Damp & Mould Questions Answered
Clear, practical answers on what causes mould and damp, how to find the source, how to fix it properly and how it can affect your health — from specialists who diagnose it every week.
The Short Version
Straight answers from CSRT-qualified surveyors who diagnose damp in London properties every week.
Every damp problem starts with water in the wrong place
Mould and damp are caused by excess moisture — from leaks, rising ground water, rain getting in, or condensation from poor ventilation. The fix depends entirely on which it is, so diagnose first, treat second. If anyone in the home is a child, elderly, or has respiratory, skin or immune conditions, don’t wait.
Book a damp survey →What Causes Mould and Damp?
Mould and damp share one root cause — water where it shouldn’t be. The hard part is finding where it’s coming from.
What causes mould around the home?
Mould is caused by excess moisture. Spores are present in all indoor air; they only grow where a surface stays damp long enough, which is why mould appears on cold external walls, behind furniture, around window reveals and in poorly ventilated bathrooms and bedrooms.
The cause is simple — water where it shouldn’t be — but finding the source of that moisture is often much harder than it sounds. That’s why a professional mould removal visit starts with diagnosis, not spraying.
What causes damp around the home?
Damp can come from many sources, including:
- Leaking pipework, radiators and guttering
- Rising damp from the ground in basements and ground floors
- Roof damage, such as slipped or leaking tiles
- Poorly sealed doors and windows
- Excessive condensation caused by poor ventilation
- Failed or bridged damp-proof courses, cracked render and failing pointing
Most properties have one main cause, but a combination is common — which is why each source has to be identified before treatment.
What are the three main types of damp, and how do I tell them apart?
Condensation is the most common: warm, moist air meeting cold surfaces. It shows as water on windows and mould in corners, behind wardrobes and on cold external walls, and is usually worse in winter. Rising damp is groundwater drawn up through masonry; it typically leaves a tide mark on ground-floor walls, often up to around a metre high, with white salt deposits. Penetrating damp is water coming through the building envelope — patches that darken after rain, or stains on ceilings below a roof or on walls near faulty gutters.
They look similar but need completely different fixes, which is why the cause should be confirmed before any treatment is bought. See our guides to rising damp, penetrating damp and condensation control.
Why does mould keep coming back after I clean it?
Cleaning removes what you can see, but if the moisture source is still active the surface will become damp again and mould will regrow, often within weeks. Biocide kills existing spores; it does not stop new growth.
If mould returns repeatedly in the same place, treat that as a sign of an unresolved cause — poor ventilation, a cold-bridged wall, a leak or rising or penetrating damp — and have it diagnosed rather than cleaned again.
Is black mould more dangerous than other mould?
Colour alone doesn’t tell you much. Any mould growth indicates a moisture problem that should be fixed, and any mould can irritate people who are sensitive to it. What matters is how much is present, how long it has been growing and who is living in the property. Rather than trying to identify the species, focus on removing the growth safely and fixing the moisture that caused it.
Finding the Source: Surveys and Moisture Readings
Accurate diagnosis is what separates a lasting fix from money spent on the wrong treatment.
How do you find the source of damp in a property?
A specialist works through the building methodically: a visual inspection inside and out (roof coverings, gutters, render, pointing, ground levels, ventilation), moisture readings with calibrated equipment, humidity and dew-point checks where condensation is suspected, and photographic evidence of what is found. The pattern of readings, the location of the damage and the property’s history are read together to establish the cause.
Our damp survey follows this process, and you can see what a written report contains in our guide to damp survey reports.
Can a moisture meter on its own tell me what type of damp I have?
No. A meter shows that moisture is present, not why. Readings can be raised by salt contamination or foil-backed wallpaper even where there is no active damp, and many DIY meters measure electrical resistance rather than true moisture content. See our damp meter reading chart for how readings are interpreted, and treat any single reading with caution.
What happens during a damp survey, and how long does it take?
Most residential surveys take around one to three hours on site, depending on the size and complexity of the property. The surveyor inspects the affected areas and the likely sources, takes readings and photographs, and explains what they have found. A written report with findings and recommended action follows within a few working days.
Is a free survey from a damp-proofing company reliable?
Be cautious. A survey from a contractor who profits from the treatment they recommend has a built-in incentive to find a problem that fits their product. We survey before we recommend, and we’ll tell you honestly if the cause is minor or no treatment is needed. If you’re buying, a written pre-purchase report is also something your solicitor can use — see our pre-purchase damp and timber survey.
Do I need a timber survey as well as a damp survey?
If there are signs of wet rot, dry rot or woodworm alongside the damp — soft or spongy timber, cracking skirting, a musty smell under floors — then yes, because the timber’s condition has to be assessed along with the moisture. A combined damp and timber report covers both.
Fixing Damp and Mould Properly
There is no one-size-fits-all fix. The right treatment depends on the property and on exactly what is causing the moisture.
How do you solve damp problems?
Solving damp starts with isolating the cause. Once that is confirmed, you fix it — and then deal with the damage it has done. For example, if damp is coming from ageing window seals, the solution is to treat the affected area and replace the failed seals so it cannot happen again. Rising damp is treated at the source with a damp-proof course, followed by suitable replastering; condensation is controlled through ventilation and humidity management.
Because there is no “one size fits all” approach, an experienced and versatile team matters. At Damp and Mould Solutions Ltd we examine the whole property first, determine exactly where your damp, mould or mildew problems stem from, and only then recommend how to solve them.
Can I paint over damp walls?
No. Paint over a damp wall traps moisture behind the surface and it will fail, usually within weeks or months. The source has to be fixed, the wall fully dried and confirmed dry with a meter, and only then redecorated with a breathable finish. Our step-by-step guide, How to Treat Damp Walls, covers the full sequence.
Can I fix damp myself, or do I need a professional?
Condensation and surface mould can often be managed yourself by improving ventilation, reducing humidity and cleaning properly. Rising damp, penetrating damp and anything you can’t confidently diagnose should be assessed by a specialist, because treating the wrong type usually costs more than the original problem. See DIY vs professional damp proofing for a full comparison.
How can I reduce condensation and mould day to day?
Use extractor fans when cooking and showering, keep trickle vents open, avoid drying washing on radiators, and keep furniture slightly away from cold external walls. Aim for indoor humidity of roughly 40–60%. Where homes are persistently damp despite good habits, a ventilation system such as positive input ventilation (PIV) can help.
How long does it take for a wall to dry after damp is treated?
It varies widely. Minor condensation may dry in days; walls affected by rising or penetrating damp, especially where plaster has been replaced, can take weeks or even months. Only moisture readings can confirm a wall is ready to decorate — never judge by appearance.
Damp, Mould and Your Health
Damp and mould are not just cosmetic. They can affect the people who live with them.
Are damp problems potentially harmful?
Yes. The NHS advice is to get the issue isolated and fixed as soon as possible.
Not everyone reacts in the same way. For some people it may only mean an unpleasant, stale smell. For others it can irritate existing respiratory conditions such as asthma. It isn’t only breathing that can be affected: damp can also trigger flare-ups in people with skin conditions like eczema, and affect those with a weakened immune system.
Who is most at risk from damp and mould?
It is especially important to get damp isolated and treated quickly if you, or anyone you live with, is:
- A child
- Elderly
- Living with respiratory issues
- Suffering with skin issues
- Has a weakened immune system, for example from treatments such as chemotherapy
Condensation and damp can have wide-reaching consequences, and the sooner the cause is treated, the less damage it does — to the property and to health.
Should I see a doctor if I think damp is affecting my health?
If you or someone in your household has breathing difficulties, worsening asthma, skin flare-ups or other symptoms you think may be linked to damp or mould, speak to your GP or pharmacist and tell them about the conditions at home. Fixing the cause of the damp should happen alongside, not instead of, medical advice.
Rights, Responsibilities and Property Transactions
Damp and mould raise legal and financial questions for landlords, tenants, buyers and sellers.
Is my landlord legally responsible for damp and mould?
Landlords have legal duties to keep homes fit to live in. Under the Homes (Fitness for Human Habitation) Act 2018 and the Housing Health and Safety Rating System, serious damp and mould can be a hazard that local authorities can act on. Social landlords also have specific duties following Awaab’s Law, with set timescales to investigate and fix hazards. If you rent, report the problem in writing and keep records. Our guide to mould advice for landlords and tenants and our summary of building regulations for damp, mould and ventilation explain more. Check GOV.UK for the current timescales for your situation.
Should I get a damp survey before buying a property?
If you spot signs of damp at a viewing, or your surveyor has flagged it, a specialist report is worth having before you exchange. It identifies the type and cause of damp, gives indicative costs and can support a price negotiation. See buying and selling a property with damp and our pre-purchase survey.
Which areas do you cover?
We cover all London boroughs and parts of Hertfordshire, with same-week surveys available in most areas. Call 020 4542 6114 to confirm availability at your address.
Written and Reviewed by Specialists
Reviewed by Micken Patel
CSRT-qualified surveyor with 15 years’ experience diagnosing and treating damp in London properties.
The answers on this page reflect the diagnostic process we use on every survey: find the source first, confirm it with evidence, then recommend only the work that is genuinely needed. This page is general guidance — every property is different, and a survey gives you an answer specific to yours.
Still Not Sure What’s Causing Your Damp?
A proper diagnosis from a CSRT-qualified specialist before spending money on any treatment — that’s where every job starts.