Damp Patch on Wall – Water Leak, Condensation, or Rising Damp?

A damp patch is one of the few household problems where the first person you call largely decides what you get sold.

Ring a damp proofing company and you’ll likely be offered a damp course. Ring a plumber and you’ll get a leak search. The patch itself doesn’t care what anyone’s van says on the side, so it’s worth spending ten minutes working out what you’re looking at before you spend money.

Do I have a water leak, condensation or rising damp?

Four things cause most damp patches, and they each leave a different signature.

Condensation. Worst in winter. Appears in cold corners, on window reveals, behind wardrobes and on external walls. Comes with black spot mould. Gets noticeably worse after showers, cooking, and drying washing indoors on radiators.

A leak. Can appear at any height, including first floor and ceilings. Usually near a bathroom, a soil stack, a radiator or the airing cupboard. Often yellow or brown at the edges. Crucially, it ignores the weather entirely.

Penetrating damp. Follows the rain. Gets worse during and after wet weather, and it maps onto something outside: a leaking gutter, missing pointing, failed flashing, a blocked downpipe.

Rising damp. Ground floor only, roughly below a metre to a metre and a bit. Has a fairly level tide mark, white salt deposits, blown plaster and rotting skirting. It’s a real thing, and it’s also one of the most over-diagnosed problems in British housing.

How do I tell condensation from a leaking pipe?

Use the foil test. It costs nothing and it’s more reliable than a meter.

Dry the patch as best you can, then tape a piece of kitchen foil flat over it and seal all four edges with tape. Leave it 24 to 48 hours, then peel it back.

• Moisture on the room side of the foil means condensation. The water is coming out of the air in the room.
• Moisture on the wall side, behind the foil, means moisture is coming through the wall or the structure.

There’s a simpler rule of thumb alongside it. A stain that grows after somebody has a shower, but does nothing during a week of rain, is a plumbing problem until proven otherwise. One that only appears after heavy rain is not.

What does rising damp actually look like?

A low, roughly level tide mark on ground floor walls, with salt crystals on the surface and plaster that’s lost its bond.

What it isn’t: a patch in a first floor bedroom, a ring around a window, or a stain on a ceiling. Water doesn’t rise up through the middle of a house.

One more thing worth knowing. Damp meters read electrical conductivity, and old plaster that’s been contaminated with salts will read high forever, even once it’s completely dry. A high meter reading on salty plaster is not proof that the wall is still wet, and it shouldn’t be the sole basis for a quote.

Why is there a damp patch under my bathroom?

Treat it as a leak until you’ve ruled one out.

The list of suspects is short and every one of them is a plumbing job:

• a toilet leaking past the flush valve, running quietly into the pan and past a worn pan connector
• a dripping tap wetting the back of a basin pedestal
• a failed seal around a shower tray or bath
• a waste pipe joint that’s weeping under a floor
• a WC pan connector that’s slipped

Painting over the patch doesn’t buy time. Chipboard flooring swells and never recovers, and mould gets going in the void where nobody can see it. We’d far rather find this in Burbage or Leicester this week than have you argue with an insurer about gradual leakage in a year’s time, which is an exclusion most policies have.

Should I get a damp course or call a plumber first?

If the patch sits below a bathroom or kitchen, near a soil stack, or if your water meter moves with everything turned off, get a plumber first.

We rule a leak in or out. That’s a small job and it settles the biggest question.

If it turns out to be condensation, the answer is heating, insulation and extraction. Injection does nothing for that. If it’s penetrating damp, the work is on the outside of the house, on gutters, pointing or flashings.

We’re WaterSafe registered plumbers, not a damp injection company, and there’s no version of this where we sell you a damp proof course for a dripping waste pipe.

How do I check my water meter for a hidden leak?

Turn off every tap and appliance in the house, including anything on a timer. Read the meter, write it down, and leave it an hour with nothing used. Read it again.

If it’s moved, water is going somewhere. That’s the single most useful test you can do before anyone comes out, and it’s worth doing before you book anything.

When is a damp patch urgent?

When it’s growing week on week, when a ceiling is stained, or when you can hear water running with everything off.

Slow leaks stop being a decorating problem and start being a structural one. Joists rot, plasterboard sags, and insurers get less sympathetic the longer it’s been visible.

We cover Hinckley, Burbage, Earl Shilton, Barwell, Leicester, Nuneaton, Coventry, Rugby, Tamworth, Lutterworth, Market Bosworth and the villages around them.