Quick answer: Condensation forms on cold surfaces, worsens in winter, and usually has no tide mark and no salt staining. Rising damp shows a tide mark low on a ground-floor wall with salt staining and blown plaster. Penetrating damp tracks rainfall and can appear at any height. Below-ground water affects basements. The fix for each is different, so if the signs do not match, get an independent survey before spending money.
Damp in a home is not one problem — it is four, and they are routinely confused with each other. This matters because none of the fixes is cheap and none of them works on the wrong problem. This guide sets out the distinguishing signs so you can form a working theory, and explains when to pay for a survey to confirm it.
Condensation — the most common by far
Where it appears: on cold surfaces. Black spot mould in the top corners of external walls, around window reveals, on ceilings, on the wall behind wardrobes and headboards pushed against outside walls. Water pooling on the inside of windows in the morning.
The tells: it is worse in winter and in unheated or lightly-heated rooms; it is worse on north-facing walls; there is usually no tide mark and no salt (efflorescence) staining. It often appears or worsens after old draughty windows are replaced with sealed double glazing and no trickle vents.
The cause: moist indoor air meeting surfaces colder than its dew point, faster than the building can remove that moisture — a combination of cold surfaces, inadequate ventilation and under-heating.
The fix: ventilation (upgraded extract, trickle vents, PIV or MVHR), insulation to cold surfaces, and adequate affordable heating. See condensation control solutions.
Rising damp — real, but over-diagnosed
Where it appears: at the base of ground-floor walls only, typically no higher than about 1.2 metres from floor level.
The tells: a horizontal "tide mark", often with a band of salt staining just above it, and plaster that is blown, crumbling or hollow-sounding at the bottom of the wall. Skirting boards may be damp or rotten.
The cause: groundwater drawing up through the wall because the original damp-proof course has failed or was bridged (for example by a raised external path or flowerbed).
The reality: genuine rising damp is far rarer than the number of "rising damp" diagnoses suggests. Many are actually condensation, penetrating damp, or a leak. Because rising-damp treatment is lucrative, a free survey from a firm that sells chemical damp-proof courses is not a neutral diagnosis.
The fix: where a genuine DPC failure is confirmed, a chemical injection damp-proof course and salt-resistant replastering — or, sometimes, simply lowering an external path or clearing a bridged cavity. See damp proofing.
Penetrating damp — follows the weather
Where it appears: wet patches on walls or ceilings, at any height, often on the side of the house that faces the prevailing weather.
The tells: it appears or worsens during and after rainfall and dries out in settled weather. It can often be traced to a specific external defect.
The cause: water getting through the building envelope — failed pointing, cracked or missing render, a leaking gutter or downpipe, a defective lead flashing, a blocked cavity, or a cracked window sill.
The fix: find and repair the external defect. See damp proofing.
Below-ground and structural water
Where it appears: basements, cellars and below-ground rooms, or against retaining structures.
The tells: persistent damp that does not respond to surface treatment, standing water, efflorescence, and damp that is present regardless of the weather.
The cause: water under pressure from the surrounding ground.
The fix: specialist waterproofing design to BS 8102 — cavity drainage, tanking or waterproof concrete. See structural damp and waterproofing.
A quick decision checklist
Work through these in order:
- Is there a horizontal tide mark at the base of a ground-floor wall, with salt staining and blown plaster, and nothing similar upstairs? That pattern is consistent with rising damp — but confirm it with an independent surveyor before agreeing to a chemical damp-proof course.
- Do wet patches come and go with the weather, at any height, on the weather-facing side of the house? That is penetrating damp. Look outside for the defect — pointing, render, gutters, flashings — before touching the inside.
- Is it black spot mould in corners, on reveals, on ceilings and behind furniture, worse in winter, with no tide mark? That is condensation. Move to ventilation, heating and insulation.
- Is it a basement or below-ground room, damp regardless of weather? That is structural water and needs a specialist waterproofing design.
- None of the above match cleanly, or a previous fix failed? Get a survey.
Why "damp specialists" so often say rising damp
The economics are worth understanding. Diagnosing and treating rising damp — chemical injection, hacking off and replastering, new skirtings — is a several-thousand-pound job with a 20 to 30 year guarantee attached. A firm whose income depends on selling that treatment, offering a "free survey", is not a neutral diagnostician. That is not an accusation of dishonesty; it is a structural conflict of interest. An independent surveyor charges a fee for the report and has nothing to sell, which is precisely why the report is worth more.
What the fixes have in common
Whichever type it turns out to be, two principles hold. First, treat the cause, not the surface: repainting over mould, or replastering over rising damp without addressing the DPC, buys a few months at most. Second, get it in writing — a written diagnosis and scope of works protects you whether you are a homeowner planning a budget or a landlord facing a disrepair claim.
Homeowner or landlord: the difference
The diagnosis is the same either way, but the stakes differ. For a homeowner, misdiagnosis mostly costs money — an unnecessary chemical damp-proof course, or a fix that does not hold. For a landlord, misdiagnosis also carries legal risk: a tenant living with unresolved damp has routes to the council's environmental health team and, potentially, a disrepair claim, and for social landlords the Awaab's Law timeframes apply to significant damp and mould now and to excess cold from 30 November 2026. In both cases the written survey report is the thing that protects you — as a budgeting tool for the homeowner, as evidence of a proper investigation for the landlord.
When to get an independent survey
If you cannot confidently place your problem in one of the four categories above, or a previous fix failed, or a contractor has proposed an expensive treatment, commission an independent damp survey. It costs £200–£400, produces a written report, and — crucially — comes from a surveyor whose fee does not depend on selling you a particular treatment. For condensation specifically, see the condensation control guide and how much condensation control costs.
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Search the directory →Frequently Asked Questions
- How can I tell condensation from rising damp?
- Condensation appears on cold surfaces, worsens in winter, and usually shows no tide mark and no salt staining — black spot mould in corners, on window reveals and behind furniture. Rising damp shows a tide mark low on a ground-floor wall, typically no higher than about 1.2 metres, often with salt staining and blown plaster at the base.
- Is rising damp as common as contractors say?
- No. Genuine rising damp from a failed damp-proof course is far rarer than it is diagnosed. Many "rising damp" call-outs are actually condensation, penetrating damp from an external defect, or a plumbing leak. An independent survey that is not tied to selling a treatment is the reliable check.
- What does penetrating damp look like?
- Wet patches on walls or ceilings that appear or worsen with rainfall and can occur at any height, often traceable to a specific external defect — failed pointing, cracked render, a leaking gutter or downpipe, or a defective flashing.
- Why does getting the diagnosis right matter so much?
- The fix for each type of damp is completely different and none of them is cheap. Injecting a chemical damp-proof course into a wall that actually has condensation wastes the money and leaves the problem unsolved. Correct diagnosis is the difference between fixing it once and paying twice.
- Should I get a survey before doing any work?
- If the cause is unclear, a previous fix failed, or a contractor has proposed an expensive treatment, yes. An independent damp survey costs £200–£400 and produces a written report you can act on and, if you are a landlord, use as evidence.