Three Problems That Look the Same
Damp on a wall looks much the same whatever caused it, which is why it gets misdiagnosed so routinely. The treatments are different, they are not interchangeable, and the wrong one costs money without fixing anything.
Rising damp
Groundwater drawn up through the structure by capillary action, where there is no working damp-proof course to stop it.
What it looks like: a tide mark on the wall, usually no higher than a metre. Often a band of salting or blown plaster along the top edge. Skirtings and the base of door frames may be affected.
How it is diagnosed: by ruling the alternatives out. We check the external ground level against the DPC, look for a path, patio or flowerbed bridging it, check for a failed or missing course, and take readings at depth rather than relying on a surface meter — surface meters read salt as enthusiastically as they read water, which is one reason rising damp gets over-diagnosed.
What fixes it: lowering ground levels or clearing the bridge where that is the cause. Where the course itself has genuinely failed, a replacement DPC, then replastering to a salt-resistant specification.
Penetrating damp
Water getting in through the fabric of the building from outside, rather than up from the ground.
What it looks like: patches rather than a tide line, often high on the wall or around a specific feature, and noticeably worse after rain or on one particular elevation.
How it is diagnosed: externally as much as internally. Failed pointing, cracked render, a blocked or leaking gutter, a slipped tile, defective flashing, a blocked cavity, or a windowsill without a drip groove. The damp patch inside tells you where to look outside.
What fixes it: the defect. Repointing, repairing render, clearing or renewing rainwater goods. Damp proofing a wall that is being rained on through a broken gutter achieves nothing.
Condensation
Moisture already inside the house, condensing on cold surfaces. The most common damp problem in British homes, and the one most often mistaken for something more expensive.
What it looks like: black spot mould, typically in corners, behind furniture, on north-facing walls, around window reveals and in bathrooms and bedrooms. Worst in winter. Not a tide mark.
How it is diagnosed: by pattern and by conditions — where in the room, which surfaces, what the humidity and surface temperatures are, and how the house is heated and ventilated.
What fixes it: ventilation, heating pattern, and insulation where cold bridging is the cause. Extraction that actually vents outside. This is usually the cheapest of the three to resolve and does not need a damp proofing contract at all.
Older and solid-walled properties are a different case
Everything above assumes standard cavity-wall construction. Solid-walled and pre-1919 buildings manage moisture by breathing, and treatments that seal them — cement render, gypsum plaster, injected courses — tend to move the problem rather than remove it. If that describes your property, see damp in period and listed properties.
Why we survey first
All three problems can be present at once, and two of the three are frequently sold as the third. A survey costs us time and costs you nothing, and it is the only way to know which you are dealing with. Book one here.
Frequently Asked Questions
How do I tell rising damp from condensation?
Rising damp shows as a tide mark low on the wall, rarely above about a metre, and is worst in winter but present year-round. Condensation appears on cold surfaces and in corners, often with black mould, and is worst in rooms where moisture is produced. If the damp is above a metre or on an internal wall away from the ground, it is very unlikely to be rising damp.
Is rising damp real?
Yes, but it is diagnosed far more often than it occurs. A great deal of what is sold as rising damp treatment is actually penetrating damp, a bridged damp-proof course, or condensation. All three are cheaper to fix. This is why we survey before quoting.
Do you need to remove the plaster?
Where salts have contaminated the plaster, yes — hygroscopic salts left in the plaster will keep drawing moisture from the air and the wall will look damp even after the cause is fixed. Where the plaster is sound and the problem is penetrating damp or condensation, often not.
How long does a wall take to dry out?
As a rough guide, a solid wall dries at about 25mm per month once the source of moisture is removed, so a 450mm wall can take the best part of a year. Any firm promising an immediate dry wall after treatment is overselling.