What is the difference between damp and mould?

Damp is excess moisture in a building that should not be there. Mould is the fungal growth that appears once a surface stays damp long enough. Mould is the symptom; damp is the underlying problem. Cleaning mould without finding and fixing the moisture source means it returns.

How do I tell condensation from rising or penetrating damp?

Condensation forms on cold surfaces — windows, external walls, corners — and is worst in winter and in kitchens and bathrooms. Penetrating damp tracks from a specific external defect such as a leaking gutter or cracked render. So-called rising damp is rare and over-diagnosed. Only a proper investigation, not a single meter reading, reliably tells them apart.

Is black mould dangerous?

It can be. Damp and mould are linked to respiratory infections, asthma and allergic reactions, and the risk is highest for children, older people and anyone with a respiratory condition. Large areas of mould should be dealt with by a professional, and the moisture cause should always be diagnosed and fixed.

Should I trust a free damp survey from a treatment company?

Treat it with caution. A company that sells damp-proofing has a commercial incentive to recommend its own products. For an impartial diagnosis — especially before buying a home or in a dispute — use an independent surveyor who charges for the survey and has no stake in the remedial works.

How quickly must a landlord deal with damp and mould?

For social landlords under Awaab's Law (in force since 27 October 2025), a significant damp and mould hazard must be investigated within 10 working days, a written summary given to the tenant within 3 working days of the investigation concluding, and safety work carried out within 5 working days. Emergencies must be made safe within 24 hours.

Damp and Mould: The Complete UK Guide (2026)

Damp and mould are among the most common — and most misdiagnosed — problems in UK homes. This guide explains what they actually are, how to tell the different types apart, how to deal with mould safely, and when to bring in a proper survey. It is written for homeowners, landlords and tenants, and for the surveyors who advise them.

What "damp" actually means

Damp is simply excess moisture where it should not be. If part of a home is not dry to the touch, smells musty, or has visible mould, there is a moisture problem to investigate. The crucial thing to understand is that "penetrating damp", "condensation" and "rising damp" are not causes — they are different ways moisture moves through or onto a building. To fix a damp problem you first have to find where the water is coming from.

The three types, and how to tell them apart

Getting this right is the whole game. The same patch of mould can have completely different causes, and each demands a different fix. Our deep-dive on condensation, damp or mould walks through the distinctions in detail; here is the short version.

Condensation

By far the most common. Warm, moist indoor air meets a cold surface — a window, an external wall, a cold corner — and the water vapour turns to liquid. It shows up around windows, on external walls and on ceilings near external walls, and it is worst in winter and in kitchens and bathrooms. Cooking, showering, drying clothes indoors and poor ventilation all feed it.

Penetrating damp

Water crossing from the outside in through a specific defect — a leaking or blocked gutter, a cracked render, failed pointing, a broken roof tile, or perished sealant around a window. It is usually traceable to a fault you can find, often by going outside in the rain and watching where water goes.

"Rising damp"

Moisture drawn up from the ground through masonry by capillary action. It is real but rare and heavily over-diagnosed — genuine failure of an original damp-proof course is unusual, and "rising damp" is too often written down as a conclusion when it is really a guess that sells a new damp-proof course. Our guides on what causes rising damp and rising versus penetrating damp explain why misclassification is the single most common cause of failed, wasted remediation.

How to reduce condensation

Because condensation is the most common form, most households can cut it back with practical changes — though you should still find the moisture source first:

Use extractor fans whenever cooking, bathing or showering, and keep them clean and correctly sized.
Ventilate in short bursts — roughly five minutes an hour in a used room, or use trickle vents — rather than leaving windows open all day.
Keep kitchen and bathroom doors closed in use so moisture doesn't spread to colder rooms.
Vent tumble dryers externally, and avoid drying laundry on radiators.
Maintain a reasonably even, gentle heat — roughly 18–21 °C — since cold surfaces are where condensation forms.
Don't block air vents in bedrooms, cupboards or under suspended floors.

What mould is, and how to deal with it safely

Mould is fungal growth that thrives on persistently damp surfaces. It is a health risk — linked to respiratory infections, asthma and allergic reactions, and most dangerous to children, older people and those with existing conditions, which is why the law now treats it so seriously. Our guide to black mould health risks covers this in full.

The first step is always to identify and fix the moisture source — cleaning treats the symptom, not the cause. For small areas of hard surface, proprietary mould cleaners are more effective and safer than bleach. Wear gloves, a mask and eye protection, keep the door shut so spores don't spread and a window open for ventilation, and keep cleaning products away from children. Large areas — a substantial part of a wall or ceiling — should be dealt with by a professional, and porous materials like carpets and soft furnishings may not be fully salvageable.

Why a single damp meter reading isn't a diagnosis

Be wary of anyone who diagnoses damp from a damp meter alone, particularly on solid walls and floors. Electrical-resistance meters are reliable for timber, but on masonry they can read high from salts and other conductive materials, not just moisture — so a high number is a prompt to investigate, not a conclusion. A proper investigation looks at the whole building: external defects, ground levels, ventilation, construction type and occupancy, not just a reading on a screen. This is exactly the difference between a PCA specialist survey and a general RICS survey, and it is worth understanding before you instruct anyone.

When to get a survey — and who from

If there is a significant or persistent problem, instruct an independent surveyor with genuine experience in building pathology — the science of how and why buildings fail. Ask about their experience with the specific issue, and prefer an investigation carried out to recognised industry standards rather than a sales visit. An impartial report gives you a professional opinion from someone with no commercial interest in selling you a product, and recommends only works that are proportionate to the building. Our UK damp survey cost guide explains what an independent survey should cost and why "free" surveys rarely are.

If you're a landlord or tenant

Damp and mould is the top-ranked hazard in the Housing Health and Safety Rating System (HHSRS), the framework councils use to assess housing conditions under the Housing Act 2004. The HHSRS is itself being reformed — from 23 June 2026 it moves to 21 hazards and High/Medium/Low bands.

For social landlords, Awaab's Law (in force since 27 October 2025) sets hard deadlines once a hazard is reported: investigate a significant damp and mould hazard within 10 working days, give the tenant a written summary within 3 working days of the investigation concluding, and carry out safety work within 5 working days, with emergencies made safe within 24 hours. Our guide to how long a landlord has to fix damp and mould covers the clocks in detail, and our Awaab's Law page explains how housing providers stay compliant. Tenants who can't get a response should raise concerns with their landlord in writing and, if needed, their local council's environmental health team.

The principle behind all of it

Every part of this guide comes back to one idea: diagnose the cause, don't just record the symptom. A good damp report identifies the moisture source, classifies the severity, and justifies the remedy with evidence — it doesn't stop at a meter reading and a photograph. That is the standard the regulators are converging on, and it is the standard a homeowner, landlord or tenant should expect from anyone they instruct.

Damp and mould: the complete UK guide