Damp and mould: diagnose it properly first

Three kinds of damp, one expensive confusion

Every damp problem belongs to one of three families. Condensation is moisture your household makes indoors, settling on cold surfaces. Penetrating damp is weather getting in from outside: a blocked gutter, failed pointing, a cracked cill. Rising damp is ground moisture climbing a wall whose damp-proof course has failed or been bridged, and it is far rarer than the treatment industry suggests.

Diagnosis matters because the fixes do not overlap. A chemical damp-proof course does nothing for condensation. A dehumidifier will not stop a leaking gutter. Money spent on the wrong family is money spent twice, because the real cause is still there when the scaffolding comes down.

Read the symptom before anyone sells you a cure

Two tells are worth knowing by heart. Rising damp leaves a tide mark with crusty salts and rarely climbs past a metre. Condensation grows mould with no stain line, in the places air barely reaches. The full comparison, including the kitchen-foil test, is in condensation or rising damp.

Condensation: the one you probably have

A household is a small weather system. Cooking, kettles, showers, breathing and washing drying on an airer release roughly 10 to 14 litres of water into the air every day. In summer it drifts out of open windows. In winter it meets cold glass and cold outside walls, condenses, and feeds mould.

The fix is a pincer movement: make less moisture and remove more of it. Lids on pans, the bathroom door shut, an extractor fan that actually pulls, trickle vents open, and a burst of fresh air each morning. The whole routine is in our guide to stopping condensation on windows.

Where mould has already taken hold, small patches (under about half a square metre) are a DIY clean with gloves and ventilation. Anything bigger, or anything that keeps returning, needs a professional. Removing black mould safely covers the method, including what never to mix.

Penetrating damp: rain finding a way in

Penetrating damp announces itself by timing: patches darken during or shortly after rain, usually on one wall at one height. The way in is nearly always mundane. Overflowing gutters soak a wall all winter; failed pointing, cracked render and worn cills each let water track through. Work through finding the way in before touching the inside, because redecorating waits until the wall has dried.

The outside of a house fails as a system: roof coverings, flashings, gutters, walls. Our outside survival guide maps the whole envelope and the twice-a-year ground-level inspection that keeps it in order.

Rising damp: real, rare, oversold

True rising damp is ground moisture wicking up through masonry, and physics caps the climb at about a metre. Nearly every house built since the late 1800s has a damp-proof course to block it, so genuine failures are unusual. Many that look like failures are bridging instead: soil, paving or render carrying moisture over the top of the DPC.

If an injected damp-proof course has been recommended, pause. Rule out condensation and penetrating causes first, then pay for an unbiased opinion. Structural symptoms deserve the same independence: cracks in walls has the sober version of that conversation.

If you rent

Damp and mould are not something tenants are expected to live with. The Homes (Fitness for Human Habitation) Act lets tenants in England and Wales hold landlords to a home fit to live in, and damp and mould count. Awaab’s Law now sets fixed timescales for social landlords to investigate and fix mould hazards, and those duties are being extended to private tenancies.

Report problems in writing with dated photos, keep copies, and be specific about rooms and symptoms. A landlord’s first response should be an inspection, not a lecture about lifestyle. This is general information rather than legal advice; for a complex or disputed case, an answer verified by a chartered professional is the unbiased route.

Your ten-minute investigation

Ten minutes of looking beats years of paying for the wrong fix. Most damp, once named, is a small job: a gutter cleared, a fan that works, a window opened before the kettle goes on. Little, boring and on time beats big, dramatic and too late.