What causes damp in a house?
Updated June 2026
Damp in a house comes from one of three sources: moisture in the air condensing on cold walls, groundwater rising through a failed damp-proof course, or rainwater getting in from outside. Each has a different cause and a different fix, so knowing which one you’re dealing with is the whole game.
Condensation: moisture from inside the home
The most common cause of damp. Cooking, showering, drying laundry indoors and even breathing release water vapour into the air. When that warm, moist air meets a cold surface it condenses, and black mould follows. Modern sealed-up homes trap this moisture, which is why condensation is so widespread. More in our guide to condensation on walls.
Rising damp: groundwater through the walls
Every ground-floor wall needs a damp-proof course to stop moisture climbing from the ground. When that barrier fails, was never fitted, or gets bridged by raised soil or render, groundwater wicks up through the brick and mortar. It shows as a low tide mark with salt deposits. See the rising damp guide.
Penetrating damp: rainwater from outside
Water gets in through a building fault and tracks through the wall: cracked render, failed pointing, blocked gutters, defective flashing or perished window seals. It shows as a patch that worsens after rain, and it’s common on exposed coastal walls. More in our penetrating damp guide.
Less common causes
Occasionally damp comes from a plumbing leak, a defective roof, construction moisture in a new build, or bridged cavities. A survey identifies these too.
Why the cause matters more than the symptom
The damp patch on your wall looks the same whichever cause is behind it, but the treatments are completely different and not cheap. Diagnosing the cause first is what stops you paying for the wrong fix. A damp survey gives you a written diagnosis before any work.
Frequently asked questions
What is the most common cause of damp?
Condensation, caused by moisture in the air meeting cold surfaces. It’s also usually the cheapest to fix.
Can a house have more than one type of damp?
Yes, and it’s fairly common, especially in older properties. A survey identifies each cause so they can all be dealt with.
Does damp mean there’s something seriously wrong with the house?
Not necessarily. Condensation is a lifestyle-and-ventilation issue, and even rising or penetrating damp is usually a fixable building fault rather than a structural disaster.
Want to know what’s causing the damp in your home? Book a free survey in Brighton, Hove or Sussex.