A dark patch appears on a bedroom wall. Black speckles creep along the window reveal. A skirting board starts to look tired.
Most people’s reaction is the same: this is damp, and damp is expensive. But “damp” isn’t one problem. It’s four, and they have almost nothing to do with each other.
Telling them apart matters, because the fix for one does nothing at all for the others.
Condensation: the common one
If you’re looking at black spotty mould, this is where to start. Condensation is behind most of the damp problems we’re called to, particularly in flats and well-sealed modern homes.
It happens when warm, moist air meets a cold surface and the moisture drops out onto it. Cooking, showering and drying washing indoors all put water into the air. Cold walls, single glazing and unheated corners give it somewhere to land.
The signs are fairly distinctive:
- Worst in winter, better in summer
- Water sitting on windows in the morning
- Black, speckled mould rather than a solid stain — often in corners, behind furniture and along window frames
- Concentrated in bathrooms, kitchens and bedrooms
- Musty smell, mould on curtains or on clothes in a wardrobe against an external wall
Condensation is usually about ventilation, heating and cold surfaces rather than water getting into the building. That said, the underlying cause can still be the building — no working extractor, thin or missing insulation, a cold bridge round a lintel — so it isn’t automatically down to how a home is being lived in.
Extractor fans that work and are actually used, trickle vents left open, wardrobes pulled a few centimetres off cold walls, and gentle background heat rather than short blasts all make a real difference.
Penetrating damp: water coming in from outside
This is rain finding a route through the fabric of the building. The usual culprits are unglamorous: failed pointing, a blocked or leaking gutter, cracked render, a defective sill, or a downpipe discharging against a wall.
The giveaway is timing and position. Penetrating damp appears or worsens after heavy rain and dries back a little between. The patch stays in one place, and it will usually correspond to something on the outside wall directly behind it.
Go and look during heavy rain rather than after. That’s when a leaking gutter joint shows itself, and half an hour in a coat saves a lot of guesswork.
Rising damp: rarer than people think
Rising damp is groundwater drawn up through masonry where the damp proof course has failed, is missing, or has been bridged.
The classic pattern is a tide mark up to around a metre from floor level, roughly level all along the wall, on ground floor walls only, sometimes with salt deposits and blown plaster.
Two things worth saying plainly. First, it’s genuinely uncommon — most homes have a damp proof course and most still work. Second, when it does occur the cause is often a bridge rather than a failure: soil piled above the DPC, a raised patio or path, render carried down past it, or debris in a cavity. Removing the bridge is a modest job. Injecting a chemical DPC into a wall that never needed one is not.
If someone diagnoses rising damp within a minute of arriving, be curious about how they reached that conclusion.
A leak: sudden and localised
Leaks behave differently from all of the above. They tend to appear quickly, stay in one tight spot, and take no notice of the weather.
Plumbing leaks are the usual suspects — a joint under a bath, failed sealant round a shower, a heating pipe under a floor. A ceiling stain directly below a bathroom is a strong hint. Roof leaks are the other half: a slipped tile or tired flashing round a chimney, showing up as a stain in a loft or on a top-floor ceiling after rain.
The quick test is timing. If the patch stays wet through a dry spell, rain isn’t the cause — you’re more likely looking at a plumbing leak or moisture coming up or through the wall.
Simple checks you can do yourself
None of these need tools:
- Note when it’s worst — winter and mornings point to condensation; after rain points to penetrating damp or a roof leak; constant points to plumbing
- Check the height and shape — high up or in corners is rarely rising damp; a level tide mark near the floor might be
- Look at the same spot from outside for gutters, cracks, pointing and ground levels
- Tape a piece of foil or clingfilm tightly over the patch for a day or two. Moisture on the outside face suggests condensation; moisture trapped behind it suggests damp coming through the wall
Why guessing wrong costs money
Because every wrong diagnosis buys a solution to a problem you didn’t have. A chemical damp proof course won’t stop a leaking gutter. Replastering won’t stop condensation returning. Mould wash treats the symptom and none of the cause, and in three months you’re back where you started, minus the money.
Landlords have another reason not to sit on it. Awaab’s Law is being extended to the private rented sector, bringing legally enforceable timeframes for dealing with serious hazards including damp and mould. Rules in this area are still developing, so check the current position rather than relying on a blog — but the direction is clear enough, and delay is getting harder to defend.
If you’d rather just know what it is
Diagnosis is the part worth getting right. upload a photo of the patch through our Get a quote page, tell us when it’s worst, and we’ll give you our honest read on it.
