What this diagnoser does
Damp inside a house has a handful of possible causes, and they are treated in completely different ways. Getting the cause wrong is expensive: the standard mistake is to pay for a chemical damp proof course to fix what turns out to be condensation, a leaking pipe or a raised flowerbed, and to still have the damp afterwards.
This diagnoser asks about where the damp is, what it looks like, when it appears and what is happening either side of the wall, then ranks the five usual causes by how well they fit. It weighs the answers that actually discriminate between causes — the height of the patch, whether it follows the weather, what has formed on the surface — more heavily than the ones that fit almost anything.
It is a starting point for an informed look at your own house, not a survey. It cannot see your wall, it does not know what is behind it, and more than one cause can be present at once. What it can do is tell you which explanation fits the pattern, what to check next, and — the part most sites leave out — what not to spend money on until you know more.
Describe the damp
The five causes, and how they differ
Almost all internal damp comes down to one of five things. Telling them apart is mostly a matter of three questions: how high up is it, does it follow the weather, and what has formed on the surface.
| Cause | Typical height | Timing | Surface |
|---|---|---|---|
| Condensation | Anywhere, especially cold corners and behind furniture | Worse in cold weather | Black speckled mould |
| Penetrating damp | Any height, often mid-wall | A day or two after heavy rain | Staining, blistering, sometimes salts |
| Plumbing or heating leak | Any height, often low | Constant, slowly worsening | A patch that never dries |
| Bridged damp proof course | Low, near the skirting | Fairly constant, worse in wet spells | Tide mark, crumbling plaster |
| Rising damp | Low, stopping below about a metre | Constant | Level tide mark with a salt band |
Water can only be drawn up masonry by capillary action so far before gravity and evaporation balance it out. In practice that limit is around a metre, occasionally a little more in a very fine-pored, salt-laden wall. Damp appearing at 1.5 m, or in an upstairs room, is not rising damp — whatever anyone tells you. That single observation rules out the most-sold diagnosis on the list, which is why this tool asks it first.
How this diagnoser works
Each answer carries a set of weights, one per cause. The weights reflect how strongly that observation points at that cause, not how bad the damp is.
Score for a cause = sum of the weights that cause receives from your seven answers
Match strength = score ÷ the highest score that cause could possibly reach, as a percentage
Causes are ranked by scoreDistinctive observations score heavily. Black speckled mould scores six for condensation because very little else produces it. A wet patch near a bathroom scores six for a leak. Weak observations score one or two, because a ground-floor external wall is consistent with almost everything.
One answer acts as a veto rather than a weight. If the damp is above about 1.2 m or at ceiling level, rising damp is set to zero and removed from the ranking, because capillary rise cannot reach that height.
The percentage is a match against that cause's own profile, not a probability. Comparing the causes against each other is meaningful; reading a single figure as a likelihood is not. A result of 70% for condensation means your answers fit the condensation pattern well — it does not mean there is a 70% chance that is what you have.
Worked example: damp above the skirting in a back room
A ground-floor room in a 1930s semi. There is a brownish tide mark up to about 600 mm above the skirting, the plaster is crumbling along the bottom, it looks much the same all year, and outside, the patio was raised two years ago and now sits roughly level with the internal floor.
Entering those observations — worst area at the skirting, external wall on the ground floor, a brownish tide mark, much the same all year, ground level at or above the indoor floor, nothing notable inside the room, built after about 1900 — the diagnoser produces:
| Cause | Match | What it is |
|---|---|---|
| A bridged damp proof course | 100% | The barrier is intact, but something is carrying moisture across it. |
| Rising damp | 91% | Groundwater drawn up through the masonry by capillary action. |
| A plumbing or heating leak | 27% | Water escaping from a pipe, fitting, appliance or shower tray. |
| Penetrating damp | 13% | Water crossing the wall from outside through a defect. |
| Condensation | 4% | Moisture produced inside the house settling on cold surfaces. |
A bridged damp proof course comes out ahead of rising damp, and the reason is the patio. The observations are otherwise identical to textbook rising damp, which is precisely why this case is so often misdiagnosed and treated with an injected damp proof course that cannot work — the new barrier would sit below the paving that is carrying water over the top of it. Lowering the patio, or cutting a drainage channel along the wall, costs a fraction of the treatment and addresses the actual mechanism.
Note that rising damp still appears, and appropriately so. If the ground level is corrected and the wall does not dry out over the following year, it becomes the next candidate — and at that point an independent survey with drilled samples is the sensible next spend.
Understanding the result
Read the gap between the top two, not the top figure alone.
- A clear leader, well ahead of the rest. Your observations form a consistent pattern. Follow the checks listed for it before spending anything.
- Two causes close together. Common and often correct — condensation frequently sits on top of another problem, and a leak will produce mould in the same way condensation does. Work through the checks for both.
- Everything clustered in the middle. Usually means the observations are ambiguous or that you have had to guess at one or two. Look again at the wall in different weather, then come back.
- Rising damp removed from the ranking. That is the height veto, and it is a useful result in itself. If anyone has quoted you for a damp proof course on a patch above a metre, ask them to explain the mechanism in writing.
Some situations need a professional now rather than more diagnosis: water running or dripping rather than seeping, damp spreading quickly over days, a ceiling that is sagging or staining from above, damp near electrical fittings or a consumer unit, or a musty smell with visible fungal growth on structural timber that could be dry rot. Damp in a bedroom used by a baby, an elderly person or anyone with asthma or a lung condition also warrants prompt attention rather than a season of watching.
Things that affect the result
- More than one cause at a time. The most common combination is a genuine defect plus condensation, because a cold wet wall is a cold surface and attracts condensation of its own.
- The season you are looking in. Condensation largely disappears in summer. Penetrating damp is invisible in a dry month. A diagnosis made in August and one made in January can differ entirely.
- Recent building work. New plaster, screed and concrete hold construction water for months — a new extension is wet for a year and that is normal, not a fault.
- Salt-contaminated plaster. Old salts left in plaster from a long-cured problem absorb moisture from the air and keep a wall reading damp indefinitely. The wall behind may be perfectly dry.
- How the house is used. Occupancy makes an enormous difference to condensation. The same house with two occupants and with six behaves differently.
- Wall construction. Solid walls, cavity walls and cavity walls that have been retrofitted with insulation all behave differently in driving rain.
- Exposure. A south-westerly elevation in Cornwall takes far more driving rain than a sheltered one inland, and penetrating damp appears on walls that would be fine elsewhere.
Common mistakes
- Accepting a free damp survey as a diagnosis. If the surveyor's employer sells the treatment, the report is a quotation. Independent advice costs money and is almost always cheaper in the end.
- Trusting a moisture meter reading taken on plaster. Electrical conductance meters read salts as well as water. On salt-contaminated plaster they read high when the wall is bone dry.
- Painting over mould. It returns, because the water is still arriving.
- Sealing the house up. Draught-proofing without ventilation reliably converts a mild condensation problem into a serious one.
- Replastering too early. A solid wall can take months to dry after the cause is fixed. Making good before then wastes the work.
- Assuming low damp equals rising damp. A bridged DPC and a leaking heating pipe both look exactly like it.
- Treating the stain rather than tracing it. Water travels. The defect is usually above and to one side of where it shows.
If you rent your home
Damp and mould in rented housing carries legal duties, and they have tightened considerably since 2025.
Awaab's Law is named after Awaab Ishak, a two-year-old who died in 2020 from a severe respiratory condition caused by prolonged exposure to mould in his home. It was created by section 42 of the Social Housing (Regulation) Act 2023, which inserted section 10A into the Landlord and Tenant Act 1985, and is given effect by the Hazards in Social Housing (Prescribed Requirements) (England) Regulations 2025. It applies to social landlords in England.
What a social landlord must do
Where a landlord becomes aware of a potential hazard, the regulations set out what has to happen and by when.
| Duty | Deadline |
|---|---|
| Investigate a potential emergency hazard, and carry out the relevant safety work | Both within 24 hours of becoming aware |
| Investigate a potential significant hazard | Within 10 working days of becoming aware |
| Provide the tenant with a written summary of the findings | Within 3 working days of the investigation concluding |
| Carry out the relevant safety work, where a significant hazard is found | Within 5 working days of the investigation concluding |
| Begin work to stop the hazard recurring (supplementary preventative work) | Within 5 working days, or as soon as reasonably practicable and within 12 weeks |
| Provide suitable alternative accommodation, at the landlord's expense, if the safety work cannot be done within these timescales | — |
| Keep the tenant updated throughout, and advise how to stay safe if the property is not vacated | — |
What is covered, and when
The duties are being introduced in three phases, with a period between each to test how the regulations work in practice.
- Phase 1, in force since 27 October 2025. All emergency hazards, and significant damp and mould hazards.
- Phase 2, in force from 30 November 2026. Extends the significant hazard duties to excess cold, excess heat, falls (on the level, on stairs and between levels), structural collapse, fire and explosions, electrical hazards, and domestic hygiene including personal hygiene and food safety.
- Phase 3. Will extend the duties to the remaining hazards under the Housing Health and Safety Rating System, apart from overcrowding. The government has not published a date for this phase.
A hazard reported before 27 October 2025 does not have to be investigated under these rules until it is reported again, or something material changes.
A landlord cannot dismiss a reported hazard as the tenant's lifestyle, and the tenant does not have to produce medical evidence for an investigation to be required.
If you rent privately, or live elsewhere in the UK, the Homes (Fitness for Human Habitation) Act 2018 still requires rented property in England to be fit to live in, and damp is one of the matters it lists. Your council's environmental health team can also inspect under the Housing Health and Safety Rating System, in which damp and mould growth is a category of hazard.
Report it in writing and keep the correspondence, photographs with dates, and a note of the response. Whatever the cause turns out to be, a written record from the first report onwards is what makes these duties enforceable. The full picture is in the damp and mould guide.
Checked against the Ministry of Housing, Communities and Local Government collection on GOV.UK and the regulations on legislation.gov.uk on 7 August 2026. The phasing is still under way, so check the current position at source before relying on it.
Frequently asked questions
How common is rising damp really?
Considerably less common than the volume of treatment sold for it implies. It requires either no damp proof course or a failed one, and most housing built after about 1900 has an intact barrier. Surveys of properties diagnosed with rising damp repeatedly find that the majority turn out to be condensation, penetrating damp, a leak or a bridged damp proof course. It does exist, particularly in older buildings, but it should be the conclusion after the alternatives are excluded rather than the assumption at the start.
A company offered me a free damp survey. Is that worth having?
It is worth understanding what it is. A free survey provided by a firm that installs damp proofing is a sales visit, and the surveyor's income depends on work being found. That does not make them dishonest, but it is not an independent opinion and should not be treated as one. If a treatment costing four figures is being proposed, paying a few hundred pounds for a report from someone with no stake in the outcome — a RICS surveyor or an independent damp and timber surveyor — is proportionate.
Why is a moisture meter reading not enough?
The handheld meters usually produced on a survey measure electrical conductance between two pins. Water conducts, but so do the mineral salts that accumulate in walls, and so does a surface that is merely cool. Pressed against plaster, such a meter can read alarmingly high on a wall that is completely dry underneath but salt-contaminated from a problem resolved decades ago. The reliable methods sample the masonry itself — drilled dust tested with a calcium carbide meter, or samples oven-dried and weighed in a laboratory — and analyse for ground salts at the same time.
Is black mould dangerous?
Mould in the home is a recognised health risk, particularly for babies and young children, older people, and anyone with asthma, a lung condition or a weakened immune system. It can cause or worsen respiratory symptoms, and severe exposure has been fatal. Small patches can be cleaned by a competent adult with the room ventilated, but cleaning treats the symptom — the moisture source has to be dealt with or it returns. Extensive growth, or growth in a home where someone is vulnerable, should be treated as urgent.
How long does a wall take to dry out?
Longer than most people expect. A rough rule for solid masonry is around a month of drying for every 25 mm of thickness once the source of water has stopped, in reasonable conditions. A 225 mm solid wall can therefore take the better part of a year. Gentle background heat and good ventilation help; sealing the surface or replastering early does not.
Can cavity wall insulation cause damp?
It can, though it usually does not. Problems arise where insulation was installed in a wall that was unsuitable — an exposed elevation taking heavy driving rain, or a wall with existing defects in the pointing or render — allowing moisture to track across the cavity that was previously a barrier. The pattern tends to be patchy damp on exposed walls that began after installation. Where insulation was installed under a guarantee scheme there is normally a route to have it inspected.
Does a dehumidifier fix damp?
It manages condensation while it is running, which can be genuinely useful during drying out or in a room that cannot be ventilated well. It does nothing about penetrating damp, a leak or a bridged damp proof course, because those keep supplying water regardless. Treat it as a tool, not a cure.
Is what I enter stored?
No. Your answers are processed entirely in your browser and are never transmitted or retained. Even so, treat any public website as an unsecured place and use your judgement about what you enter.
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References
These organisations publish the guidance behind this page.
- Awaab's Law in the social rented sector, GOV.UK — the government collection setting out the duties, timescales and phasing
- Hazards in Social Housing (Prescribed Requirements) (England) Regulations 2025 — the regulations themselves
- Approved Documents, GOV.UK — Approved Document C covers resistance to moisture; Approved Document F covers ventilation
- NHS — health effects of damp and mould, and who is most at risk
- Building Research Establishment — research and good repair guidance on diagnosing and treating damp in dwellings
- Historic England — guidance on moisture in older, solid-walled and traditionally built properties
- Royal Institution of Chartered Surveyors — professional standards for building surveys and independent damp investigation
- Property Care Association — the trade body for damp and timber contractors, whose members generally supply the treatments discussed here
- Shelter — tenants' rights on damp and mould, including Awaab's Law
- Housing Ombudsman — complaints about how a social landlord has handled a damp and mould report
External guidance changes. Check the current position at the source before relying on it for a decision that matters.