, you are told where and why before it happens rather than afterwards.

The damp is in the hall but the bathroom is upstairs at the back. Can it be the same leak?

Very often, yes. Water follows whatever it lands on, a joist, a pipe run, the top of a ceiling board, until it finds a low point or a joint to come through, and that can be several metres from where it escaped. It is the main reason chasing the stain wastes time and money. What gets followed is the pipe run, not the mark on the plaster.