Damp and Mould Problems: Signs, Causes and What to Do

What causes damp and mould in a house?

Every damp problem has a water source. Mould spores are present in almost all air, indoors and out, and they only grow when a surface stays damp for long enough. So when mould appears, the useful question is not “how do I kill it?” but “where is the water coming from?” The answer is nearly always one of these:

• CondensationWarm air holds more moisture than cold air. When moist indoor air meets a surface colder than its dew point – window glass, a reveal, an outside wall corner – water forms on it. This is the most common cause of black mould in UK homes.
• Penetrating dampWater getting in from outside through walls, roofs, windows and doors: blocked or leaking gutters and downpipes, defective pointing or render, failed flashings, cracked sills and roof leaks.
• Rising dampGround moisture drawn up through brick and mortar where there is no effective damp proof course, or where the course has been bridged.
• Plumbing leaksWeeping joints, failed shower seals, leaking waste pipes and overflows, or water from the flat above.

Several causes often happen together. An older Liverpool terrace might have condensation mould in the back bedroom, a damp chimney breast from a leak at roof level and a bridged damp proof course in the front room. Treating only one will leave the others behind.

How can you tell condensation, penetrating damp and rising damp apart?

The pattern of the damp – where it is, what it looks like and when it changes – tells you more than any single reading. Use this as a starting point:

If you are still unsure, a damp survey looks at all of these clues together, inside and outside, and explains the findings in plain English.

Where do damp and mould show up, room by room?

Bathrooms

Black mould in silicone sealant and grout, on the ceiling above the shower and in the corners nearest the outside wall. Bathrooms produce bursts of very wet air, and a fan that is too weak, blocked, switched off by a broken pull cord or ducted through a long flexible pipe can leave the room wet for hours. Mould that has grown into sealant cannot be cleaned out and the sealant needs renewing. Our page on bathroom mould removal covers what that involves.

Bedrooms

Mould behind wardrobes and beds, in the top corners of outside walls and around the window. Bedrooms are often kept cooler than living rooms, and two people breathing overnight in a closed room add a lot of moisture. Furniture pushed against a cold outside wall stops air moving and keeps that patch of wall colder still. Condensation control looks at moisture, heating and ventilation together to bring this kind of mould under control.

Kitchens

Mould on the ceiling, behind units, around the window and on the back of wall cupboards. Cooking, kettles and washing machines add moisture, and many small kitchens in terraces and flats have weak or recirculating cooker hoods that do not take damp air outside.

Windows

Streaming glass in the morning, black mould on the frames, seals and reveals, and wet sills. Single glazing, old double glazing and cold reveals are often the coldest surfaces in the room. Replacement windows fitted without trickle vents, or with vents kept closed, remove a route for background ventilation.

Ceilings under the loft

Mould on top-floor ceilings, often near the edges where loft insulation is thin or missing over the wall plate. These are cold spots where condensation forms first.

Ground-floor walls and floors

Tide marks, salts and blown plaster low on walls, peeling skirting paint, rotting skirting boards and a musty smell. Old solid floors without a working membrane can be damp, and laminate, vinyl or rubber-backed carpet can trap that moisture and grow mould underneath. See our page on damp floor repair.

Cellars

Damp walls, salts, a musty smell and sometimes standing water. Below-ground walls hold back moisture from the surrounding ground, so a dry cellar needs a waterproofing system rather than ordinary damp proofing. Many Victorian terraces and villas in Liverpool have cellars – our page on cellar damp proofing explains the options.

Is black mould dangerous?

“Black mould” is a common name for several dark-coloured moulds found indoors. You cannot tell which species it is by looking, and you do not need to know the species to deal with it: the approach is the same – remove the growth safely and stop the moisture.

The UK Government’s guidance Understanding and addressing the health risks of damp and mould in the home (published September 2023 for rented housing providers) says damp and mould can affect anyone’s health. It highlights respiratory effects, especially for people with asthma or other lung conditions, people with weakened immune systems, children and older people. The guidance also tells landlords to deal with the underlying cause rather than just painting over mould, not to wait for medical evidence before acting, and not to blame residents’ lifestyles.

If you are worried about symptoms, speak to your GP or NHS 111, or call 999 in an emergency. This guide is not medical advice.

Why is rising damp so often misdiagnosed?

Rising damp is real, but it is less common than its reputation suggests. Three things lead to mistakes:

1. Damp meters are easily misread. Standard electrical-resistance meters are calibrated for timber. On plaster and masonry they give only relative readings, and salts, foil-backed wallpaper and surface condensation can all make a dry wall read “wet”. A high reading low on a wall is not proof of rising damp on its own.
2. Condensation can collect low on walls. Cold ground-floor walls behind sofas and in corners can be damp from condensation, which looks similar at first glance.
3. Bridged damp proof courses. Many “rising damp” cases are caused by a path or flower bed raised above the damp proof course, render or plaster taken down over it, or debris piled against the wall. Fixing the bridge can solve the problem without injection.

Proper diagnosis looks at the pattern of the damp, the presence of salts, the construction of the building and, where needed, more reliable tests such as drilled masonry samples. BS 6576 is the code of practice for diagnosing rising damp and installing chemical damp proof courses. Our rising damp treatment page explains how we work through it.

What are wet rot and dry rot?

Both are fungal decay of timber, and both need moisture. Wet rot attacks timber that stays wet – skirting boards against a damp wall, floor joists near a leak, window frames where water sits. The wood goes soft, dark and cracked, and the decay stays where the water is. Dry rot (Serpula lacrymans) is more serious: it can spread through masonry towards fresh timber and shows as cotton-wool-like growth, grey strands and rust-coloured spore dust. In both cases, removing the moisture source is the key step, and affected timber is cut out and replaced. Read more about wet rot repair and dry rot treatment.

Can changes to your home cause damp and mould?

Yes, and it is one of the most common stories behind new mould in a home that never used to have it. Look out for:

• New windows without trickle vents, or with smaller ones. The 2021 guidance in Approved Document F expects replacement windows not to reduce background ventilation.
• Blocked or removed vents, such as wall vents covered over during decorating or an extractor fan taken out in a refit.
• A new bathroom or shower added to a back extension or loft room without enough extraction.
• Hard floors laid straight onto an old solid floor without a suitable membrane.
• Raised paths, patios or flower beds built up against the outside wall above the damp proof course.
• Cement render or gypsum plaster on an older solid-walled house, which can trap moisture that used to evaporate.
• Changes in heating, such as heating only some rooms or for short periods. Cold, unheated rooms are where condensation settles. Many households are cutting back on heating because of cost, and that is no one’s fault, but it helps to know that a low, steady background heat usually causes fewer problems than short bursts.

What can you safely do yourself?

There is plenty you can do to reduce condensation and keep an eye on the problem:

• Run extractor fans during cooking and bathing and for a while afterwards; keep kitchen and bathroom doors closed while they run.
• Open trickle vents and keep them clear.
• Put lids on pans, vent tumble dryers outside and dry washing in a ventilated room where possible.
• Leave a gap between furniture and cold outside walls.
• Wipe condensation off windows and sills in the morning.
• Use an inexpensive hygrometer to see how humid rooms get. Humidity that stays high – often quoted as above around 60 to 70% – encourages condensation and mould. Our page on humidity monitoring explains how longer-term readings can help.
• For a small patch of mould on a hard surface, wipe it with a damp cloth and a suitable mould-removal product, following the label, with the room ventilated.
• Look from the ground for overflowing gutters, dripping downpipes and wet patches on outside walls after rain.

What if you rent your home?

Report damp and mould to your landlord, letting agent or housing provider in writing, with dated photos and a note of which rooms are affected and since when. For private rented homes in England, damp and mould growth is one of the hazards councils assess under the Housing Health and Safety Rating System, and dampness is one of the matters listed in the Homes (Fitness for Human Habitation) Act 2018. Social landlords must now follow Awaab’s Law timescales for significant damp and mould hazards, which came into force on 27 October 2025. The government is consulting until 18 December 2026 on extending Awaab’s Law to private rented tenancies – those rules are proposed, not yet final, so check the latest GOV.UK guidance.

We do not give legal advice. For questions about your rights, contact your council’s private sector housing team, Shelter, the Housing Ombudsman (for social housing tenants) or a solicitor. Landlords and agents can find out about our landlord damp and mould service, which includes written notes and photos.

When should you call a professional?

Get the problem looked at if:

• the mould covers a large area, often quoted as more than about one square metre;
• it keeps coming back after cleaning;
• it is in the room of a baby, young child, older person or anyone with asthma or another breathing condition;
• a damp patch grows after rain, or water is visibly coming in;
• plaster is crumbling, skirting is soft or there are signs of rot;
• you have been told you have rising damp and want the diagnosis checked before paying for injection;
• you are buying, selling or letting a property and need a clear written record.

Our mould removal and damp treatment across Liverpool starts with finding the cause, and the damp and mould cost guide explains what affects the price of each kind of repair.