As the weather turns and the heating goes on, damp and mould start to appear in homes across London. A black bloom in the corner of a bedroom, a musty smell in a wardrobe, dark spots around the window seals. It looks alarming, but in most cases it has a clear cause and a clear fix. Understanding what causes damp and mould is the first step to getting rid of it for good.
What causes damp and mould?
Mould needs moisture to grow. In other words, wherever moisture sits on a surface for long enough, mould follows. In London homes, that moisture comes from one of three sources, and telling them apart matters because each has a different fix.
1. Condensation
This is by far the most common cause of damp and mould in flats and houses. In fact, most of the black spots people see are down to it. Everyday living, cooking, showering, drying clothes and even breathing, puts warm, moist air into the home. When that warm air meets a cold surface such as a window, an outside wall or the back of a wardrobe, it turns into water. Do that every day through winter and you get the black spots that most people recognise as mould.
2. Penetrating damp
By contrast, this is water getting in from outside. A cracked roof tile, a blocked or leaking gutter, failed pointing in the brickwork or a gap around a window can all let rain soak into the walls. It usually shows as a damp patch that appears or grows after heavy rain, often in the same spot each time.
3. Rising damp
Less common, however, this is moisture rising up from the ground through the walls, usually because the damp-proof course has failed or been bridged. It leaves a tide mark low down on the wall, often with flaking plaster and a white, salty residue.
How to tell which type you have
In most cases, a quick look narrows down the type of damp and mould:
- Condensation: worst in winter, on cold outside walls, around windows and behind furniture. Water on the glass in the morning is the giveaway.
- Penetrating damp: appears after rain, often higher up a wall or on a ceiling, and comes and goes with the weather.
- Rising damp: stays near the bottom of the wall, with a tide mark and crumbling plaster.
Getting this right saves money, because treating condensation as if it were rising damp, or the other way round, wastes time and never solves the problem.
How to stop condensation and mould
Because condensation causes most damp and mould, most of the fix is about controlling moisture and improving airflow. None of it is complicated. However, it has to be done every day.
- Ventilate while you make moisture. Use the extractor fan during and after showering and cooking, and keep lids on pans.
- Open windows a little, even in winter. Ten minutes in the morning swaps wet air for dry air. Leave trickle vents open.
- Don’t dry washing on radiators. Wet clothes release litres of water into the air. Dry them near an open window or use a dehumidifier.
- Keep the heating low and steady. Cold walls are what turn vapour into water, so a steady background temperature helps.
- Leave a gap behind furniture. Pull wardrobes and sofas a few centimetres off outside walls so air can move.
- Keep extractor fans and air bricks clear. Blocked ventilation is a common hidden cause.
Cleaning mould safely
For small patches, you can clean the damp and mould off yourself. Wear gloves, open a window, and wipe it away gently with a cloth and a suitable mould cleaner rather than brushing it, which spreads the spores. Dry the surface fully afterwards. Anyone with asthma or a weak immune system should leave mould cleaning to someone else. Larger areas, or mould that keeps coming back in the same place, point to a deeper problem that cleaning alone won’t fix.
When damp needs a professional
If the cause of the damp and mould is the building rather than day-to-day living, then no amount of ventilation will solve it. Penetrating and rising damp need the source repaired: clearing gutters, re-pointing brickwork, fixing the roof or renewing a damp-proof course. After that, once the water is stopped and the wall has dried, the damaged plaster usually needs replacing, because damp plaster never takes paint well and the problem looks like it has returned.
Damp and mould in rented homes
For landlords, meanwhile, damp and mould are not just a decorating problem. A home must be free from serious damp and mould to be fit to live in, and tenants are more willing to report it than they used to be. So dealing with the cause quickly, and keeping a record, protects both the property and the tenancy.
Get damp and mould sorted
If mould keeps coming back, or you have a damp patch that won’t dry, it is worth finding the real cause. Our team repairs the sources of damp, from gutters and brickwork to roofs, then our plastering team makes good the damaged walls so the finish lasts. Call 020 3855 2692 or book online for a free assessment.
Damp and mould questions
What causes damp and mould in a house?
Damp and mould grow wherever moisture sits on a surface. The three causes are condensation from everyday living, penetrating damp from water getting in outside, and rising damp from the ground. Condensation is by far the most common.
How do I know if it’s condensation or a leak?
Meanwhile, condensation is worst in winter, forms on cold walls and windows, and leaves water on the glass. A leak or penetrating damp appears after rain and often returns in the same spot.
How do I stop mould coming back?
First, control moisture and improve airflow: use extractor fans, open windows briefly each day, avoid drying washing indoors, keep the heating steady and leave gaps behind furniture. If it still returns, the cause may be in the building.
Is it safe to clean mould myself?
Generally, small patches are safe to clean with gloves, a cloth and a mould cleaner, with a window open. Anyone with asthma or a weak immune system should avoid it, and large areas should be looked at by a professional.
Whose responsibility is damp and mould in a rented home?
Damp caused by the building, such as a leak or rising damp, is the landlord’s responsibility to fix. Condensation from daily living is usually for the tenant to manage with ventilation.
